GeoHECHMS can simulate and analyze the hydrologic processes of a watershed. The common HEC-HMS workflows performed in GeoHECHMS are flexible and can be adapted to address specific hydrologic modeling needs based on the characteristics of the study area and the objectives of the analysis.

Following are some common workflows that can be performed using GeoHECHMS:

• Watershed Delineation
  Based on topographic data and digital elevation models (DEMs), GeoHECHMS can delineate the boundaries of a watershed. This process helps in defining the spatial extents of the study area.
  
• Rainfall-Runoff Modeling
  GeoHECHMS allows users to develop rainfall-runoff models to simulate the response of a watershed to precipitation events. It can incorporate various methods such as the Soil Conservation Service (SCS) Curve Number method, Unit Hydrograph theory, etc., to estimate runoff volumes and hydrographs.
  
• Flood Routing
  GeoHECHMS offers flood routing capabilities to simulate the movement of water through river channels and reservoirs. It can utilize different routing methods like Muskingum-Cunge, Muskingum, and Lag routing to analyze the propagation of flood waves.
• Reservoir Operations
  GeoHECHMS allows for the simulation of reservoir operations, including storage routing, flood control analysis, and determination of release patterns based on inflow forecasts, operating rules, and constraints.
• Design Storm Analysis
  GeoHECHMS automatically determines the design storm and corresponding precipitation based upon the project location anywhere within the USA, Canada, Austria, and Germany. It will determine the design rainfall for the 1, 2, 5, 10, 25, 50, 100, and 500-year storms by querying the Precipitation Frequency Data Server based on the project location.
• Scenario Analysis
  GeoHECHMS facilitates scenario-based analysis by allowing users to evaluate the effects of various factors on the hydrologic response. This may involve investigating the impacts of land use changes, infrastructure development, etc.

In GeoHECHMS, different types of outputs help users to understand watershed behavior, evaluate the performance of hydrologic models, support decision-making processes related to water management and infrastructure design, etc.

Following is the list of common outputs that GeoHECHMS can generate:

1. Hydrographs
   • Time series hydrographs of streamflow or discharge at specified locations within the watershed.
   • Peak flow rates and timing.
   • Rising and falling limb characteristics of the hydrographs.
   • Hydrograph comparison plots for multiple scenarios.
2. Storage-Discharge Curves
   • Relationship between reservoir storage levels and corresponding outflow or discharge rates.
   • Spillway discharge rates and corresponding water levels.
3. Infiltration Losses
   • Quantification of infiltration losses from rainfall.
   • Depth of water infiltrated into the soil.
   • Infiltration rates and their variation over time.
4. Runoff Hydrographs
   • Simulated hydrographs representing the flow of water in rivers or channels.
   • Peak flow rates, timing, and duration.
   • Baseflow and direct runoff separation.
5. Reservoir Operations
   • Reservoir storage levels and variations over time.
   • Reservoir releases and outflow rates.
   • Spillway discharges and flood routing through the reservoir.
6. Project Summaries
   • Summary statistics (i.e., Generated Report) of simulated results, such as peak flows, volumes, durations, and lag times.
   • Error measures for comparing simulated and observed data.

The Export HEC-HMS to HydroCAD command of GeoHECHMS allows the user to export the current HEC-HMS project to HydroCAD input data files.

The following table details the conversions that will occur when exporting a HEC-HMS model to HydroCAD:

Follow the steps below to export a HEC-HMS model to HydroCAD:

1. From the Input ribbon menu, click the Export Data dropdown menu and then select the Export HEC-HMS to HydroCAD command.
   
2. The Export HydroCAD Project dialog box will be displayed.
   
3. From the Select HydroCAD File Type to Export section, select either of the following options:
   • HydroCAD input data files: After selecting this option, click the [...] browse button to specify the directory and the file name for the HydroCAD data to be exported. By default, this option is selected.
   • HydroCAD archive (Zip) file: After selecting this option, click the [...] browse button to specify the directory and the file name. All the HydroCAD project input data files will be archived into a single ZIP file.
4. From the Select Scenario (Plans) to Export section, select either of the following options:
   • All scenarios: Select this option to export all the scenarios contained within the model. By default, this option is selected.
   • Select scenarios: Select this option to export a specific scenario. Click the dropdown combo box adjacent to this option to select the corresponding scenarios to be exported.
     
5. When all the options have been properly defined, click the [Export] button to export the HEC-HMS project to HydroCAD input data files.

The Import HydroCAD Project command allows the user to import HydroCAD input data files to a GeoHECHMS project. This is useful when an existing hydrologic model has already been developed in HydroCAD and needs to be transitioned into GeoHECHMS for further analysis, editing, or reporting. This eliminates the need to manually re-enter the model data from scratch.

A HydroCAD model is represented by a network of the following types of nodes:

• Subcatchment: A relatively homogenous area of land that typically drains into a reach or pond. Each subcatchment generates a runoff hydrograph.
• Pond: A pond, swamp, dam, catch basin, manhole, drywell, or other impoundment that fills with water from one or more sources and empties in a manner determined by a weir, culvert, or other outlet device(s).
• Catch Basin: A special type of pond that provides an insignificant amount of storage but otherwise has all the properties and capabilities of a pond.
• Reach: A uniform stream, channel, or pipe that conveys water from one point to another and operates under open channel flow.
• Link: A link may be used to 1) enter a hydrograph generated outside HydroCAD, 2) interconnect several routing diagrams, 3) scale a hydrograph, 4) split a hydrograph into two components for independent routing, or 5) define a fixed or tidal tailwater elevation.

The following table details the conversions that will occur when importing HydroCAD input data files to a GeoHECHMS project.

Follow the steps below to use the Import HydroCAD Project command:

1. From the Input ribbon menu, click the Import Data dropdown menu and choose the Import HydroCAD Project command.
   unknown node
2. The Import HydroCAD Project dialog box will be displayed.
   unknown node
3. From the Select HydroCAD Project section, click the […] browse button to select the HydroCAD project file.
4. The software will display the Import HydroCAD Project File dialog box. From the displayed dialog box, browse to the folder where the HydroCAD project file is located and select it. Click the [Open] button.
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5. The user will be returned to the Import HydroCAD Project dialog box, and the complete directory path and the file name will be shown in the HydroCAD Project File entry.
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6. Click the [Import] button to import the HydroCAD input data files.
7. The imported HydroCAD model elements will be shown on the Map View, as shown below.
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The Import HEC-HMS Project command allows a preexisting HEC-HMS stormwater model to be imported and then displayed on the Map View.

Follow the steps below to use the Import HEC-HMS Project command.

1. From the Input ribbon menu, click the Import Data menu item and choose the Import HEC-HMS Project command.
2. The Import HEC-HMS Project dialog box will be displayed.
3. From the Select HEC-HMS Project section, click the […] browse button to select the HEC-HMS project file.
4. The software will display the Import HEC-HMS Project File dialog box. From the displayed dialog box, browse to the folder where the HEC-HMS project file is located and select it. Click the [Open] button.
5. After selecting the HEC‑HMS project file, you will be returned to the Import HEC‑HMS Project dialog box, and the complete directory path and the file name will be shown in the HEC-HMS project file entry.
6. From the Select Scenario (Plans) to Import section, select either of the following options:
   • All scenarios: Select this option to import all the scenarios contained within the model. (Note that by default, this option is selected.)
   • Select scenarios: Select this option to import the desired scenarios by selecting them from the Select scenarios dropdown combo box.
7. After specifying all the required details, click the [Import] button to import the HEC-HMS project.

The Import HEC-1 Project command allows a preexisting HEC-1 data file to be imported and converted into a HEC-HMS model, and then displayed on the Map View.

Follow the steps given below to use the Import HEC-1 Project command:

1. From the Input ribbon menu, click the Import Data menu item and choose the Import HEC-1 Project command.
2. The Import HEC-1 Project dialog box will be displayed.

The below sections describe the Import HEC-1 Project command and how to interact with the above dialog box.

Selecting HEC-1 Data Files

This section allows the user to select a HEC-1 data file and a DSS file to be imported. The following options are provided:

• HEC-1 data file: This field is used to select the HEC-1 data file to import. Clicking on the […] browse button displays the Import HEC-1 Project File dialog box. The user can select the HEC-1 data file. After selecting the file, the user will be returned to the Import HEC-1 Project dialog box, and the complete directory path and the file name will be shown in the HEC-1 data file field.
• Associated DSS file: If the HEC-1 file includes ZR records for loading time-series data from a HEC-DSS file, then the user should specify the DSS file that contains the data. Clicking on the […] browse button displays the Import HEC DSS File dialog box. The user can select the DSS file. After selecting the file, the user will be returned to the Import HEC-1 Project dialog box, and the complete directory path and the file name will be shown in the Associated DSS file field.

  Note that without specifying the file, the user will have to manually connect to the time-series data.

Defining Associated HEC-HMS Component Names

This section allows the user to enter the name of the basin model, meteorologic model, and control specifications imported from the HEC-1 model.

Importing a HEC-1 Model

Once the user has specified the HEC-1 file name to import, and the names of the three components, click the [Import] button. The software will read the HEC-1 file and create the appropriate parts of the three components.

GeoHECHMS is a data wrapper to the HEC-HMS software. It is quite easy to export the HEC‑HMS project that the software generates. The Export HEC-HMS Project command allows a HEC-HMS project to be exported to HEC-HMS input data files. This allows the HEC-HMS project to be used with the US Army Corps HEC-HMS software.

Follow the steps below to export a HEC‑HMS project:

1. From the Input ribbon menu, select the Export Data dropdown menu and then choose the Export HEC-HMS Project command.
2. The Export HEC-HMS Project dialog box will be displayed.

The following sections describe the Export HEC-HMS Project command and how to interact with the above dialog box.

Selecting HEC-HMS File Type to Export

The Select HEC-HMS File Type to Export section allows the user to select the HEC-HMS file type to export. The following options are available in this section:

• HEC-HMS input data files: Selecting this radio button option allows the user to specify the directory and the file name for the HEC‑HMS data to be exported. Clicking the [...] browse button displays the Export HEC-HMS Input Data Files dialog box where the available file types are “HEC-HMS Project Files (*.hms)”. After the user defines the HEC-HMS file name, the complete directory path and HEC-HMS project file name is shown in the adjacent string field.
• HEC-HMS archive (ZIP) file: Selecting this radio button option allows the user to archive all the HEC-HMS project input data files into a single ZIP file. Clicking the [...] browse button displays the Export HEC-HMS Archive (ZIP) File dialog box where the available file types are “Archive File (*.zip)”. After the user defines the archive file name, the complete directory path and ZIP file name is shown in the adjacent string field.

Archive (ZIP) Export Options

This section is only enabled when the HEC-HMS archive (Zip) file radio button option is selected. The following options are available in this section:

• Include associated elevation DEM files
  This checkbox option moves all the elevation DEM files that are contained in the Map Data Layers panel into the archive ZIP file.
• Include output results
  This checkbox option moves all the HEC-HMS project analysis output files into the archived ZIP file.

Selecting Scenarios (Plans) to Export

The Select Scenarios (Plans) to Export section contains a radio button group that controls what plan data from the current HEC-HMS project file is to be exported. The following options are provided:

• All scenarios: Selecting this radio button option causes all scenarios (or plans) contained within the current HEC-HMS project to be exported to equivalent scenarios.
• Select scenarios: Selecting this radio button option allows the user to select a single scenario (or plan) contained within the current HEC-HMS project to be exported to an equivalent scenario.

After specifying all the required details, click the [Export] button to export the HEC-HMS project.

In order to create a HEC-HMS model and compute a model analysis, the following project modeling stages must be completed:

1. Collect and organize data
   The first step is to collect and organize the required data for analysis, such as rainfall data, stream gauge data, land use data, soil data, and topographical data. It is important to ensure that the data is accurate, consistent, and in the correct format.
2. Project setup
   Next, the user needs to set up the project in HEC-HMS by defining the watershed boundary, specifying the units of measurement, and setting the time interval for the simulation.
3. Define basin characteristics
   The user then needs to define the physical characteristics of the watershed, such as land use, soil type, and topography. This information is used to calculate the runoff, which is a critical input for the model.
4. Create hydrologic model
   The next step is to create the hydrologic model, which involves selecting the appropriate hydrologic method and defining the parameters of the model. Some of the supported hydrologic methods are SCS Curve Number, Green-Ampt, etc.
5. Model calibration
   After creating the hydrologic model, it is important to calibrate the model using observed streamflow data.

Once the model has been calibrated, the user can compute the model analysis to generate the predicted hydrological response of the watershed. The output of the model includes a range of hydrological variables, such as rainfall, runoff, etc.

In GeoHECHMS, the Compute Analysis command is used to simulate the analysis for single or multiple scenarios (plan) of the model.

Follow the steps below to compute the analysis of the current scenario:

1. From the Analysis ribbon menu, click the Compute Analysis dropdown menu and then select the Current Scenario option. Alternatively, press the [F5] key.
   
2. The Compute Analysis - Current Scenario dialog box will be displayed.
   
3. The name of the scenario selected for analysis is displayed in the Scenario being analyzed read-only field. Once the analysis is performed, the elapsed analysis time will be displayed in the Elapsed analysis time read-only field.
4. Click the [Compute] button to start the analysis.
5. Once the analysis is done, the completion status will be displayed in the Analysis Progress read-only field as the Exit status.
   
   Note: If the Exit status is equal to 0, it means the analysis is successful. If the Exit status is equal to -1, it means the analysis has failed, and the corresponding modeling error(s)/warning(s) will be displayed under the Analysis Progress read-only field.
6. The user can click the [Copy to Clipboard] button to copy the computed analysis progress into the clipboard.

Similarly, the Multiple Scenarios command can be used to perform computations of multiple plans sequentially (one immediately after the other). Refer to this article in our knowledge base to learn more about multiple plan analysis.

Once the analysis is computed successfully, the user can evaluate the results of the simulation to ensure that they are reasonable and consistent with the observed hydrological conditions.

GeoHECHMS can perform analysis on multiple scenarios at the same time. This is useful, for example, when comparing 100-year or 500-year storm events.

The Multiple Scenarios command can be used to perform computations on multiple plans sequentially (one immediately after the other). The user can specify which scenarios to compute, and the order in which they should be computed.

Follow the steps below to use the Multiple Scenarios command:

1. From the Analysis ribbon menu, click the Compute Analysis dropdown menu and select the Multiple Scenarios command.
   
2. The Compute Analysis – Multiple Scenarios dialog box will be displayed. The user can select the scenarios to be analyzed and click the [Compute] button to allow the HEC‑HMS engine to perform the multiple scenario analysis.
   

The Scenario Summary command in GeoHECHMS displays model information for all scenarios (plans) contained within the project. The current scenario plan and the corresponding basin model, meteorological model, and controls specifications are highlighted.

Follow the steps given below to use the Scenario Summary command:

1. From the Input ribbon menu, click the Scenario Manager dropdown combo box and select the Scenario Summary command.
   
2. The Scenario Summary dialog box will be displayed.
   

Note that the above dialog box can also be displayed by clicking the [Summary] button of the Scenario Manager dialog box.

The following sections describe the model information presented in the Scenario Summary dialog box.

General Information

This section displays the project name (Project name), the project run file name (Project file name), and the directory path for the project (Project directory). The text area on the right displays the project description added by the user.

Scenario (Simulation Run)

This section displays Plan Name of all the scenarios within the project, including the associated Basin Models, Meteorologic Models, and Control Specifications in a tabular form. The current scenario of the project is highlighted.

Meteorologic Models

This section displays meteorologic models associated with the project. It consists of meteorological data (i.e., rainfall, distribution, etc.) that define the boundary conditions applied to the model.

Control Specifications

This section displays Control Specifications (i.e., start and stop times, time steps, etc.) associated with the project that are used in running the stormwater analysis for the defined model.

Current Plan Detailed Information

This section provides detailed information on the current scenarios, such as Element Details, Modeling Specifications, and Meteorology Data contained within the model.

Element Details

The Element Details panel lists all the elements (i.e., subbasins, reaches, junctions, storage areas, diversions, sources, sinks, etc.) that make up the HEC-HMS model.

Modeling Specifications

The Modeling Specifications panel describes how the HEC-HMS model is setup.

Meteorology Data

The Meteorology Data panel lists rainfall and other related boundary conditions that make up the HEC-HMS model.

The Delete Scenarios command in GeoHECHMS is useful for deleting data that is no longer relevant to a project and should not be included in a project submittal. Additionally, the function helps to compress the database by deleting data that is no longer in use, resulting in a smaller profile file size.

Follow the steps given below to use the Delete Scenarios command:

1. From the Input ribbon menu, click the Scenario Manager dropdown combo box and select the Delete Scenarios command.
   
2. The Delete Scenarios dialog box will be displayed. It comprises Scenario (Run) Data, Basin Data, Meteorology Data, and Control Specification Data of the HEC-HMS project.
   
3. The user can select the checkbox(es) corresponding to the plan(s), basin model(s), meteorological model(s), and control specification(s) that need(s) to be removed from the HEC‑HMS project and then click the [Delete] button.
   

Note that after the scenario data has been deleted from the HEC‑HMS project, it cannot be recovered with the Undo command. However, if the project is not saved, then the user can reload the project and the scenario data will still be present. Alternatively, if the project has been saved, then the project backup file can be used to recover the previous state of scenario data.

The Duplicate Current Scenario command in GeoHECHMS allows the user to make an identical copy of the current scenario (plan). This copy is independent of and does not interact with the original after it has been made. The user also has the option to make a copy of the current basin, meteorological, and control specification data.

Follow the steps given below to use the Duplicate Current Scenario command:

1. From the Input ribbon menu, click the Scenario Manager dropdown menu and select the Duplicate Current Scenario command.
   
2. The Duplicate Current Scenario dialog box will be displayed.
   
3. Enter the scenario name in the Scenario (run) checkbox entry field to create an identical copy of the current scenario (run). By default, the Scenario (run) checkbox is checked. If unchecked, then the Scenario (run) entry and Description fields are disabled.
4. Enter the description in the Description field to describe the scenario.
5. By default, the software utilizes the current scenario’s basin model, meteorological data, and control specifications for the duplicated scenario.
6. Additionally, the user can check the Basin model, Meteorology data, and Control specification checkboxes to make identical copies of the current scenario’s basin, meteorological, and control specification data. Note that this is useful when creating alternative designs for comparison, such as when comparing detention and retention pond designs for different storm events.
7. Click the [Apply] button, and a copy of the current scenario will be created.
   

Note: When the Basin model checkbox is selected, the software creates copies of any paired data associated with the basin model and assigns each duplicated scenario its own unique paired-data references.

This behavior ensures that the original and duplicated scenarios remain completely independent. As a result, any changes made to the basin model or its associated data in the duplicated scenario do not affect the original scenario.

For example, as shown in the Storage Area Data dialog box, a storage area using the Elevation–Area storage method references an Elevation–Area curve (Elevation Area 01). When the current scenario is duplicated with the Basin model checkbox selected, the referenced elevation–area curve is also duplicated. The Elevation–Area curve dropdown list then displays both the original curve (Elevation Area 01) and a copied version (Elevation Area 01 – Copy (1)). The duplicated storage area is linked to the copied elevation–area curve, allowing users to modify storage characteristics in the new scenario without impacting the original scenario.

GeoHECHMS provides the ability to work with multiple scenarios (or plans) within a single project. Each scenario associates a specific basin model, meteorological model, and set of control specifications with a specific condition or circumstance, such as pre-developed and post-developed phases of a project.

Elements of a HEC-HMS Project

A HEC‑HMS project is comprised of one or more scenarios (plans). Scenarios are formulated by selecting particular basin data, meteorological data, and control specifications.
HEC-HMS uses the following data (separate files) to construct a complete model within a scenario:

• Basin data – Defines the geometry representation of the model.
• Meteorology data – Defines the precipitation data.
• Control data – Defines the configuration for start time, end time, and time interval.
• Run data – References the above files when performing an analysis.

Elements of HEC-HMS Scenario

The Input ribbon menu shows the currently selected scenario and a description of the scenario.

The ribbon menu dropdown combo box allows the user to quickly switch between scenarios. Moving from one scenario to another will cause the contents of the Map View to change to represent the geometry of the selected scenario.

From the Input ribbon menu, expand the Scenario Manager dropdown combo box. It will display four commands: Scenario Manager, Duplicate Current Scenario, Delete Scenarios, and Scenario Summary.

Scenario Manager

Selecting the Scenario Manager command will display the below dialog box. This dialog box displays the details of the currently selected scenario in the project.

The Scenario Manager dialog box is segmented into the following sections:

• Scenario (Simulation Run)
• Basin Model, Meteorologic Model, Control Specifications, and Model Calibration panels
• Scenario Options

Each of these sections contains individual elements that are used to define the corresponding section data. The Scenario Manager dialog box is used to manage the basin data, meteorological data, and control specifications (i.e., basin, meteorological, and control files) associated with the current scenario. In addition, a new scenario, basin model, meteorological model, and control specification can be created, as well as copied from the existing one.

Scenario (Simulation Run)

The Scenario (Simulation Run) section allows you to create new scenarios as well as manage existing scenarios. The Scenario (run data) dropdown combo box lists all the scenarios in the project. The user can select the preferred scenario to view or modify its related data. The remaining sections of the dialog box will be updated based on the user’s selection.

To create a new scenario, the user can select the Add New Scenario option from the Scenario (run data) dropdown combo box. Alternatively, the user can click the [New] button under the Scenario (Simulation Run) section. The default settings will be applied to the remaining sections of the Scenario Manager dialog box. The user can then specify the details for the basin model, meteorologic model, control specifications, model calibration, and other scenario options in their respective sections.

The user can click the [Copy] button to copy the current scenario plan to be further used in another scenario plan. The [Delete] button can be used to delete the currently selected scenario plan.

Basin Model

The Basin Model panel defines the network element data (i.e., subbasins, reaches, junctions, etc.) that define the physical model.

The Basin data dropdown combo box lists all the existing basin models in the current project. Similarly, the Terrain surface and Map layers dropdown combo boxes list the existing terrain surfaces in the project and the corresponding map layers related to the selected terrain surface.

Note that the user can select the blank entry from the Terrain surface dropdown combo box to dissociate a terrain from the project.

Any descriptive information about the basin model can be included in the Description text box area.

The user can select from the already existing basin models to apply to the scenario plan or click the [New] button to define a new basin model. The user can also copy the details of existing basin models by clicking the [Copy] button and later using this information in another basin model. To delete the selected basin model, the user can click the [Delete] button.

Meteorologic Model

The Meteorologic Model panel defines the meteorological data (i.e., rainfall, distribution, etc.) that defines the boundary conditions applied to the model.

The Rainfall data dropdown combo box lists all the existing meteorological models in the current project. Any descriptive information about the meteorological model can be included in the Description text box area.

The user can define a new meteorological model, copy the details of an existing meteorological model to use in another meteorological model, and delete the selected meteorological model using the [New], [Copy], and [Delete] buttons, respectively.

Control Specifications

The Control Specifications panel defines the control specifications (i.e., start and stop times, time steps, etc.) that are used in running the stormwater analysis for the defined model.

The Analysis data dropdown combo box lists all the existing control specifications in the current project. Any descriptive information about the control specification can be included in the Description text box area.

The user can define a new control specification, copy the details of an existing control specification to use in another control specification, and delete the selected control specification using the [New], [Copy], and [Delete] buttons, respectively.

Model Calibration

The following panel is used to define the model calibration data.

This section describes the calibration data used for the HEC-HMS model:

• Calibration type
  This dropdown combo box allows the user to select the type of calibration to aid the simulation run. The following options are provided:

  1. None (default)
  2. Flow
  3. Precipitation

• Calibration multiplier (ratio)
  The spin control defines the multiplication ratio to be applied to the data. By default, this entry is listed as 1.000. The spin control will go up and down in increments of 0.010.

• Apply to subbasins
  This checkbox is enabled for both Flow and Precipitation calibration types. Checking this checkbox will apply the defined calibration multiplier to the subbasins.

• Apply to sources
  This checkbox is enabled for only the Flow calibration type. Checking this checkbox will apply the defined calibration multiplier to the sources.

If the Flow option is selected, then the ratio will be applied to the outflow computed from the subbasin and source elements contained within the basin model before routing the flows downstream through the routing reaches. The same ratio is applied to each basin and source element throughout the basin model.

If the Precipitation option is selected, then the ratio is applied to the precipitation computed by the meteorologic model before the precipitation is applied to the basin model.

Scenario Options

The Scenario Options section allows the user to replace the missing time series data.

Checking the Replace missing time series data checkbox causes the software to substitute a zero value for the missing data values in the time series data during the simulation run. Note that if this option is left unchecked, any missing data will cause the simulation to stop, and an error message to be displayed.

Duplicate Current Scenario

The Duplicate Current Scenario command allows the user to make an identical copy of the current scenario (plan). Once the copy has been made, it is independent of the original and they do not interact.

To learn more about the Duplicate Current Scenario command, refer to this article in our knowledge base.

Delete Scenarios

The Delete Scenarios command allows the user to delete data that no longer applies to the project and should not be submitted as part of the project. The following types of data can be removed: Scenario (Run), Basin, Meteorology, and Control Specification.

To learn more about the Delete Scenarios command, refer to this article in our knowledge base.

Scenario Summary

In the Scenario Manager dialog box, clicking on the [Summary] button will display an informational dialog box that provides information for all scenarios (plans) contained in a project. It includes a complete summary of all the scenarios, basin models, meteorologic models, control specifications, and other details of the current plan.

Alternatively, the user can view the scenario summary by selecting the Scenario Summary command from the list of Scenario Manager dropdown commands.

To learn more about the Scenario Summary command, refer to this article in our knowledge base.

Multiple Plan Analysis

GeoHECHMS can perform analysis on multiple scenarios (plans) at the same time. It is useful, for instance, when comparing storms that occur once every 100 years or 500 years. Computations on the multiple plans can then be performed sequentially (one immediately after the other) using the Multiple Scenarios command.

To learn more about the Multiple Scenarios command, refer to this article in our knowledge base.

Displaying Multiple Scenario Results

After the computations have been performed, the software can display the results from multiple scenarios for all the HEC-HMS elements (i.e., subbasins, reaches, junctions, etc).

To view the output results, select the Plot Hydrographs command from the Results ribbon menu.

The Plot Hydrographs window will be displayed.

The above window has separate panels (i.e., Subbasins, Storage Areas, Junctions, Reaches, etc.) to show output results for each HEC-HMS element type present in the project. Within each element type panel, the user can select the preferred scenario from the Scenarios dropdown combo box of the Select Items panel.

The Variables dropdown combo box allows the user to select the desired output variables associated with an element type whose results are to be displayed.

Note that the Variables dropdown combo box is not available in the Result Summary panel. The Result Summary panel displays the summary of the current HEC-HMS analysis results for each element (Subbasins, Storage Areas, Junctions, etc.) in tabular form.

All the elements (corresponding to the element type) present in the HEC-HMS model of the selected scenario will be displayed under the Elements section. The user can check the checkbox corresponding to the preferred element(s) from the Elements section to see output results associated with them. Alternatively, the user can click the [Pick Elements] button to select the element(s) from the Map View. The user can click the [Clear Elements] button to clear the selected element(s). The output results related to the selected element(s) will be displayed under the Plots, Data Tables and Data Summary panels.

The Global Copy command can be used to copy project elements from one scenario to other existing scenarios of the same project or between two separate (loaded) projects as long as the two projects have the same coordinate reference system (CRS).

This allows the user to quickly copy elements to other scenarios and create new models based on existing ones. This is particularly useful for engineers who want to study and visualize the effects of modifications made to a model. This helps an engineer make faster and better design decisions.

Using Global Copy Command in GeoSTORM

You can use the Global Copy command to copy these stormwater elements:

• Cross sections
• Manholes
• Pipes
• Roadway crossings
• Roadway segments
• Routing junctions
• Routing reaches
• Storage areas
• Subbasins
• Terminal outfalls

Using Global Copy Command in GeoHECHMS

You can use the Global Copy command to copy these HEC-HMS elements:

• Cross sections
• Diversions
• Junctions
• Reaches
• Sinks
• Sources
• Storage areas
• Subbasins

Note that when any of the above elements are copied, then the sub-elements associated with that element are also copied automatically. Hence, the tally for copied elements in the status panel might differ from selected elements.

This article describes the Global Copy command in GeoHECHMS. This command works similarly for the GeoSTORM software.

Follow the steps below to use the Global Copy command in GeoHECHMS:

1. From the Input ribbon menu, click the Global Editing dropdown menu and select the Global Copy command.
2. The Global Copy dialog box will be displayed. This dialog box allows the user to copy the selected elements to a target project. The target project can be the current project or any other stormwater project currently loaded. Note that if there are no elements preselected on the Map View, then the Copy panel is disabled. However, the user can switch to the Find panel to select elements to be copied. Refer to this article in our knowledge base to learn how to use the Find panel.
3. From the GeoHECHMS (loaded) project dropdown combo box, select the target project where elements will be copied.
4. From the Basin model dropdown combo box, select the preferred basin model geometry of the target project. Note that if the current project is selected, then the current basin model geometry is excluded from the listing. However, when a different project is selected, then this entry defaults to the current basin model geometry for the loaded project.
5. Optionally, check the Overwrite existing geometry elements checkbox option to overwrite existing geometry elements with the new geometry elements.
6. Click the [Copy].
7. The copy element process will start. Once the process is complete, the Status panel will display the number of copied elements.
8. Click the [Close] button to close the dialog box.

In GeoHECHMS, you can change the display properties of the following HEC-HMS elements shown on the Map View:

• Cross Sections
• Diversions
• Junctions
• Reaches
• Sinks
• Sources
• Storage Areas
• Subbasin TOC / Lag Time Flow Paths
• Subbasins
• and more…

To view or edit the display properties of different HEC-HMS elements, follow these steps:

1. In the Map Data Layers panel, click the […] Properties button next to the HEC-HMS layer.
   
2. The HEC-HMS Properties dialog box will be displayed.
   
3. In the General Options tab, scroll down through the dialog box to see all the available display options. The following sections are available:
   • Subbasin Display Properties
     Use this section to configure the display properties of subbasin elements. A subbasin represents the drainage catchment that receives rainfall during a storm event.
     
     You can configure display properties of these items:
     1. Subbasin: Turn display on/off.
     2. Subbasin symbol: Turn display on/off, change the size.
     3. Subbasin connection line: Turn display on/off, change the thickness, color and style.
     4. Subbasin border: Turn display on/off, change color, change line thickness and style.
     5. Subbasin IDs: Turn display on/off and change color.
     6. Filled-in subbasin: Turn display on/off, change color and transparency.
   • Subbasin TOC / Lag Time Flow Path Display Properties
     Use this section to configure the display properties of the subbasin TOC (time of concentration) and lag time flow paths. Typically, this flow path is the path that a drop of water would follow when travelling from the most hydrologically remote point to the outlet point of a catchment.
     
     You can configure display properties of these items:
     1. Flow path: Turn display on/off, change line thickness and style.
     2. Flow path IDs: Turn display on/off, change color and location of the label.
     3. Flow path direction arrow: Turn display on/off and change color.
     4. Sheet flow segment color: Change color.
     5. Shallow concentrated flow segment color: Change color.
     6. Channel flow segment color: Change color.
     7. Zero travel time segment color: Change flow segments color through a reservoir, lake, pond, or other structure that has a zero travel time.
   • Junction Display Properties
     Use this section to configure the display properties of junction elements. Junctions function to combine stream flow from multiple upstream hydrologic elements.
     
     You can configure display properties of these items:
     1. Junction: Turn display on/off, change color, change icon’s size, type, border, and border color.
     2. Junction IDs: Turn display on/off and change color.
   • Reach Display Properties
     Use this section to configure the display properties of the routing reaches. Reaches are used for defining conveyance links, such as pipes, ditches, streams, rivers, etc.
     
     You can configure display properties of these items:
     1. Reach: Turn display on/off, change line color, thickness, and style.
     2. Reach IDs: Turn display on/off and change color.
     3. Reach direction arrow: Turn display on/off and change color.
   • Cross Section Display Properties
     Use this section to configure the display properties of routing reach cross sections. A cross section represents the geometry of the underlying terrain surface for the corresponding reach.
     
     You can configure display properties of these items:
     1. Cross section: Turn display on/off, change color and location of the label.
     2. Cross section IDs: Turn display on/off and change color.
     3. Cross section direction arrow: Turn display on/off and change color.
     4. Roughness segments: Turn display on/off and change line thickness.
     5. Bank stations: Turn display on/off, change color, change icon’s size, type, border, and border color.
   • Storage Area Display Properties
     Use this section to configure the display properties of storage area elements. A storage area element is used to model the detention and attenuation of a hydrograph caused by a reservoir or detention pond.
     
     You can configure display properties of these items:
     1. Storage area: Turn display on/off.
     2. Storage area symbol: Turn display on/off, change the size.
     3. Storage area connection line: Turn display on/off, change the thickness, color and style.
     4. Storage area border: Turn display on/off, change color, change line thickness and style.
     5. Storage area IDs: Turn display on/off and change color.
     6. Filled-in storage areas: Turn display on/off, change color and transparency.
   • Diversion Display Properties
     Use this section to configure the display properties of diversion elements. Diversion elements are used to model stream flow leaving (or diverted from) the main channel.
     
     You can configure display properties of these items:
     1. Diversion: Turn display on/off, change icon’s size, type, border, and border color.
     2. Diversion IDs: Turn display on/off and change color.
   • Source Display Properties
     Use this section to configure the display properties of source elements. Source elements are used to introduce flow into a basin model.
     
     You can configure the display properties of these items:
     1. Source: Turn display on/off, change icon’s size, type, border, and border color.
     2. Source IDs: Turn display on/off and change color.
   • Sink Display Properties
     Use this section to configure the display properties of sink elements. Sink elements are used to represent an outlet from the basin model—such as a discharge point into a HEC-RAS river model.
     
     You can configure display properties of these items:
     1. Sink: Turn display on/off, change icon’s size, type, border, and border color.
     2. Sink IDs: Turn display on/off and change color.
   • Other Display Properties
     Use this section to configure the following display properties of text labels:
     
     1. Label auto scaling: Change the scaling of label size based on the zoom level.
     2. Text label size: Change size of text labels.
     3. Text label offset: Change distance of text labels from the item.
     4. Text label alignment: Change alignment of text labels with respect to the item.
     5. Stamp geometry polylines: Turn display on/off.
4. After configuring the above display settings, click the [OK] button to apply them on the Map View.

Notes:

• To restore the HEC-HMS elements to their default display settings, click the [Default] button.
• To restore the HEC-HMS element labels to their default locations, click the [Reset Label Locations] button.

Modifying HEC-HMS Element Properties From the Ribbon Menu

HEC-HMS element properties can also be modified directly from the ribbon menu. Selecting a HEC-HMS layer in the Map Data Layers panel displays a dedicated Display Options ribbon menu, enabling quick and precise customization of its element properties.

Computing subbasin slopes is a critical aspect of hydrologic modeling and watershed analysis. Some of the runoff methods being used in the stormwater project utilize the computed subbasin slope.

This article describes the computational method used to compute the subbasin slope and how the Compute Slopes command works in the CivilGEO software.

Subbasin Slope Computational Method

When calculating the slope for subbasins, the software uses an elevation grid, or Digital Elevation Model (DEM), and applies an algorithm that divides the subbasin area into smaller grid cells. The slope of each grid cell is then calculated based on the rate of change in both the horizontal and vertical directions from the center of the neighboring grid cells.

The illustration below demonstrates how the algorithm computes the slope of a subbasin on a cell-by-cell basis, analyzing the cells from left to right and top to bottom.

• The algorithm of the software does not store the slope for every grid cell. Instead, a slope accumulator keeps track of the total slope, and a cell count accumulator counts how many cells have had their slope calculated.
• The slope is only calculated in four directions (shown as blue lines in the illustration). The other four directions (grey lines) do not need to be calculated again because they were already processed in the previous row of cells.
• Only the cells within the subbasin boundary are considered. Cells outside the boundary are skipped.
• For each cell, the slope is calculated in all four directions, and the absolute value of each slope is determined. The slope can represent either an upward or downward gradient, but by taking the absolute value, the algorithm is only concerned with the steepness or rate of change, not the direction (up or down). For example, a slope of -5 (downward) and a slope of +5 (upward) both have an absolute value of 5, focusing purely on the steepness.
• The direction with the steepest slope is assigned to the cell whose slope is being computed. This maximum slope value is added to the slope accumulator, and the cell count accumulator is increased by one.
• The above algorithm is repeated for every cell in the subbasin.

For the above illustration, the following equations are used to compute the cell slopes:

Where, abs = Absolute slope value

Note: The subbasin slope is equal to the slope accumulator divided by the cell count accumulator.

Compute Slopes Command

Follow these steps to use the Compute Slopes command:

1. From the Watershed ribbon menu, select the Compute Slopes command.
   
2. The Compute Slopes dialog box will be displayed.
   

The following sections describe how to use the Compute Slopes command and interact with the above dialog box.

Selecting Subbasins

The Select Subbasins section includes a table that lists all the subbasins contained within the current scenario.

This section is used to manually select subbasins from the Map View. If a subbasin is already selected on the Map View prior to running this command, the same subbasin will be shown selected within the table.

Alternatively, click the [Pick] button to interactively select subbasins from the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select subbasins from the Map View. Upon returning to the Compute Slopes dialog box, the total number of selected subbasins will be displayed in the Total selected entry.

Selecting Terrain Surface

The Terrain surface dropdown combo box lists all the elevation grids (i.e., DEMs) loaded in the project. Select the elevation grid to be used in computing the subbasin slope.

Computing Subbasin Slopes

Once the data have been defined in the Compute Slopes dialog box, click the [Compute] button. The software will then look at each subbasin selected and compute the average overland flow slope for each subbasin.

After the slopes are computed, the values will be shown in the Computed Slope (ft/ft) editable column, as shown below. The software also allows the user to modify these computed values.

Once the slope is computed, all stormwater runoff methods that require a subbasin slope will have the data populated into the appropriate fields.

The Delineate Catch Basins command allows the user to delineate the drainage areas corresponding to selected catch basin inlets for the defined terrain surface.

Follow the steps given below to use the Delineate Catch Basins command:

1. From the Watershed ribbon menu, select the Delineate Catch Basins command.
   
2. The Delineate Catch Basins dialog box will be displayed.
   

The following sections describe the Delineate Catch Basins command and how to interact with the above dialog box.

Elevation Data

This section allows the user to select the elevation terrain surface. The Terrain surface dropdown combo box lists all the terrain surfaces that are already added to the project.

Defining Drainage Right-of-Way Using Roadway Centerline

This section allows the user to define the drainage right-of-way using the roadway centerline. The user can define the drainage right-of-way by manually drawing the roadway centerline or selecting the already drawn roadway centerline from the Map View.

To draw the roadway centerline on the Map View, follow the steps below:

1. Select Define Drainage Right-of-Way Using Roadway Centerline radio button. Note that this radio button option is selected by default.
2. Select the Draw roadway centerline radio button option and click the [Draw] button.
   
3. The Delineate Catch Basins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
4. Draw the roadway centerline on the Map View. While drawing the roadway centerline, the user can use the [Ctrl] key for the straight segment.
5. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
6. The Delineate Catch Basins dialog box will be redisplayed and the status of the Draw roadway centerline read-only field will be changed from Not Drawn to Drawn.
   
7. The [Clear] button can be used to remove any previously defined roadway centerline in order to redo the process.

To assign the roadway centerline from the Map View, follow the steps below:

1. Select the Assign roadway centerline radio button option and then click the [Pick] button.
   
2. The Delineate Catch Basins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Select the roadway centerline from the Map View.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Catch Basins dialog box will be redisplayed and the status of the Assign roadway centerline read-only field will be changed from Not Selected to Selected.
   
6. The [Clear] button can be used to remove any previously selected roadway centerline in order to redo the process.

The Drainage right-of-way width input field allows the user to define the buffer value for the width of the roadways to compute the drainage area. Using this value, the software can detect the roadway centerline and maintain the correct drainage network. The following image represents a typical “Right-of-Way”.

By default, the software uses a value of 200 ft. The user can enter the buffer value manually or click the […] button to measure the approximate right-of-way width that would be encountered from the Map View.

Defining Drainage Right-of-Way Boundary

This section allows the user to define the drainage right-of-way boundary. The user can define the drainage right-of-way boundary by manually drawing the polygon or selecting the already drawn polygon from the Map View.

To draw the drainage right of way boundary on the Map View, follow the steps below:

1. Select the Define Drainage Right-of-Way Using Boundary radio button to enable this section.
2. Select the Draw boundary radio button option and then click the [Draw] button.
   
3. The Delineate Catch Basins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
4. Draw the drainage boundary on the Map View. While drawing the drainage boundary, the user can use the [Ctrl] key for the straight segment.
5. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
6. The Delineate Catch Basins dialog box will be redisplayed and the status of the Draw boundary read-only field will be changed from Not Drawn to Drawn.
   
7. The [Clear] button can be used to remove any previously defined right-of-way polygon in order to redo the process.

To assign a drainage right of way boundary from the Map View, follow the steps below:

1. Select the Assign boundary radio button option and then click the [Pick] button.
   
2. The Delineate Catch Basins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Select the polygon from the Map View.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Catch Basins dialog box will be redisplayed and the status of the Assign boundary read-only field will be changed from Not Selected to Selected.
   
6. The [Clear] button can be used to remove any previously selected right-of-way polygon in order to redo the process.

Defining Drainage Catch Basin Nodes

This section allows the user to define the catch basin nodes. The user can define the catch basin nodes by manually drawing the nodes or selecting the already drawn nodes from the Map View to assign them as catch basin nodes.

To draw the catch basin nodes on the Map View, follow the steps below:

1. Select the Draw nodes radio button option and then click the [Draw] button.
   
2. The Delineate Catch Basins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Draw the catch basin nodes on the Map View.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Catch Basins dialog box will be redisplayed, and the count of drawn inlet points will be shown in the Draw nodes read-only field.
   
6. The [Clear] button can be used to remove any previously defined catch basin nodes in order to redo the process.

To assign catch basin nodes from the Map View, follow the steps below:

1. Select the Assign nodes radio button option and then click the [Pick] button.
   
2. The Delineate Catch Basins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Select the inlet points from the Map View.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Catch Basins dialog box will be redisplayed, and the count of selected inlet points will be shown in the Assign nodes read-only field.
   
6. The [Clear] button can be used to remove any previously selected catch basin nodes in order to redo the process.

Computational Options

This section provides multiple options that affect the catch basin’s appearance and delineation process.

The following options are provided under the Computational Options section:

• The Drainage contributing area entry field allows the user to define the extent of the drainage accumulation area. This is the area that must be amassed for runoff to be considered as gutter flow. By default, the software uses a value of 5000 square feet. The user can enter a different value for the drainage accumulation area or click the […] button to interactively measure the catch basin contributing area threshold from the Map View.
• Checking the Computed drainage layer group checkbox allows the user to group the catch basin, drainage, and gutter flow layers into one layer group in the Map Data Layers panel. If this checkbox is left unchecked, the software will put the catch basin, drainage, and gutter flow layers separately in the Map Data Layers panel. The user can define the names of these layers by clicking the corresponding pen icons next to the layer name.
• Checking the Delete previously computed layers checkbox causes the software to delete any previously computed drainage layers. By default, this checkbox is checked.
• Checking the Smooth drainage boundaries checkbox causes the software to smooth the boundaries of delineated catch basins. By default, this checkbox is checked.
• Checking the Smooth gutter flow alignments checkbox causes the software to smooth the gutter flow alignment network. By default, this checkbox is checked.
• Checking the Merge adjacent drainage area checkbox causes the software to merge smaller adjacent drainage areas into a larger drainage area.

Computing Catch Basin Delineation

When all the options in the Delineate Catch Basins dialog box have been defined, click the [Compute] button. The delineation process will begin. On completion, the delineated catch basin network will be displayed in the Map View, as shown below.

A subbasin represents the drainage catchment that receives rainfall during a storm event. In hydrology, a subbasin is used to represent the physical watershed.

A watershed could be defined as an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. The word "watershed" is sometimes used interchangeably with drainage basin or catchment.

Commands for Generating Subbasins

There are several commands available in CivilGEO software which allow the user to draw subbasins manually or automatically on the Map View. Some of the commands work only in the US.

Delineate Subbasins Command

This command uses the project’s local DEM gridded elevation data for the delineation process. It works globally and allows the user to automate the subbasin delineation process. Refer to this article in our knowledge base to learn about how to use this command.

Delineate Watershed Command

This command uses the USGS WMS (Web Map Service) service and the NED (National Elevation Data) to automate the watershed delineation process. It computes the contributing watershed drainage network that drains to a selected map location (outlet point) on the Map View. Refer to this article in our knowledge base to learn about how to use this command.

USGS StreamStats Command

This command works only in the US. It allows the user to click on a stream location in the Map View, prompting the software to delineate the watershed basin boundary, compute the basin characteristics, and provide estimates for flow based upon site-specific USGS regression equations. The software will generate GIS shapefiles containing the determined watershed boundary and basin characteristics, stream flow path, as well as PDF results report for the selected site. Refer to this article in our knowledge base to learn about how to use this command.

Draw Subbasins Command

This command allows the user to interactively draw polygons on the Map View as subbasins. After drawing subbasins, the user can name individual subbasins with a unique ID. Refer to this article in our knowledge base to learn about how to use this command.

In CivilGEO's software, the Delineate Subbasins command delineates upstream drainage subbasins from user-selected locations, detailing the subbasin boundaries and upstream drainage tributaries that contribute runoff to the selected locations. If the selected locations are not on an existing stream, the locations are automatically "snapped" to the nearest stream, and then the corresponding subbasin is delineated.

Note that this command is similar to the Delineate Watershed command of the Watershed ribbon menu. These commands are different, however, in that the Delineate Subbasins command uses the user's local elevation terrain surface while the Delineate Watershed command relies on the USGS WMS (Web Map Service) service and NED (National Elevation Data). To learn more about the Delineate Watershed command, refer to this article in our knowledge base.

To use the Delineate Subbasins command, follow the steps below:

1. From the Watershed ribbon menu, select the Delineate Subbasins command.
2. The Delineate Subbasins dialog box will be displayed.

The Delineate Subbasins command operates in three stages, as illustrated below. At each stage, the Delineate Subbasins dialog box provides the user with different options.

The below sections explain the above three stages and describe how to interact with the Delineate Subbasins dialog box.

Clip and Smooth Surface

At this stage, the software performs clipping, thinning, and smoothing of the terrain surface. The following Delineate Subbasins dialog box will be displayed at this stage.

In the above dialog box, the user needs to specify the following:

Terrain Elevation Source

This section allows the user to select the elevation terrain surface. The Terrain surface dropdown combo box lists all the terrain surfaces that are already added to the project.

Terrain Surface Processing Limits

This section allows the user to define the area extent of the terrain surface that will be used to process terrain data.

The user can select from three available options, as described below:

• Selecting the Elevation source extents option causes the software to process the entire terrain data.
• Selecting the User-defined limits option allows the user to select a rectangular region of the terrain surface for processing terrain data.To select a rectangular region, follow the steps below:
  1. Click the [Pick] button.
  2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select a rectangular region for processing the terrain data.
  3. Select a rectangular region from the terrain surface in the Map View. The Delineate Subbasins dialog box will be redisplayed. The status of the User-defined limits read-only field will change to Defined.
  4. The software will consider the selected rectangular region for processing terrain data.

• Selecting the Assign clipping polygon option allows the user to select polygon shape regions from the terrain surface for processing terrain data.To select polygon shape regions, follow the steps below:
  1. Click the [Pick] button.
  2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the clipping polygons.
  3. Select the clipping polygons on the terrain surface from the Map View. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
  4. The Delineate Subbasins dialog box will be redisplayed. The status of the Assign clipping polygons read-only field will change to Selected.
  5. The area underneath selected clipping polygons will be considered by the software for processing terrain data.

Terrain Surface Thinning Options

This section allows the user to define the cell size that will be used for thinning the terrain surface. The smaller the cell size value, the longer the processing time.

The user can select from three available options to define the cell size, as described below:

• Selecting the Compute optimal cell size option causes the software to use the default cell size value computed by the software. By default, the software selects this option.
• Selecting the Define cell size option allows the user to manually define the cell size value. The user can use the spin control button to define the cell size value.
• Selecting the Use terrain surface cell size option causes the software to use the default cell size value of the terrain surface.

Once all the options in the Delineate Subbasins dialog box are configured, click the [Next] button. The processing of the Clip and Smooth Surface stage will begin. On completion, the control will automatically pass to the Stream Delineation stage.

Stream Delineation

At this stage, the software analyzes possible pits, filters, and extracts dissolved pits, as well as locates possible trenches, and performs stream network delineation. The following Delineate Subbasins dialog box will be displayed at this stage.

The software provides three options to choose from to perform stream delineation, as described below:

Automatic Stream Delineation

Selecting this option causes the software to automatically perform stream delineation using stream drop analysis.

The Bridge width threshold input field allows the user to define the threshold value for the width of geographic obstructions such as roadways, bridges, and culverts that are burned on the terrain surface. These obstructions can affect the surface flow and break stream networks. Using the threshold value, the software can detect these obstructions and maintain the correct stream network. The user can enter the threshold value manually in the Bridge width threshold input field or click the [Measure] button to measure the threshold value of geographic obstructions from the Map View.

Checking the Smooth stream trenches checkbox causes the software to level the trenches found on the terrain surface. By default, this check box is checked.

Manual Stream Delineation

Selecting this option allows the user to manually select or draw polylines as delineated streams.

The Manual Stream Delineation option is useful when the user wants the delineation process to be more precise. For example, the user can quickly "burn-in" streams along bridge or culvert connections where one is missing. The software will honor the manually drawn stream alignment and force the delineation to follow that culvert or bridge connection to show the actual water flow.

To assign already drawn polylines as delineated streams, follow the steps below:

1. Click the [Pick] button.
2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the stream alignment polylines.
3. Select the polylines representing the stream network on the terrain surface in the Map View. The status bar will display the count of selected polylines.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Subbasins dialog box will be redisplayed. The stream count will appear in the Stream Polylines read-only field.
   To manually draw polylines as delineated streams, follow the steps below:

1. Click the [Draw] button.
2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to draw stream alignment polylines.
3. Draw the polylines representing the stream network in the Map View. The status bar will display the count of drawn polylines.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Subbasins dialog box will be redisplayed. The stream count will appear in the Stream Polylines read-only field.
   
   Checking the Smooth stream trenches checkbox causes the software to level the stream trenches on the terrain surface. By default, this checkbox is checked.

Do Not Apply Stream Delineation

Selecting this option allows the user to skip the Stream Delineation stage. Select this option when the relevant elevation terrain surface already has a stream network burned on it.

Once all the options in the Delineate Subbasins dialog box are configured, click the [Next] button. The Stream Delineation process will begin. On completion, the software will automatically proceed to the Watershed Delineation stage.

Watershed Delineation

This is the last stage of the delineation process. At this stage, the software performs multiple tasks such as forming stream network and drainage subbasins, smoothing subbasin boundaries, and computing subbasin hydrology. The following Delineate Subbasins dialog box will be displayed at this stage.

In the above dialog box, the user needs to specify the following:

Watershed Delineation

This section allows the user to define the extent of the drainage accumulation area. This is the area that must be amassed for runoff to be considered as streamflow. Smaller values result in minor and major streams; larger values result in only major streams. By default, the software uses a value of 100 acres. The user can enter a different value for drainage accumulation area in the Subbasin contributing area input field or click the […] button to interactively measure it from the Map View.

Manually Define Subbasin Outlets

This section allows the user to manually draw outlets or select the already drawn outlets from the Map View. Select the Manually Define Subbasin Outlets checkbox to enable this section.

Note that it is recommended to use the default settings. For example, if you specify the minimum subbasin area (Subbasin contributing area) to be too large and manually define the outlets, the smaller subbasins may not appear. The software discards the subbasins with smaller areas than the defined minimum subbasin area. As a result, rather than specifying large values for the minimum subbasin area, it is advisable to use the Merge adjacent subbasins option from the Computational Option section. If the subbasins still don't show up, it is better to go with the default settings and let the software automatically find the placement of subbasins.

To draw the outlets on the Map View, follow the steps below:

1. Click the [Draw] button.
2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to place the outlet points.
3. Click on the Map View to place downstream outlets points. The status bar will display the count of placed outlet points.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Subbasins dialog box will be redisplayed. The count of drawn outlet points will appear in the Draw outlets read-only field.
6. The [Clear] button can be used to remove any previously defined outlets in order to redo the process.

To assign outlets from the Map View, follow the steps below:

1. Click the [Pick] button.
2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select outlets points from Map View.
3. Click on the outlets on Map View to select them (selected outlets will be highlighted). The count of selected outlets will be displayed on the status bar.
4. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
5. The Delineate Subbasins dialog box will be redisplayed. The count of selected outlets will appear in the Assign Outlets read-only field.
6. The [Clear] button can be used to remove any previously defined outlets in order to redo the process.

The user can define a threshold value for maximum distance to traverse for snapping the outlet point to the nearest stream in the Snap to stream distance input field. Alternatively, the user can click the […] button to measure the threshold value from the Map View. By default, the software uses a value of 300 ft.

Computational Options

This section provides multiple options to affect the subbasin's appearance and delineation process.

The following options are provided under the Computational Options section:

• Checking the Results layer group checkbox allows the user to group the junctions, subbasin and stream network layers into one layer group in the Map Data Layers panel. If this checkbox is left unchecked, the software will put the junctions, subbasin, and stream network layers separately in the Map Data Layers panel. The user can define the names of these layers by clicking the corresponding pen icons next to the layer name.
• Checking the Delete previously delineated subbasins checkbox causes the software to delete any previously delineated subbasins. By default, this checkbox is checked.
• Checking the Compute basic hydrologic properties checkbox causes the software to compute the basic hydrologic properties of each delineated subbasin. By default, this checkbox is checked.
• Checking the Compute detailed hydrologic properties checkbox causes the software to compute the detailed hydrologic properties of each delineated subbasin.
• Checking the Smooth subbasin boundaries checkbox causes the software to smooth the boundaries of delineated subbasins. By default, this checkbox is checked.
• Checking the Smooth stream alignments checkbox causes the software to smooth the stream alignment network. By default, this checkbox is checked.
• Checking the Merge adjacent subbasins checkbox causes the software to merge smaller adjacent subbasins into a larger subbasin.
• Checking the Fix junctions checkbox causes the software to accurately align the junction points towards the actual intersection point of the delineated subbasins. By default, this checkbox is checked.
• The Subbasin Colorization section allows the user to define and edit the color scheme for delineated subbasins. The Scheme dropdown combo box allows the user to choose a subbasin colorization scheme from multiple color schemes listed under it.

Once all the options in the Delineate Subbasins dialog box are configured, click the [Next] button. The Watershed Delineation process will begin.

Note that the user can switch back and forth between completed stages and change their choices using the [Back] and [Next] buttons. To reset the entire delineation process, the user can click the [Reset All] button.

To learn more about the basic and detailed hydrologic properties computed during the Watershed Delineation stage, refer to this article in our knowledge base.

On completion of the Watershed Delineation stage, the delineated subbasins network will be displayed in the Map View.

When the software imports a model, it automatically places the subbasins on the Map View. However, if the original model was not spatially georeferenced, the subbasins will not align with any loaded background base map. While the software can operate without any issues in this situation, the user may prefer to have the subbasins georeferenced to the background base map. Therefore, it may become necessary to georeference the imported subbasins.

The Georeference Subbasins command is used to manually georeference each of the subbasins to the background base map displayed in the Map View. The process of georeferencing subbasins to the Map View can be a trial-and-error process—especially when the exact location of the original subbasins is not known. Using the Georeference Subbasins command, the georeferencing process can be accelerated.

Follow the steps below to georeference an existing subbasin:

1. From the Input ribbon menu, click on the Drainage Subbasins dropdown menu and then select the Georeference Subbasins command.
   
2. The Georeference Subbasins dialog box will be displayed.
   
3. Click the [Pick] button adjacent to the Subbasin ID dropdown combo box.
   
4. The Georeference Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
5. Select subbasin on the Map View.
6. The Georeference Subbasins dialog box will be redisplayed, and the selected subbasin will be displayed in the Subbasin ID dropdown combo box.
   
   Alternatively, choose the subbasin from the dropdown combo box adjacent to the Subbasin ID entry.
   
   Note that if a subbasin has been preselected prior to running this command, the selected subbasin will be displayed in the Subbasin ID dropdown combo box.
7. Click the [Clear] button to cancel all the previous selections and redo the entire process.

Once the subbasin has been selected, the user can choose between the following options to georeference the subbasin.

• Snap to Alignment Polyline/Polygon
• Draw on Map View

Snap to Alignment Polyline/Polygon

If an existing river alignment centerline for the selected subbasin exists on the Map View, the Snap to Alignment Polyline/Polygon can be used to snap the HEC-HMS subbasin to the alignment polyline/polygon.

Follow the steps below to snap the subbasin to the selected alignment polyline or polygon:

1. Choose the Snap to Alignment Polyline/Polygon radio button option and click the [Pick] button adjacent to the Select alignment polyline/polygon read-only field.
   
2. The Georeference Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Select the alignment polyline or polygon on the Map View.
4. The Georeference Subbasins dialog box will be redisplayed, and the status of the Select alignment polyline/polygon read-only field will be changed from Not Selected to Selected.
5. Click the [Clear] button to cancel all the previous selections and redo the entire process.
6. Click the [Snap] button.
   
7. The software will snap the subbasin to the selected alignment polyline or polygon.

Draw on Map View

This section allows the user to draw the subbasin polygon on the Map View.

Follow the steps below to draw the storage area on the Map View:

1. Choose the Draw on Map View radio button option and click the [Draw] button. Alternatively, turn on the Create curvilinear polygon checkbox option to draw the polygon using curvilinear segments.
   
2. The Georeference Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Draw the subbasin polygon on the Map View.
4. When drawing the subbasin polygon, click on a starting location on the subbasin outline and click on locations where the subbasin outline changes direction. While drawing the subbasin on the Map View, a rubber banding line will be shown to indicate where the subbasin is being placed. Finally, click an ending location for the subbasin outline.
5. When finished, press the [Enter] key or right-click and choose Done from the displayed context menu.

   Note that only one subbasin can be drawn at a time, although the user can revise the drawn subbasin by reselecting the [Draw] button.

6. The Georeference Subbasins dialog box will be redisplayed.
7. The software will automatically snap the subbasin accordingly.

Additional Georeferencing Options

This section contains the Assign polygon area as subbasin drainage area checkbox option. The user can turn on this checkbox for assigning the polygon area as the subbasin’s drainage area. By default, this checkbox option is unchecked when the dialog box is displayed.

The Subbasin Table Edit command allows the user to edit the subbasin parameters in one dialog box for common hydrology infiltration and runoff methods. When the user selects this command, a modeless dialog box will be displayed. This dialog box contains a table with editable columns listing all the subbasins in the current scenario, along with the associated subbasin parameters. When the user selects a row in the table, the corresponding subbasin will be magnified on the Map View (if not visible) and highlighted. In addition, the user can select a subbasin from the Map View, and the corresponding row in the table will be shown selected.

Follow the steps below to use the Subbasin Table Edit command:

1. From the Input ribbon menu, click the Drainage Subbasins dropdown menu and then select the Subbasin Table Edit command.
   
2. The Subbasin Table Edit dialog box will be displayed.
   

The following sections describe how to use the Subbasin Table Edit command and interact with the above dialog box.

Selecting Infiltration and Runoff Methods

The user can select one of the following infiltration and runoff methods from the Options backstage page. Upon selecting the infiltration and runoff method, the parameters displayed in the Subbasin Parameters table changes.

Subbasin Parameters

In the Subbasin Table Edit dialog box, the Subbasin Parameters section contains a table with editable columns listing the subbasin parameters.

Sorting the Subbasin Parameters Table

The data in the Subbasin Parameters table can be sorted by multiple columns. The user can sort the data by holding down the [Shift] key while clicking the Up or Down arrows in the column headers. This allows for multi-column sorting without losing the previous sort. The sorting order is determined by the sequence in which the column headers are selected.

Follow the steps below to sort the data by using multiple columns:

1. Select the header of the first column and click the Up or Down arrow to sort the data in ascending or descending order.
2. Hold down the [Shift] key, then select the header of the second column and click the Up or Down arrow to sort the second column.
3. Repeat step 2 to sort the data of any additional column.

For example, first, select the Initial Abstraction column and click the Up arrow to sort the data in ascending order. Then, while holding the [Shift] key, select the Curve Number column and click the Down arrow to apply a secondary sort.

Copying and Exporting Subbasin Parameters

The data in the Subbasin Parameters table can be copied to the clipboard or exported as a Microsoft Excel or PDF document by using the right-click context menu commands, such as Copy, Copy Table to Clipboard, Export Table to Excel, or Export Table to PDF.

Selected Cells Group Editing

The Selected Cells Group Editing section allows the user to edit multiple cells in the Subbasin Parameters table at once.

The user can select the required cells from the table and then edit the subbasin parameters using one of the following options:

• No change: This radio button option is selected by default. When this option is selected, the data remains unchanged.
• Add constant: This radio button entry allows the user to add a constant value to the existing data. The new value will be the sum of the previous value and a constant value provided by the user.
• Multiply by factor: This radio button entry allows the user to multiply the existing data by a factor. The new value will be the product of the previous value and a constant value provided by the user.
• Apply value: This radio button entry allows the user to replace the existing data with a new value. The new value entered in the adjacent entry field will replace all previous values.

After selecting the preferred option, click on the [Apply Change] button to update the new values in the cells.

In CivilGEO's software, the Reshape Subbasins command allows the user to easily and quickly cleanup the watershed boundary for a single subbasin or between two adjacent subbasins.

This article describes how to use the Reshape Subbasins command.

Reshaping a Single Subbasin

Follow the steps below to reshape a single subbasin:

1. From the Watershed ribbon menu, select the Reshape Subbasins command.
   
2. The Reshape Subbasins dialog box will be displayed.
   
3. From the Single Subbasin panel, click the [Pick] button under the Select Subbasin to Reshape section. The Reshape Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to select the subbasin to be reshaped.
4. Select the subbasin on the Map View to be reshaped.
5. The Reshape Subbasins dialog box will be redisplayed. The status of the Pick subbasin read-only field will change to Selected.
   

Draw Intersecting Reshape Polyline

This option allows the user to draw a polyline on the Map View that the reshaped subbasin should follow. The drawn polyline must intersect the subbasin two or more times. If both polyline endpoints are inside the subbasin, the intersecting area is added to the subbasin. If both polyline endpoints are outside the subbasin, the intersecting area is removed from the subbasin.

1. Choose the Draw Intersecting Reshape Polyline radio button option and click the [Draw] button. Note that the Draw Intersecting Reshape Polyline radio button option is selected by default.
2. The Reshape Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to draw a trace polyline.
3. Draw the reshape polyline on the Map View representing the change in shape for the selected subbasin. Then, press the [Enter] key or right-click and select Done from the displayed context menu.
4. The Reshape Subbasins dialog box will be redisplayed. The status of the Draw reshape polyline read-only field will change to Drawn.
   
5. Click the [OK] button.
6. The software will reshape the selected subbasin.

Use Intersecting Trace Polyline

This option allows the user to select an already existing polyline on the Map View that the reshaped subbasin should follow. The selected polyline must intersect the subbasin two or more times. If both polyline endpoints are inside the subbasin, the intersecting area is added to the subbasin. If both polyline endpoints are outside the subbasin, the intersecting area is removed from the subbasin.

1. Choose the Use Intersecting Trace Polyline radio button option and click the [Pick] button.
2. The Reshape Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to select the trace polyline that intersects with the subbasin to be reshaped.
3. Select the trace polyline on the Map View.
4. The Reshape Subbasins dialog box will be redisplayed. The status of the Pick trace polyline read-only field will change to Selected.
   
5. Click the [OK] button.
6. The software will reshape the selected subbasin.

Reshaping Adjacent Subbasins

Follow the steps below to reshape the subbasin boundary between two adjacent subbasins:

1. From the Reshape Subbasins dialog box, select the Adjacent Subbasins panel.
2. From the Select Adjacent Subbasins to Reshape section, click the [Pick]. The Reshape Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to select two adjacent subbasins with a common boundary to be reshaped.
3. Select two adjacent subbasins on the Map View to be reshaped.
4. The Reshape Subbasins dialog box will be redisplayed. The status of the Pick two adjacent subbasins read-only field will change to Selected.
   

Note that the Draw Intersecting Reshape Polyline and Use Intersecting Trace Polyline options work similar to what is provided in the Single Subbasin panel. These options allow the user to either draw or select an already existing intersection trace polyline from the Map View.

After clicking the [OK] button, the software will reshape the common boundary of two adjacent subbasins along the intersecting trace polyline.

The Split Subbasins command allows the user to split one or more selected subbasins into separate subbasin polygons using a cut polyline. When splitting subbasins, make sure that the cut polyline intersects completely through the selected subbasin polygons.

Follow the steps below to use the Split Subbasins command:

1. From the Watershed ribbon menu, select the Split Subbasins command.
   
2. The Split Subbasins dialog box will be displayed.
   

The following sections describe the Split Subbasins command and how to interact with the above dialog box.

Selecting Subbasins to Split

The Select Subbasins to Split section allows the user to select the subbasins to be split.

The user can click the [Pick] button to select the subbasins from the Map View. After selecting the subbasins, the Pick subbasins read-only field will display the number of selected subbasins.

Alternatively, the user can preselect the subbasins on the Map View prior to running this command. Upon running the command, the number of preselected subbasins will be displayed in the Pick subbasins read-only field.

Defining Cut Polyline

The Define Cut Polyline section allows the user to either draw or select an existing cut polyline from the Map View.

The following options are provided for defining the cut polyline:

Draw Cut Polyline

The Draw cut polyline radio button option allows the user to interactively draw a cut polyline on the Map View.

Follow the steps below to draw a cut polyline on the Map View:

1. Select the Draw cut polyline radio button option and then click the [Draw] button. Alternatively, check the Create curvilinear polyline checkbox to draw the cut polyline using curvilinear segments.
   
2. The Split Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Draw the cut polyline on the Map View. While drawing polylines, the user can use the [Ctrl] key to switch between linear and curvilinear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.
4. After drawing the cut polyline, right-click and select Done from the displayed context menu.
5. The dialog box will be redisplayed, and the status of the Draw cut polyline read-only field will change from Not Drawn to Drawn.

Pick Cut Polyline

The Pick cut polyline radio button option allows the user to select an existing cut polyline from the Map View.

Follow the steps below to select a cut polyline from the Map View:

1. Select the Pick cut polyline radio button option and then click the [Pick] button.
   
2. The Split Subbasins dialog will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
3. Select the cut polyline from the Map View. After selecting the cut polyline, right-click and select Done from the displayed context menu.
4. The dialog box will be redisplayed, and the status of the Pick cut polyline read-only field will change from Not Selected to Selected.

After the cut polyline has been defined, the user can click the [OK] button. The software will split the selected subbasin(s) along the defined cut polyline.

The Merge Subbasins command is used to merge two (or more) existing subbasins as a single subbasin that shares internal polygon boundaries.

Notes:

• When merging subbasins, non-geometric attributes (i.e., area, perimeter, etc.) are assigned from the first selected subbasins.
• If the selected subbasins overlap or are adjacent to one another (share a common edge), then a single subbasin is created.

Follow the steps given below to use the Merge Subbasins command:

1. From the Watershed ribbon menu, select the Merge Subbasins command.
   
   Alternatively, the user can select the Merge Subbasins command from the Drainage Subbasins dropdown menu of the Input ribbon menu.
   
2. The Merge Subbasins dialog box will be displayed.
   
3. Click the [Pick] button adjacent to the Pick subbasin polygons field.
4. The Merge Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select subbasin polygons from the Map View.
5. Click on the polygons representing the subbasins on the Map View to select them.
6. When finished, press the [Enter] key or right-click and select Done from the displayed context menu.
7. The Merge Subbasins dialog box will be redisplayed, and the total number of selected polygons will be displayed in the Pick subbasin polygons field, as shown below.
   
   Note that if the subbasins polygon(s) have been preselected prior to running this command, the count of selected polygon(s) will be displayed in the Pick subbasin polygons field.
8. Check the Compute basic hydrologic properties checkbox to compute the basic hydrologic properties of the selected subbasins. By default, this checkbox is checked.
9. Check the Compute detailed hydrologic properties (optional) checkbox to compute the detailed hydrologic properties of the selected subbasins. By default, this checkbox is unchecked.
10. When all the data have been defined, click the [Merge] button.
    
11. The software will merge the selected subbasins into a single subbasin.

Note that if the user has selected the subbasins polygons that are not adjacent (or directly connected) to each other, the following informational dialog box will be displayed on clicking the [Merge] button.

Subbasins define the drainage area polygons that produce runoff to the other elements in the model. In GeoHECHMS, the Subbasin Data command allows users to add new subbasin and edit subbasin data in a project.

Follow the steps below to view or modify the subbasin data:

1. From the Input ribbon menu, select the Subbasin Data command.
   Alternatively, the user can either double-click on the subbasin polygon from the Map View or choose the Subbasin Data command from the Drainage Subbasins dropdown menu of the Input ribbon menu.
   
2. The Subbasin Data dialog box will be displayed.
   

The following sections describe the Subbasin Data command and how to interact with the above dialog box.

Selecting Subbasin

The Select Subbasin section allows the user to select the subbasin for defining the subbasin data. The user can create a new subbasin, copy existing subbasin data to a new subbasin, and delete a subbasin. In addition, the user can navigate between subbasins and enter a description detailing the defined subbasin.

The following entries are provided in this section:

• Subbasin ID
  This dropdown combo box lists all the subbasins that are defined in the project. The user can select the desired subbasin from the dropdown combo box. Click the pencil icon to edit the subbasin ID. The user can navigate between the previous and next subbasins using the up and down arrow buttons. Alternatively, the user can click the […] button to select the subbasin from the Map View. Note that the up and down arrow buttons will be disabled (i.e., grayed out) when the model contains only a single subbasin.
• Description
  This text field allows the user to enter additional information to describe the selected subbasin.
• New
  The [New] button allows the user to create a new subbasin. On clicking this button, the dialog box will temporarily disappear. A prompt will be displayed on the status bar directing the user to draw a new subbasin on the Map View. Once finished, press the [Enter] key, or right-click and select Done from the displayed context menu. While drawing a subbasin, the user can press the [Esc] key to abort the creation of a new subbasin, which returns the dialog box to its previous state.
  
  The dialog box will be redisplayed. Next, enter the subbasin name in the Subbasin ID entry field and click the [Accept Changes] button. The software checks that the defined ID is unique. If not, a warning dialog box is displayed, and the user is then returned to the Subbasin ID field to change the ID.
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• Copy
  The [Copy] button allows the user to copy existing subbasin data to a new subbasin. When this command is executed, the software automatically provides a unique default name for the duplicated subbasin. The cursor is then placed into the Subbasin ID. The user can go with the default name or enter a different valid and unique ID before moving on to add any other data.
• Delete
  The [Delete] button allows the user to delete a selected subbasin and its associated data from the project.

Subbasin Specifications

The following sections describe the subbasin. Click on the dropdown selector at the Subbasin Specifications entry to display the various data panels that define the subbasin data.

General Specifications

This panel allows the user to define the infiltration, runoff, baseflow, surface storage, and canopy methods to be used for the current subbasin.

Note that the infiltration, runoff, baseflow, surface storage, and canopy methods are defined for the project in the Scenario Manager, which will be displayed automatically in the corresponding dropdown entries. However, the user can change the desired method for the selected subbasin from this dialog box. Refer to this article in our knowledge base to learn how to use the Scenario Manager dialog box.

The following options are provided in this data panel:

• Infiltration (loss) method
  This dropdown combo box allows the user to select the infiltration method for the selected subbasin. Infiltration methods are used to calculate precipitation loss due to infiltration. It is impossible to precisely predict infiltration in heterogeneous soil over commonly used modeling scales, such as hundreds to thousands of square miles. The dropdown combo box provides the following infiltration methods:
  1. None
  2. Deficit and Constant
  3. Exponential
  4. Green and Ampt
  5. Initial and Constant
  6. Layered Green and Ampt
  7. SCS Curve Number
  8. Smith Parlange
  9. Soil Moisture Accounting Loss
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     Refer to this article in our knowledge base to learn about various infiltration methods and how to use them to compute surface runoff.
• Runoff (transform) method
  This dropdown combo box allows the user to select the runoff method for the selected subbasin. Runoff (transform) methods are used to transform the excess precipitation (after the losses have been subtracted) into surface runoff (direct runoff). The dropdown combo box provides the following runoff methods:
  1. None
  2. Clark Unit Hydrograph
  3. Kinematic Wave
  4. SCS Unit Hydrograph
  5. Snyder Unit Hydrograph
  6. User-Specified S-Graph
  7. User-Specified Unit Hydrograph
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     Refer to this article in our knowledge base to learn about various runoff methods and how to use them to compute surface runoff.
• Baseflow method
  This dropdown combo box allows the user to select the baseflow method for the selected subbasin. Baseflow methods are used to represent contributions to subbasin runoff due to excess groundwater. Baseflow is comprised of runoff that infiltrates into the ground, flows down gradient or slope, and then reaches back on the land surface and flows back into small channels. The smaller channels merge into a larger channel, then into the stream and river and so on. The dropdown combo box provides the following runoff methods:
  1. None
  2. Bounded Recession
  3. Linear Reservoir
  4. Monthly Constant
  5. Nonlinear Boussinesq
  6. Recession
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     Refer to this article in our knowledge base to learn about various baseflow methods and how to use them to compute surface runoff.
• Surface storage method
  This dropdown combo box allows the user to select the surface storage method for the selected subbasin. Surface storage methods are used to compute precipitation losses due to local depressions in the ground surface, cracks and crevices in parking lots or roofs. The surface component is included to represent the ground surface where water may accumulate in surface depression storage. Selecting a surface method is optional and generally only used for continuous simulation applications. The dropdown combo box provides the following runoff methods:
  1. None
  2. Simple Storage
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     Refer to this article in our knowledge base to learn about various surface storage methods and how to use them to compute surface runoff.
• Canopy method
  This dropdown combo box allows the user to select the canopy method for the selected subbasin. The canopy component represents the presence of plants in the landscape. The dropdown combo box provides the following runoff methods:
  1. None
  2. Simple Canopy
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     Refer to this article in our knowledge base to learn about various canopy methods and how to use them to compute surface runoff.
• Drainage area
  This entry field defines the area of the selected subbasin. The user can either enter the value manually or click the [...] button to measure the subbasin area from the Map View. Clicking on the [...] button allows the user to draw a measurement polyline representing the outline of the subbasin area. However, initially the software will assign the area from the defined subbasin polygon. If the user graphically edits the subbasin polygon boundary, then the drainage area value automatically updates based upon the Recompute dimensions on element edits checkbox setting in the Options backstage page. Refer to this article in our knowledge base to learn about the Options backstage page.

Clicking on the [Recalc] button causes the software to automatically update this field value with the corresponding subbasin polygon area. Similarly, the [Recalc All] button causes the software to update the subbasin areas for all subbasin polygon areas.

• Downstream connection
  This entry field defines the downstream element that the selected subbasin drains to. Clicking on the [...] button allows the user to select the downstream element from the Map View. The following elements can be selected:
  1. Diversion
  2. Junction
  3. Reach
  4. Reservoir
  5. Sink

Note that a subbasin cannot connect to another subbasin.

• Runoff (flow) adjustment ratio
  This spin control entry field allows the user to specify an adjustment ratio for calibrating the runoff from the selected subbasin. By default, the software uses a value of 1.00, meaning that there is no adjustment. However, the user can enter a different value ranging from 0 to 1.

Computational Results

This section allows the user to see the analysis results for the current subbasin that was computed by HEC-HMS.

DSS Data

This section includes an external DSS file for referencing the HEC-HMS computational results. It allows the user to easily copy the references and paste them into GeoHECRAS.

• DSS file name: This entry defines the external DSS file to be used for reading the data.
• Data path: This entry denotes the data path within the DSS data file which contains the paired data.

Baseflow Data

The Baseflow Data panel is used to enter the data for the selected baseflow method for the defined subbasins. This panel is displayed when the Baseflow Data option is selected in the Subbasin Specifications dropdown combo box. Note that the Baseflow Data panel content changes based upon the baseflow method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

Canopy Data

The Canopy Data panel is used to enter data for the selected canopy method for the defined subbasins. This panel is displayed when the Canopy Data option is selected in the Subbasin Specifications dropdown combo box. Note that the Canopy Data panel content changes based upon the canopy method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

Infiltration (Loss) Data

The Infiltration (Loss) Data panel is used to enter data for the selected infiltration method for the defined subbasins. This panel is displayed when the Infiltration (Loss) Data option is selected in the Subbasin Specifications dropdown combo box. Note that the Infiltration (Loss) Data panel content changes based upon the infiltration (loss) method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

Runoff (Transform) Data

The Runoff (Transform) Data panel is used to enter data for the selected runoff method for the defined subbasins. This panel is displayed when the Runoff (Transform) Data option is selected in the Subbasin Specifications dropdown combo box. Note that the Runoff (Transform) Data panel content changes based upon the runoff (transform) method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

Surface Storage Data

The Surface Storage Data panel is used to enter data for the selected surface storage method for the defined subbasins. This panel is displayed when the Surface Storage Data option is selected in the Subbasin Specifications dropdown combo box. Note that the Surface Storage Data panel content changes based upon the surface storage method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

Basin Hydrologic Properties Computation

After the subbasins have been delineated, the software computes the hydrologic properties (attributes) for each subbasin. The GIS hydrologic attribute variables are represented by square brackets (shown below), allowing the user to run queries on this data.

Basic Properties

The following hydrologic properties are always computed for subbasins, regardless of the detailed hydrologic properties checkbox option.

• Basin area [BASINAREA]
  Area of the drainage basin.
• Basin perimeter [BASINPERIM]
  Perimeter length the drainage basin boundary (divide).
• Overland flow slope (mean) [OVRFLSLAVG]
  Computed by summing the slope of each cell within the subbasin, and then dividing by the number of cells within the subbasin. Reported as a decimal percent slope.
• Overland flow slope (maximum) [OVRFLSLMAX]
  Computed by reviewing the slope of each cell within the drainage subbasin and returning the maximum value. Reported as a decimal percent slope.
• Stream slope (average) [STRMSPLAVG]
  Average slope of all streams within the subbasin, from the top of each stream to where the stream exits the subbasin at the outlet. If more than one stream exists within a subbasin, then this value represents the average slope of the stream with the maximum flow length (i.e., STREAMFLOWLEN). Reported as a decimal percent slope.
• Stream slope (maximum) [STRMSPLMAX]
  Maximum slope of all streams within the subbasin, from the top of each stream to where the stream exits the subbasin at the outlet. If more than one stream exists within a subbasin, then this value represents the maximum slope of the stream with the maximum flow length (i.e., STREAMFLOWLEN). Reported as a decimal percent slope.
• Stream length (maximum) [STRMLENMAX]
  Computed by determining the flow distance traveled from the top of each stream within the subbasin and to where the stream exits the basin at the outlet. If more than one stream exists within a subbasin, then this value represents the maximum flow distance of the evaluated streams.
• Stream length (total) [STREAMLEN]
  Computed by determining the flow distance traveled from the top of each stream within the subbasin and to where the stream exits the subbasin at the outlet. If more than one stream exists within a subbasin, then this value represents the summation of all flow distances of the evaluated streams.

Advanced Properties

The following hydrologic properties are only computed for the subbasins when you select the Compute detailed hydrologic properties checkbox as shown below.

• Basin azimuth [BSNAZIMUTH]
  The compass direction of a straight line from the drainage subbasin outlet to the farthest point on the subbasin parameter. Reported as decimal degrees, measured clockwise from North at 0°.
• Basin compactness ratio [BSNCOMPRAT]
  Computed as the ratio of the subbasin perimeter to the circumference of a circle of equal area (i.e., BASINPERIM / CIRCLEPERIM).
• Basin elevation (mean) [BSNELVMEAN]
  Mean elevation within the subbasin, computed by adding up the elevations of each of the cells contained within the subbasin and then dividing by the number of cells within the subbasin.
• Basin elevation (maximum) [BSNELEVMAX]
  Computed by reviewing the elevation of each cell within the subbasin and returning the maximum value.
• Basin elevation (minimum) [BSNELEVMIN]
  Elevation of the cell at the subbasin cell.
• Basin elevation (relief) [BSNELEVRLF]
  Elevation change contained within the subbasin, computed by BSNELEVMAX – BSNELEVMIN.
• Basin elevation (relative relief) [BSNELEVREL]
  Normalized subbasin relief, computed by BSNELEVRLF / BASINPERIM.
• Basin length [BSNLENGTH]
  Computed by measuring the distance in a straight line from the subbasin outlet to the farthest point on the subbasin parameter. Sometimes referred to as valley length.
• Basin shape factor [BSNSHPFCTR]
  Computed by dividing basin length by basin width. Sometimes referred to as basin slenderness ratio.
• Basin width [BASINWIDTH]
  Computed by dividing basin area by subbasin length. Sometimes referred to as valley width.
• Overland flow distance [OVRFLWDIST]
  Computed by averaging the overland distance traveled from the centroid of each cell within the subbasin to the nearest stream within the subbasin.
• Percent North facing [PRCNTNORTH]
  The percentage of the subbasin whose aspect is directed North, where North is defined as the positive Y direction. Reported as a decimal percent.
• Percent South facing [PRCNTSOUTH]
  The percentage of the subbasin whose aspect is directed South, where South is defined as the negative Y direction. Reported as a decimal percent.
• Stream density [STRMDENSTY]
  Computed as the total stream length divided by the subbasin area (i.e., STREAMLEN / BASINAREA). Reported as miles per mile² or km per km².
• Stream order (Shreve method) [STRMORSHRV]
  Stream order computed by Shreve method of the stream at the subbasin outlet. This method of stream ordering by magnitude, proposed by Shreve in 1967. All links with no tributaries are assigned a magnitude (order) of one. Magnitudes are additive downslope. When two links intersect, their magnitudes are added and assigned to the downslope link.
  
• Stream order (Strahler method) [STRMORSTHR]
  Stream order computed by Strahler method of the stream at the subbasin outlet. This method of stream ordering was proposed by Robert E. Horton (1945) and Arthur Newell Strahler (1952, 1957). All streams with no tributaries are assigned a base order of 1 and the stream order only increases when streams of the same order intersect. Therefore, the intersection of a first-order and second-order link will remain a second-order stream, rather than create a third-order stream.
  
• Stream first order [STRMFRORDR]
  Number of first order stream segments contained within the subbasin. This value is the same for either the Strahler or Shreve stream ordering method.
• Stream sinuosity [STRMSNOSTY]
  Computed by maximum stream length within the subbasin divided by the subbasin length (STREAMLEN / BASINLEN).
• Total flow length [TLFLOWLEN]
  Total flow distance within the subbasin, including both overland and stream flow.
• Total flow slope (average) [TLFLSLPAVG]
  Average flow slope within the subbasin, including both overland and stream flow. Reported as a decimal percent slope.
• Total flow slope (maximum) [TLFLSLPMAX]
  Maximum flow slope within the subbasin, including both overland and stream flow. Reported as a decimal percent slope.
• Total flow slope ratio [TLFLSLPRAT]
  Computed as the average stream slope divided by the average overland flow slope.
• Total stream length [TLSTRMLEN]
  Total length of all streams within the subbasin.

Stream Hydrologic Properties Computation

After the subbasins have been delineated, the software computes the hydrologic properties (attributes) for each stream. The hydrologic attribute variables are represented by square brackets (shown below), allowing the user to run queries on this data.

Basic Properties

The following hydrologic properties are always computed for the streams, regardless of the detailed hydrologic properties checkbox option.

• Stream segment length [SSTRMLEN]
  Length of the stream segment.
• Stream segment slope (average) [SSTRMSLAVG]
  Average slope of the stream segment. Reported as a decimal percent slope.
• Stream segment slope (maximum) [SSTRMSLMAX]
  Maximum slope of the stream segment. Reported as a decimal percent slope.

Advanced Properties

The following hydrologic properties are only computed for the streams when you select the Compute detailed hydrologic properties checkbox as shown below.

• Stream segment elevation (average) [SSTRMELAVG]
  Average elevation of the stream segment, computed by averaging the maximum and minimum elevation of the stream segment.
• Stream segment elevation (maximum) [SSTRMELMAX]
  Maximum elevation of the stream segment.
• Stream segment elevation (minimum) [SSTRMELMIN]
  Minimum elevation of the stream segment.
• Stream segment elevation (relief) [SSTRMELREL]
  Elevation change along the stream segment, computed by SSTRMELMAX – SSTRMELMIN.
• Stream segment order (Shreve method) [SSTRMORSHR]
  Stream order computed by Shreve method of the stream segment. This method of stream ordering by magnitude, proposed by Shreve in 1967. All links with no tributaries are assigned a magnitude (order) of one. Magnitudes are additive downslope. When two links intersect, their magnitudes are added and assigned to the downslope link.
  
• Stream segment order (Strahler method) [SSTRMORSTR]
  Stream order computed by Strahler method of the stream segment. This method of stream ordering was proposed by Robert E. Horton (1945) and Arthur Newell Strahler (1952, 1957). All streams with no tributaries are assigned a base order of 1 and the stream order only increases when streams of the same order intersect. Therefore, the intersection of a first-order and second-order link will remain a second-order stream, rather than create a third-order stream.
  

The Recompute Properties command is used to compute the hydrologic properties (attributes) of stormwater elements (subbasin polygons and reach polylines). Refer to this article in our knowledge base to learn more about hydrologic properties.

Note that if the user has not delineated the subbasins using the Delineate Subbasins command before running this command, the software will prompt the user to perform those computations. Hydrologic properties of only those stormwater elements delineated using the Delineate Subbasins command are computed. To learn more about the Delineate Subbasins command, refer to this article in our knowledge base.

Follow the steps below to use the Recompute Properties command:

1. From the Watershed ribbon menu, select the Recompute Properties command.
   
2. The Recompute Properties dialog box will be displayed.
   
3. Click the [Pick] button to select the stormwater elements for computation of hydrologic properties.
   
4. The Recompute Properties dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to select elements from the Map View.
5. Click the elements on the Map View to select them.
6. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
7. The Recompute Properties dialog box will be redisplayed. The Select elements read-only field will display the number of selected elements.
   
8. The Compute basic hydrologic properties checkbox is used to compute the basic hydrologic properties of the selected elements. By default, this checkbox is selected.
9. Select the Compute detailed hydrologic properties (optional) checkbox to compute the detailed hydrologic properties of the selected elements.
10. Click the [Compute] button.
    
11. The software will compute the hydrologic properties of the selected elements. The progress bar at the bottom will display the progress of the execution gradually.

Note that if the user has selected polylines and polygons that do not have the necessary data fields for storing the hydrologic properties, the command will skip over those elements.

The Compute Reach Parameters command of GeoHECHMS allows the user to compute invert elevation, slope, length, and other parameters using reach alignment along the elevation terrain surface. These parameters are useful for quantifying the physical traits of reaches.

Follow the steps below to use the Compute Reach Parameters command:

1. From the Input ribbon menu, select the Routing Reaches dropdown menu, and then choose the Compute Reach Parameters command.
2. The Compute Reach Parameters dialog box will be displayed.

The following sections describe how to use the Compute Reach Parameters command and interact with the above dialog box.

Selecting Reaches

The Select Reaches section includes a table that lists all the reaches contained within the current scenario along with their elevations, lengths, and slopes.

This section is used to manually select reach(s) from the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select reaches from the Map View.

Upon returning to the Compute Reach Parameters dialog box, the total number of selected reach(s) will be displayed in the Total selected read-only field.

Note that the user can also preselect the reach(s) from the Map View prior to running this command. If the reach(s) were preselected on the Map View, the same reach(s) will be shown selected within the table.

Extract Elevation Data

This optional section is used to define the elevation data source to be used for extracting the reaches. Depending upon the elevation data source type that is selected, different options are provided to specify additional elevation data information.

Refer to this article in our knowledge base to learn more about the types of terrain elevation data that can be used to construct reaches.

If the section checkbox is unchecked, then the Downstream invert elevation and Reach Slope options of the Compute Parameters section will be unavailable (i.e., grayed out). The software will only compute the reach length.

The user can also apply the elevation offset by enabling the Apply elevation offset check box entry. To lower the reaches for specific circumstances—a manhole bottom elevation, for example—define a negative offset value.

Compute Parameters

This section is used to compute the selected parameters for the reach. The Reach length checkbox is checked by default. The software automatically computes the reach length based upon the assigned polyline length.

If the user has checked the Extract Elevation Data checkbox section, the software will also allow the user to compute the following parameters for the reach:

• Downstream invert elevation
• Reach slope

Computing Reach Parameters

Once the data has been defined in the Compute Reach Parameter dialog box, click the [Compute] button. The software will then look at each selected reach and compute the chosen parameters.

Subbasins define the drainage area polygons that produce runoff to other elements in the model. CivilGEO's software automatically calculates and assigns the drainage area for the subbasins from the defined subbasin polygon. The user can keep using the polygons or revise the polygons to represent the actual shape of the drainage area. If the user revises the subbasin polygons, then the drainage area value can be recalculated for the corresponding subbasins using the Compute Subbasins Areas command. This command allows the user to compute the drainage area of multiple subbasins simultaneously.

Follow the steps below to use the Compute Subbasin Areas command:

1. From the Input ribbon menu, select the Drainage Subbasins menu item and select the Compute Subbasin Areas command.
2. The Compute Subbasin Areas dialog box will be displayed.
3. Select the subbasins for which the drainage area needs to be calculated.
4. Alternatively, click the [Pick] button to select the subbasins from the Map View interactively. On clicking the [Pick] button, the Compute Subbasin Areas dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to select the subbasins. Select the subbasins from the Map View. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu. The dialog box will be redisplayed. The number of selected subbasins will be displayed in the Total selected read-only field.
5. Once the subbasins are selected, click the [Compute] button. The software will compute the drainage area for the selected subbasins and update their drainage area values.

An impervious surface is a hard surface that prevents or stops water from entering the soil mantle or causes water to run off the surface in higher amounts or at a faster rate. Common impervious surfaces include rooftops, walkways, patios, driveways, parking lots, storage areas, concrete or asphalt paving, and gravel roads.

During stormwater modeling, impervious surfaces serve as one of the critical factors influencing the rainfall-runoff relationship in urban areas. Impervious surface increases the amount and speed of water entering rivers and other water bodies during a stormwater event and limits the amount of water that can soak into the soil. Impervious surfaces also affect water quality, streamflow, and flooding characteristics of local streams.

In GeoHECHMS, the below listed stormwater loss methods require the user to define the percent of land area that is impervious (e.g., concrete, asphalt, etc.):

• Deficit and Constant
• Exponential
• Green and Ampt
• Initial and Constant
• Layered Green and Ampt
• SCS Curve Number
• Smith Parlange
• Soil Moisture Account

Area of land that is impervious is assumed to have no infiltration losses and the loss method is applied only to the remainder of the area.

Within GeoHECHMS, the user can automatically compute the percent impervious value (i.e., percent of land area that is impervious) for each subbasin using the Compute % Impervious command. This command utilizes the USGS NLCD web map service data and/or the user-defined GIS polygon shapefile representing the impervious areas to compute the percent impervious value. Additionally, it allows the user to manually provide a default percent impervious value that will be assigned to the remaining areas not covered by NLCD and polygon layers.

The software uses a grid approach to determine the composite imperviousness for each subbasin. In this approach, an internal uniformly spaced cell-centered grid is created to examine fixed points in the subbasins and investigate the corresponding NLCD imperviousness and GIS polygon shapefile value at each point. The grid is traversed row by row, investigating each cell within a row. After processing all the cells, the composite impervious parameter value for each subbasin is computed by the software. While determining the imperviousness value, the GIS polygon layer impervious value has the highest priority, followed by the NLCD impervious value, and then the default impervious value. The command also allows the user to compute intermediate results in the form of shapefile.

After computation, the software automatically populates the computed percent impervious value into the appropriate fields of each HEC-HMS stormwater loss method. Refer to this article in our knowledge base to learn more about HEC-HMS stormwater loss methods.

Follow the steps below to use the Compute % Impervious command:

1. From the Watershed ribbon menu, select the Compute % Impervious command.
   
2. The Compute % Impervious dialog box will be displayed.
   

The following sections describe how to compute the % Impervious and interact with the above dialog box.

Selecting Subbasins

The Select Subbasins section lists all the subbasins contained within the current scenario.

The user can select the subbasins for which the percent impervious value is to be computed. If a subbasin is already selected on the Map View prior to running this command, the same subbasin will be shown selected within the table.

Alternatively, the user can click the [Pick] button to interactively select the subbasins from the Map View. On clicking the [Pick] button, the Compute % Impervious dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select subbasins from the Map View. After selecting the subbasins, the user can press the [Enter] key or right-click and choose Done from the displayed context menu. The software will accept the selection of the subbasin and then the dialog box will be redisplayed. The total number of selected subbasins will be displayed in the Total selected entry.

Impervious Land Cover Data

This section consists of four data panels that are used to define the impervious land cover data.

NLCD Layer

This panel allows the user to utilize the USGS Percent Developed Impervious web map service to compute the percent impervious value for a subbasin.

The name (i.e., NLCD Urban Impervious) displayed in the NLCD map layer read-only field is the default NLCD layer name.

The NLCD impervious data source dropdown combo box allows the user to select which impervious data source to use. By default, the command utilizes the latest available impervious data source. The NLCD impervious data source dropdown combo box contains the following options:

• NLCD Impervious 2021
• NLCD Impervious 2019
• NLCD Impervious 2016
• NLCD Impervious 2013
• NLCD Impervious 2011
• NLCD Impervious 2008
• NLCD Impervious 2006
• NLCD Impervious 2004
• NLCD Impervious 2001
  

If the “Overwrite existing data” checkbox option is checked, then the new impervious data selected in the NLCD impervious data source dropdown combo box will overwrite the existing impervious data (if one exists).

Note that for countries other than the USA, the NLCD Layer data panel will be replaced by the GMIS Layer data panel. The software will utilize the GMIS urban impervious database for computing % impervious values in these regions.

Polygon Layer

The user may have a GIS polygon layer that can be used to represent areas of imperviousness. By default, it is assumed that the polygon areas contained within the GIS polygon layer are 100% impervious. However, similar to the USGS Percent Developed Impervious web map service, the software also supports partial imperviousness.

Land use layer
This dropdown combo box allows the user to select the GIS polygon layer included within the project.

Impervious value
This dropdown combo box allows the user to select the corresponding GIS shapefile field that represents the amount of imperviousness for each polygon area.

Value type
This dropdown combo box is used to determine how to apply the defined impervious value for computing the subbasin imperviousness. If the user specifies an impervious value in the above entry, then this dropdown combo box provides the following options:

• Decimal Fraction
• Percentage (default)

Default Value

This panel allows the user to define the default percent impervious value for the subbasins. By default, the software uses a value of 20 %. However, if desired, the user can change this value as needed.

Intermediate Results

In this panel, the user can compute and save the intermediate results of the subbasin polygons in the form of shapefile. The Compute Intermediate Results checkbox option should be checked before computing the intermediate results. If Load intermediate results option is checked, then the software will create a % Impervious Intermediate Results layer in the Map Data Layers panel that will contain the intermediate results for the selected subbasin(s).

Computing Subbasin % Impervious

Once the required data has been defined in the Compute % Impervious dialog box, click the [Compute] button.

The software will then compute the percent impervious value for each of the selected subbasins. After the computation is done, the computed values will be shown in the Computed % Impervious editable column. The software also allows the user to modify these computed values.

Once the computation is done, all HEC-HMS stormwater loss methods that require the percent impervious value will have the data populated into the appropriate fields.

Computed Intermediate Results

After computing the % impervious with the intermediate results, the corresponding intermediate results of the subbasin polygons can be viewed in the % Impervious Intermediate Results layer under the Map Data Layers panel.

Curve numbers provide a practical way to estimate how much rainfall becomes runoff from a watershed or subbasin. In urban areas, the calculation deserves extra attention because rooftops, streets, parking lots, sidewalks, compacted soils, and storm drainage systems can quickly change how rainfall moves across a site.

What is a Curve Number?

A curve number, commonly abbreviated as CN, is an empirical value used in hydrology to estimate direct runoff from rainfall. It ties together three major site characteristics: land use or land cover, the underlying hydrologic soil group, and the moisture or runoff condition assumed for the storm event. In most stormwater applications, CN values are treated as practical runoff indicators: the higher the CN, the greater the expected runoff potential.

The curve number method was developed by the USDA Soil Conservation Service, now the Natural Resources Conservation Service (NRCS), and became widely used because it gave engineers a practical way to estimate runoff from storm events using information that is usually available for a project: soils, cover, and drainage conditions. TR-55, first issued by the Soil Conservation Service in 1975 and revised in 1986, helped make the method especially familiar for small and urbanizing watersheds.

In simple terms, the curve number is not a randomly selected modeling input. It is looked up or computed from a combination of what is on the ground surface and what is underneath. A paved parking lot on soil with poor drainage will produce a much higher CN than a forested area on sandy soil. That is why the same rainfall depth can produce very different runoff results from two areas in close proximity to one another.

Why Urban Areas Need Special Treatment

Urbanization changes the natural water cycle by replacing absorbent ground with surfaces that do not allow much infiltration. Roads, roofs, parking lots, driveways, sidewalks, and compacted soils reduce infiltration and deliver runoff more rapidly to streams or drainage infrastructure. EPA summarizes this as a common hydrologic effect of urbanization: decreased infiltration, increased surface runoff, and faster delivery of runoff through storm drainage systems.

This is why curve numbers are so important in stormwater modeling. A site may look simple in plain view, but hydrologically it can behave very differently depending on whether runoff is allowed to spread across grassed areas or is routed directly into a storm drain. Recent urban hydrology research summarized by the USGS notes that roughly 90 percent of rainfall on impervious surfaces and drainage infrastructure can become runoff, which helps explain why developed watersheds can experience increased flood risk, water quality impacts, and reduced groundwater recharge.

Data Used to Determine Curve Numbers

To determine a curve number, the software needs two main pieces of information: the land use or land cover and the hydrologic soil group. These two datasets are combined spatially, and the appropriate CN value is selected from standard curve number relationships for each land use and soil group combination.

For example, a grassy open space on a well-drained soil will usually have a lower CN than that same grassy area on a poorly drained soil. If the area is paved, the CN will be higher still because most of the rainfall becomes runoff. This is the central idea behind CN lookup tables: each land cover and soil group combination represents a different runoff response.

Understanding Hydrologic Soil Groups

The NRCS classifies soils into hydrologic soil groups based on how easily water can move through the soil when it is wet. These groups have a direct impact on curve numbers because they describe the infiltration side of the runoff equation.

A helpful way to think about the progression is this: Group A behaves more like sand, where water can soak in quickly. Group D behaves more like clay or a shallow restrictive soil, where water is much more likely to remain near the surface and become runoff. Groups B and C fall between those two conditions.

Dual Hydrologic Soil Groups: A/D, B/D, and C/D

Some soils are assigned dual hydrologic soil groups, such as A/D, B/D, or C/D. These are usually wet soils that behave like Group D soils in their natural, undrained condition because of a shallow seasonal water table. If those soils are adequately drained, they may behave like the first letter in the dual classification.

For example, an A/D soil can behave like Group A if it is adequately drained, but like Group D if it remains undrained. The first letter represents the drained condition, and the second letter represents the undrained condition. For this reason, it is important to confirm whether the project condition being modeled reflects drained or undrained soil behavior.

Connected and Unconnected Impervious Areas

In urban watersheds, it is not enough to know how much impervious area exists. It also matters how that impervious area is connected to the drainage system. TR-55 specifically identifies the percentage of impervious area and the means of conveying runoff from impervious areas to the drainage system as important factors in computing urban curve numbers.

Connected Impervious Areas

An impervious area is considered connected when runoff flows directly into a drainage system. This can include runoff flowing into a curb and gutter, storm sewer inlet, channel, pipe, ditch, or another defined drainage path. TR-55 also treats impervious runoff as connected when it travels as concentrated shallow flow across a pervious area and then enters the drainage system.

Urban CN tables were developed for typical land use relationships using assumed percentages of impervious area. They also assume that pervious urban areas behave like pasture in good hydrologic condition and that impervious areas have a CN of 98 when directly connected to the drainage system. If those assumptions do not match the project site, a composite CN should be computed instead of relying only on the default table value.

Unconnected Impervious Areas

An impervious area is considered unconnected when its runoff is spread over a pervious surface as sheet flow before entering the drainage system. This allows part of the runoff to slow down, infiltrate, or be filtered by vegetation before it reaches a gutter, inlet, or channel.

TR-55 provides a separate procedure for an unconnected impervious area when the total impervious area is less than 30 percent. In that situation, the composite CN accounts for both the total impervious area and the ratio of unconnected impervious area to total impervious area. When the total impervious area is 30 percent or greater, TR-55 recommends using the connected impervious area relationship because the remaining pervious area is not expected to significantly reduce runoff.

The chart below shows the unconnected impervious area composite CN procedure used when the total impervious area is less than 30 percent.

Why This Distinction Matters

The difference between connected and unconnected impervious areas can affect the computed CN. TR-55 gives a simple example: a half-acre residential lot with 20 percent total impervious area and a pervious CN of 61 produces a composite CN of 68 if all impervious area is connected, but a lower composite CN of 66 when 75 percent of the impervious area is unconnected.

Rooftop Disconnection

Rooftop disconnection is a common example of an unconnected impervious area. Instead of directing a roof leader or downspout directly into a storm sewer, the discharge is routed onto a vegetated area. This gives runoff a chance to spread out, slow down, and infiltrate before reaching the drainage system.

When rooftop disconnection is allowed by the reviewing agency, it can reduce the amount of directly connected impervious area used in the composite CN calculation. However, this credit should not be applied automatically. Local plumbing codes, stormwater ordinances, separation distances, soil limitations, and grading requirements should always be checked before applying a rooftop disconnection credit.

Local requirements reminder

Rooftop disconnection requirements vary by jurisdiction. Before applying a disconnection credit, confirm that the proposed discharge location, flow path length, slope, soil conditions, and setback requirements comply with the governing stormwater and plumbing requirements.

Common Rooftop Disconnection Checks

The following checks are commonly seen in local stormwater guidance. They should be treated as examples only; always follow the requirements of the governing jurisdiction.

• The contributing roof area to each disconnected discharge point is limited, often to 500 square feet or less.
• The receiving pervious area is not a hydrologic soil group D, or an equivalent very poorly draining soil.
• The pervious flow path has a gentle slope, commonly 5 percent or less.
• The flow path is continuous, vegetated, and free of intervening impervious surfaces.
• The flow path remains separated from ground-level impervious areas, such as driveways and walkways, where required by local guidance.
• Each discharge point has adequate energy dissipation or spreading, so the flow remains sheet flow instead of causing erosion or concentrated flow.

Example Partial Rooftop Disconnection Credit

Some local stormwater ordinances use a partial-credit relationship based on the length of the pervious flow path. The example below is representative of one commonly used approach and should be checked against local requirements before use.

As an example, consider a 1,000 square foot roof served by two roof leaders, with each leader draining 500 square feet to a lawn area. If the receiving area has a hydrologic soil group B, a slope of 3 percent, and a 65 foot pervious flow path before reaching the street, the table above would treat 80 percent of the contributing roof area as disconnected under that example standard. The remaining connected portion would still be treated as directly connected impervious area.

Automated Curve Number Computation in CivilGEO Software

CivilGEO software can automate much of the curve number computation by combining land use or land cover data with hydrologic soil type data. The software evaluates each selected subbasin, overlays the land cover and soil group datasets, and computes the curve number based on the resulting land use and soil group combinations.

For United States projects, this workflow commonly uses online land cover data, such as NLCD, together with NRCS soil survey data. For projects outside the United States, available land cover and soil datasets may vary by country or region. Users can also bring in their own GIS polygon layers when more detailed local land use or soil data are available.

For site-specific projects, user-supplied GIS polygon data are often the best approach because the data can represent precise boundaries for rooftops, parking lots, roadways, landscaped areas, open space, and other important site features. Pixel-based land cover datasets can work well for regional studies, but they may not capture enough detail for smaller urban sites.

In CivilGEO software, the Compute CN command can be used to automatically compute the curve number based on the underlying land use and hydrologic soil type data. To learn more about the Compute CN command, refer to this article in our knowledge base.

Intermediate Results and QA Review

A useful quality-check step is to review the intermediate land use and soil group combinations used by the software. When intermediate results are generated, each unique land use and soil group combination can be represented as a separate GIS polygon. This helps the modeler verify which land cover types and soil groups are driving the final composite curve number for each subbasin.

This is especially helpful for urban projects where small areas can have a large hydrologic effect. For example, a parking lot on Group C soil, a lawn on Group B soil, and a rooftop routed directly to a storm sewer may all exist in the same subbasin. Reviewing the intermediate results makes it easier to confirm that the final CN reflects the intended spatial data and assumptions.

Practical Modeling Guidance

Curve numbers are simple enough to use efficiently, but they still require engineering judgment. The following checks can help avoid common mistakes:

• Confirm that land use categories match the project condition being modeled, such as existing conditions, proposed conditions, or post-development conditions.
• Use site-specific GIS land use polygons when a project requires more detail than an online land cover dataset can provide.
• Verify hydrologic soil groups, especially where soils are disturbed, compacted, filled, drained, or mapped as dual hydrologic soil groups.
• Do not treat all impervious areas the same. Determine whether impervious runoff is connected or unconnected to the drainage system.
• Apply rooftop disconnection credits only when the site meets the governing jurisdiction’s criteria and local code requirements.
• Review intermediate results when available to confirm that the spatial overlay and computed CN values make sense.

Conclusion

The curve number method remains popular because it is practical, familiar, and able to translate land cover, soil behavior, and drainage assumptions into a single runoff parameter. In urban watersheds, the most important modeling step is often not just identifying how much impervious area exists, but understanding how that impervious area drains. Connected impervious areas typically produce more runoff, while properly unconnected areas may allow some infiltration and runoff reduction. By combining accurate land use data, reliable hydrologic soil group data, and careful review of connected versus unconnected impervious areas, the computed CN values can provide a more defensible basis for stormwater modeling and design.

What are Curve Numbers?

A curve number (CN) is a hydrologic parameter used to describe the stormwater runoff potential for the drainage area. It is a function of land use, soil type, and soil moisture. The curve number values range from approximately 30 (for permeable soils with high infiltration rates) to 100 (for water bodies, impervious surfaces, and soil near-zero infiltration rates).

This video demonstrates how to compute the curve numbers.

Curve Number Computation

The process of computing CN involves land use and soil types. Our software automates this process by using the following data:

• Drainage area
• Land use and soil maps data
• Look-up tables for software reference

For a watershed that consists of several soil types and land use, a composite CN is calculated as:

Compute CN Command

The Compute CN command is used to compute the curve number (CN) for each subbasin, which is used to determine how much runoff occurs based upon the Land Use (or Land Cover) and the underlying Hydrologic Soil Type. For example, urban areas that are characterized by pavement and hard surfaces will generate significant runoff, regardless of the underlying hydrologic soil type. To learn more about curve number computation for urban areas, refer to this article in our knowledge base.

Wooded areas tend to absorb a lot of rainfall but produce very little runoff, unless the underlying hydrologic soil type cannot absorb the rainwater, such as hardpan clay soils. Therefore, the command will combine the two coverages (i.e., Land Use and Hydrologic Soil Type) and develop a representative curve number for each subbasin.

Follow the steps below to compute the curve number for subbasins using the Compute CN command:

1. From the Watershed ribbon menu, select the Compute CN command.
2. The Compute CN dialog box will be displayed.
   Note that the [Compute] button will be grayed out until the user has selected a subbasin.

The following sections describe how to use the Compute CN command and interact with the above dialog box.

Selecting Subbasins

The Select Subbasins section includes a table that lists all the subbasins contained within the current scenario.

The user can select the subbasins for which the curve number value is to be computed. If a subbasin is already selected on the Map View prior to running this command, the same subbasin will be shown selected within the table.

Alternatively, the user can click the [Pick] button to interactively select the subbasins from the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select HEC-HMS subbasins from the Map View. Upon returning to the Compute CN dialog box, the total number of selected subbasins will be displayed in the Total selected read-only field as shown below.

Note that the user can select multiple subbasins from the Map View prior to running the Compute CN command by holding the [Ctrl] key while selecting the desired subbasins.

Defining Computational Data

The Define Computational Data section contains three panels: Land Use Data, Hydrologic Soil Type Data, and Intermediate Results. These panels are used to define the land cover data and hydrologic soil group data that will be used for computing the curve number for the selected subbasins.

Land Use Data

The Land Use Data panel contains three subpanels, which are described below:

NLCD Layer

The NLCD Layer subpanel is used to determine land type with data from the National Land Cover Database (NLCD), which is represented by 75 ft cells throughout the USA. The NLCD map service is used to determine the curve number based upon the hydrologic soil group (i.e., A, B, C, D, A/D, B/D, C/D) as shown below.

Note that for countries other than the USA, the software provides similar land cover databases. The content of this data panel will change to represent the one that is available for the project area. Currently, our software supports land cover databases from Africa, Australia, Canada, Europe, India, New Zealand, and the USA.

The Layer cover data source dropdown combo box allows the user to select which land use data source to utilize for the newly created layer. By default, the software selects the most recent land use data. The following options are available in the dropdown combo box:

• NLCD Land Cover 2021 (default)
• NLCD Land Cover 2019
• NLCD Land Cover 2016
• NLCD Land Cover 2013
• NLCD Land Cover 2011
• NLCD Land Cover 2008
• NLCD Land Cover 2006
• NLCD Land Cover 2004
• NLCD Land Cover 2001

If the “Overwrite existing data” checkbox option is checked, then the new land cover data selected in the Land cover data source dropdown combo box will overwrite the existing land cover data (if one exists).

The user can click the [Edit] button to display the NLCD Land Cover dialog box that provides an editable data grid, allowing the user to change the CN values of different hydrologic soil groups.

After editing the required cell values, the user can click the following buttons:

• [Default] – This button will restore the default CN values and close the dialog box.
• [Save] – This button will save the user changes and close the dialog box.
• [Cancel] – This button will discard any user changes and close the dialog box.

The NLCD Layer subpanel will be replaced by other subpanels based on the modeling region, as described below:

GIS Polygon Layer

The GIS Polygon Layer subpanel is used for user-defined land use polygon data. The land use data can either directly assign the CN value for each polygon or provide a standard land use type for the software to cross reference. When the user makes the Standardized land use type attribute field radio button entry active, then the Soil Group CN attribute field dropdown combo box entries become enabled. The Soil Group CN attribute field dropdown combo box entries show text, float, and integer fields contained within the selected land use layer. Then, the software determines which CN value should be applied based upon the corresponding land type and CN value.

Default Values

The Default Values subpanel is used to define the default CN value or standardized land use type for the software to cross-reference. The user can enter integer or float values into these default value fields.

Hydrologic Soil Type Data

The Hydrologic Soil Type Data panel contains three subpanels, which are described below:

NRCS Layer

The NRCS Layer subpanel uses data from the NRCS Soil Survey Database to determine the hydrologic soil group data for the watershed region.

The Natural Resources Conservation Service (NRCS) has divided soils into four hydrologic soil groups (i.e., A, B, C, and D) according to water’s ability to infiltrate. In addition to standard hydrologic soil groups, the software supports dual hydrologic soil groups (i.e., A/D, B/D, C/D). The dual hydrologic soil groups are used for certain wet soils because they might have a high-water table within 60 cm of the surface. Once drained, these soils can be assigned a less restrictive hydrologic soil group (i.e., “A” instead of “D” in a HSG of “A/D”). The first letter in the dual hydrologic soil group refers to the drained condition and the second letter refers to the undrained condition.

Note that for countries other than the USA, the software provides similar soil survey databases. The content of this data panel will change to represent the one that is available for the project area. Currently, our software supports land cover databases for Africa, Australia, Canada, Europe, India, New Zealand, and the USA.

The NRCS Layer subpanel will be replaced by other subpanels based on the modeling region, as described below:

GIS Polygon Layer

The GIS Polygon Layer subpanel is used for the user-defined soil map polygon data. The soil map data provides hydrologic soil group data for each defined polygon.

Default Value

The Default Value subpanel is used to define a default hydrologic soil type for the software to cross-reference as shown below.

Intermediate Results

The Intermediate Results panel allows the user to compute the intermediate results of the selected subbasin(s) in a shapefile, save the results at the user-specified location, and load the results in a project. By default, the content of this panel is disabled (i.e., grayed out). Select the Compute Intermediate Results checkbox to enable the content of this panel.

Clicking the […] browse button adjacent to the Intermediate results shapefile path entry field allows the user to define the location where the intermediate computed results will be saved.

Note that the Load intermediate results checkbox should be checked to load the computed intermediate results in a project.

Computing Curve Number

When the data have been defined in the Compute CN dialog box, click the [Compute] button. The software will then look at each selected subbasin and compute the curve number for each subbasin.

After the curve numbers are computed, the values will be shown in the Compute CN editable column, as shown below. The software also allows the user to modify these computed values.

Once curve numbers are computed, all stormwater methods that require a curve number will have the data populated into the appropriate fields.

Note that while the computation is running, the [Close] button changes to [Cancel] so that the user can abort applying the computed curve numbers if desired.

In the Map Data Layers panel, the software provides a data legend for computed intermediate results of each subbasin layer that lists the land covers included within that layer. The user can expand the layer to view the data legend.

Clicking on the […] button adjacent to the computed intermediate results of each subbasin layer will display the GIS polygon properties dialog box, allowing the user to adjust the color and style of the polygon(s) according to the land and soil type. To learn more about the GIS polygon properties dialog box, refer to this article in our knowledge base.

The canopy component represents the presence of plants in the landscape. Plants intercept precipitation, reducing the amount of precipitation that arrives at the ground surface. Intercepted water evaporates between storm events. Plants also extract water from the soil in a process called transpiration. Evaporation and transpiration are often combined as evapotranspiration.

In GeoHECHMS, the Simple Canopy method is used to compute the precipitation loss due to evapotranspiration. Selecting a canopy method is optional and generally used for continuous simulation applications. If no canopy method is selected, the subbasin will not compute any losses due to evapotranspiration and all precipitation will be treated as direct precipitation.

The Simple Canopy method provides a simple representation of a plant canopy. In this method, all the precipitation is intercepted until the canopy storage capacity is full. Once the canopy storage is filled, all further precipitation falls to the surface or directly to the soil if no surface representation is included. All potential evapotranspiration will be used to empty the canopy storage until the water in storage has been eliminated. The potential evapotranspiration is multiplied by the crop coefficient to determine the amount of evapotranspiration from canopy storage and later from surface and soil components. Only after the canopy storage has been emptied will the surface, and soil components, use the unused potential evapotranspiration.

Note that the Dynamic Canopy and Gridded Simple Canopy methods are not supported in GeoHECHMS since they are mostly used by researchers or the Army Corps of Engineers.

Follow the steps given below to select a canopy method:

1. From the Input ribbon menu, select the Subbasin Data command.
   
2. The Subbasin Data dialog box will be displayed.
   
3. From the Subbasin ID dropdown combo box, select the subbasin to assign a canopy method.
   
4. From the Canopy method dropdown combo box, select the canopy method.
   
5. To enter the parameters for the selected canopy method, select the Canopy Data option from the Subbasin Specifications dropdown combo box.
   
   Note that the Canopy Data option will be grayed out (i.e., unavailable) when None is selected for the canopy method.
6. The Canopy Data panel will be displayed.
   
7. Specify the initial condition of the canopy storage using the Initial storage spin control button. The initial condition is specified as the percentage of the canopy storage that is full of water at the beginning of the simulation.
8. In the Maximum storage field, specify the maximum amount of water that can be held on leaves before through-fall to the surface begins. The amount of storage is specified as an effective depth of water.
9. Specify the Crop coefficient, which is the ratio applied to potential evapotranspiration. It is used for calculating the quantity of water extracted from the soil during crop development.
10. From the Evapotranspiration dropdown combo box entry, select one of the following options:
    • Dry Periods Only
      Use this option to set the canopy to evaporate water from storage and extract water from the soil only during dry periods with no precipitation.
    • Wet & Dry Periods
      Use this option to set the canopy to evaporate water from storage and extract water from the soil during both dry and wet periods.
      
      Note that the choice for simultaneous precipitation and evapotranspiration can improve results when using a long interval.
11. From the Uptake method dropdown combo box entry, select the method for extraction of water from the soil.
    
    The Simple method extracts water at the potential evapotranspiration rate and can be used with the deficit constant or soil moisture accounting infiltration loss methods. In contrast, the Tension Reduction method can be used with the soil moisture accounting method and extracts water at the potential evapotranspiration rate from the gravity zone, but reduces the rate when extracting from the tension zone. When None is selected, there will be no soil water extraction. To learn how to select an infiltration method for a subbasin, refer to this article in our knowledge base.
12. When the data has been defined, click the [Close] button to close the Subbasin Data dialog box.

Canopy Methods along with the Kinematic Wave Runoff Method

If Kinematic Wave is selected as the runoff method, then the Canopy Data panel will be divided into two surface area sections as shown below.

By default, Surface area #1 is active with the Subbasin area field set as 100 %. However, changing the Subbasin area in Surface area #1 activates the Surface area #2 fields.

In addition, changing the Subbasin area in either Surface area #1 or Surface area #2 adjusts the Subbasin area in the other counterpart such that the total Subbasin area adds to 100%.

To learn how to select a runoff method for a subbasin, refer to this article in our knowledge base.

Pros and Cons of Simple Canopy Method

The surface storage component of the subbasin element represents the ground surface where water may accumulate in surface depression storage. The depression storage of an impervious surface such as a parking lot is generally close to zero. However, the depression storage for an agricultural field can be quite large if conservation tillage practices are used.

In GeoHECHMS, the simple storage method is used to compute the precipitation losses due to depression storage. Selecting a surface storage method is optional and generally only used for continuous simulation applications.

The simple storage method provides a simple representation of the soil surface. In this method, all the precipitation or precipitation through-fall from the canopy arriving on the soil surface is captured in storage until the storage capacity of the surface is full. Water in surface storage infiltrates into the soil whenever it is present, i.e., water will infiltrate even when the storage capacity is not full. The surface runoff will begin when the storage capacity is filled, and the precipitation through-fall rate exceeds the infiltration rate.

Note that the grided surface storage method is not supported in GeoHECHMS as it is mostly used by researchers or the Army Corps of Engineers.

The user can select a surface storage method for a subbasin from the General Specifications section of the Subbasin Data dialog box.

Follow the steps given below to select a surface storage method:

1. From the Input ribbon menu, select the Subbasin Data command.
2. The Subbasin Data dialog box will be displayed.
3. From the Subbasin ID dropdown combo box, select the subbasin to assign a surface storage method.
4. From the Surface storage method dropdown combo box, select the surface storage method.
5. To enter the parameters for the selected surface storage method, select the Surface Storage Data option from the Subbasin Specifications dropdown combo box. Note that the Surface Storage Data option will be grayed out (i.e., unavailable) when None is selected for the surface storage method.
6. The Surface Storage Data panel will be displayed.
7. Specify the initial condition of the surface storage using the Initial storage spin control button. Initial condition is specified as the percentage of the surface storage that is full of water at the beginning of the simulation.
8. Specify the maximum amount of water that can be held on the soil surface in the Maximum storage entry field. This is the amount of water that can be accumulated in the depression storage before surface runoff begins. The amount of storage is specified as an effective depth of water. Click the […] button to display the Maximum Storage lookup table.
9. When the data has been defined, click the [Close] button to close the Subbasin Data dialog box.

Pros and Cons of Simple Storage Method

Subbasins define the drainage area polygons that produce runoff to the other elements in the model. While a subbasin element conceptually represents infiltration, surface runoff, and subsurface processes interacting together, the actual infiltration loss calculations are performed by an infiltration method contained within the subbasin.

There are many different infiltration methods provided with HEC-HMS. Some of the methods are designed primarily for simulating events, while others are intended for continuous simulation. All of the methods conserve mass. That is, the sum of infiltration and precipitation left on the surface will always be equal to total incoming precipitation.

The table below shows the various methods' suitability for an event and continuous simulation.

In GeoHECHMS, the infiltration method for a subbasin can be selected from the General Specifications section of the Subbasin Data dialog box.

Follow the steps given below to select an infiltration method:

1. From the Input ribbon menu, select the Subbasin Data command.
   
2. The Subbasin Data dialog box will be displayed.
   
3. From the Subbasin ID dropdown combo box, select the subbasin to assign an infiltration method.
4. From the Infiltration (loss) method dropdown combo box, select the infiltration method.
   
5. To enter the data for the selected infiltration method, select the Infiltration (Loss) Data option from the Subbasin Specifications dropdown combo box.
   
6. The Infiltration (Loss) Data panel will be displayed. Note that the Infiltration (Loss) Data panel content changes based upon the infiltration (loss) method selected in the General Specifications section.
   

The following sections describe the different infiltration methods and how to define the data for each method in the Infiltration (Loss) Data panel.

Infiltration Method: None

When None is selected for the Infiltration (Loss) Method, then the Infiltration (Loss) Data dropdown combo box entry is disabled (i.e., grayed out).

Infiltration Method: Deficit and Constant

The deficit and constant infiltration method uses a single soil layer to account for continuous changes in moisture content. This method is used for continuous simulation.

It should be combined with a canopy method that will extract water from the soil in response to potential evapotranspiration computed in the meteorologic model. The soil layer will dry out as the canopy extracts soil water between precipitation events. There will be no soil water extraction unless a canopy method is selected. To learn how to select a canopy method for a subbasin, refer to this article in our knowledge base.

It may also be combined with a surface storage method that will hold water on the land surface. The water in surface storage infiltrates the soil layer. The infiltration rate is determined by the capacity of the soil layer to accept water. To learn how to select a surface storage method for a subbasin, refer to this article in our knowledge base.

When Deficit and Constant is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Initial moisture deficit
  This entry field defines the initial condition for the method. At the start of the simulation, it is the amount of water required to fill the soil layer to the maximum storage.
• Maximum moisture deficit
  This entry field is used to specify the total amount of water the soil layer can hold as an effective depth.
• Constant loss rate
  This entry field defines the infiltration and percolation rates when the soil layer is saturated. Clicking on the […] lookup button will display a Soil Saturated Hydraulic Conductivity lookup table dialog box.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious. No loss calculations are carried out on the impervious area. All precipitation on that portion of the subbasin becomes excess precipitation subject to surface storage and direct runoff. Note that the user can automatically compute the percent imperviousness for each subbasin using Compute % Impervious command. Refer to this article in our knowledge base to learn more about this command.

Infiltration Method: Exponential

The exponential infiltration method is empirical and should not be used without calibration. This method should only be used for event simulation. It represents incremental infiltration as an exponentially decreasing function of accumulated infiltration. It includes the option for increased initial infiltration when the soil is particularly dry before the arrival of a storm. Before using this method, consideration should be given to the Green and Ampt method (explained below) because it produces a similar exponential decrease in infiltration and uses parameters with better physical interpretation.

When Exponential is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Initial range
  This entry field specifies the amount of initially accumulated infiltration during which the loss rate increases. This parameter is a function of antecedent soil moisture deficiency and is usually storm-dependent.
• Initial coefficient
  This entry field specifies the starting loss rate coefficient on the exponential infiltration curve. It is assumed to be a function of infiltration characteristics and consequently may be correlated with soil type, land use, vegetation cover, and other properties of a subbasin.
• Coefficient ratio
  This entry field specifies the rate at which the exponential decrease in infiltration capability proceeds. It may be considered a function of the ability of the surface of a subbasin to absorb precipitation and should be reasonably constant for large, homogeneous areas.
• Precipitation exponent
  This entry field defines the influence of precipitation rate on subbasin-average loss characteristics. It reflects the manner in which storms occur within an area and may be considered a characteristic of a particular region. It varies from 0.0 up to 1.0.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious.

Infiltration Method: Green and Ampt

The Green and Ampt infiltration method is essentially a simplification of the comprehensive Richard's equation for unsteady water flow in soil. This method should only be used for event simulation and not for a continuous simulation where there is an extended dry period(s) between precipitation events. It assumes the soil is initially at uniform moisture content, and infiltration occurs with so-called piston displacement. It is also assumed that the soil layer is infinitely deep, so reaching saturation can be defined purely by the Green and Ampt equation. The method automatically accounts for ponding at the soil-air interface.

When Green and Ampt is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Soil moisture method
  This dropdown combo box allows the user to select a method for specifying the initial soil moisture state. Two available options are Initial Content and Initial Deficit.
• Initial moisture content
  This entry field allows the user to specify the initial soil moisture state in terms of moisture content. Note that this field is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
• Saturated moisture content
  This entry field specifies the maximum water holding capacity in terms of volume ratio. It is often assumed to be the total porosity of the soil. Note that this field is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
• Initial moisture deficit
  This entry field allows the user to specify the initial soil moisture state in terms of a deficit. Note that this field is only available when Initial Deficit is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
• Wetting front suction head
  This entry field is used to specify the wetting front suction. It is generally assumed to be a function of the soil texture. Clicking on the […] lookup button will display a Soil Suction lookup table dialog box.
• Hydraulic conductivity
  This entry field is used to specify the hydraulic conductivity. It can be estimated from field tests or approximated by knowing the soil texture. Clicking on the […] lookup button will display a Soil Saturated Hydraulic Conductivity lookup table dialog box.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious.

Infiltration Method: Initial and Constant

The initial and constant infiltration method is very simple but appropriate for watersheds lacking detailed soil information. It is also suitable for certain types of flow-frequency studies. This method should only be used for event simulation.

When Initial and Constant is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Initial loss
  This entry field specifies the amount of incoming precipitation infiltrated or stored in the watershed before surface runoff begins. There is no recovery of the initial loss during periods without precipitation. Clicking the [Calc] button will cause the software to automatically compute the initial loss for the selected subbasin. Click the [Calc All] to compute the initial loss for all the subbasins.
• Curve number
  This entry field defines the curve number for the subbasin used to determine how much runoff occurs based upon the Land Use (or Land Cover) and the underlying Hydrologic Soil Type. Note that the user can automatically compute the curve number for each subbasin using Compute CN command. Refer to this article in our knowledge base to learn more about this command. Clicking the […] lookup button will display the SCS Curve Number lookup table dialog box.
• Constant loss rate
  This entry field specifies the rate of infiltration that will occur after the initial loss is satisfied. The same rate is applied regardless of the length of the simulation.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious.

Infiltration Method: Layered Green and Ampt

The layered Green and Ampt infiltration method uses two soil layers (layer 1 and layer 2) to account for continuous changes in moisture content. This method is based on algorithms originally developed for the Guelph Agricultural Watershed Storm-Event Runoff (GAWSER) model. This method should only be used for continuous simulation.

Two soil layers are used to represent the dynamics of water movement in the soil. Surface water infiltrates into the upper layer, called layer 1. Layer 1 produces seepage to the lower layer, called layer 2. Both layers are functionally identical but may have separate and distinct parameters. Separate parameters can be used to represent layered soil profiles and allow for a better representation of stratified soil drying between storms. Each layer is described using bulk depth and water content values for saturation, field capacity, and wilting point. Soil water in layer 2 can percolate out of the soil profile.

This method is intended to be used in combination with the linear reservoir baseflow method. When used in this manner, the percolated water can be split between baseflow and aquifer recharge. Refer to this article in our knowledge base to learn how to select a baseflow method for a subbasin.

This method can also be used in combination with a canopy method and surface storage method.

When Layered Green and Ampt is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Layer 1 / Layer 2 initial moisture content
  These entry fields are used to set the amount of soil water at the beginning of a simulation for the two soil layers. The values should be specified in terms of volume ratio.
• Hydraulic conductivity
  This entry field is used to specify the hydraulic conductivity. It can be estimated from field tests or approximated by knowing the surface soil texture. Clicking on the […] lookup button will display a Soil Saturated Hydraulic Conductivity lookup table dialog box.
• Max seepage
  This entry field is used to specify the maximum seepage at the bottom of layer 1. Field tests or the soil texture at the bottom of layer 1 can be used to estimate this value.
• Max percolation
  This entry field is used to specify the maximum percolation at the bottom of layer 2. Field tests or the soil texture at the bottom of layer 2 can be used to estimate this value.
• Wetting front suction head
  This entry field is used to specify the wetting front suction. It is generally assumed to be a function of the soil texture. Clicking on the […] lookup button will display a Soil Suction lookup table dialog box.
• Dry duration
  This entry field is used to set the amount of time that must pass after a storm event to recalculate the initial condition for the Green and Ampt equation. It has been found that 12 hours often works well.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious.
• Layer 1 thickness
  This entry field defines the bulk depth of soil measured from the ground surface down to the bottom of layer 1.
• Layer 2 thickness
  This entry field defines the bulk depth of soil measured from the bottom of layer 1 down to the bottom of layer 2.
• Layer 1/ Layer 2 saturated moisture content
  These entry fields are used to specify the maximum water holding capacity in terms of volume ratio for the two soil layers. These values define the total porosity of the soil.
• Layer 1 / Layer 2 field capacity
  These entry fields specify the point where the soil naturally stops seeping under gravity. The values should be specified in terms of volume ratio.
• Layer 1 / Layer 2 wilting point
  These entry fields specify the amount of water remaining in the soil layers when plants are no longer capable of extracting it. The values should be specified in terms of volume ratio.

Infiltration Method: SCS Curve Number

The Soil Conservation Service (SCS) curve number method implements the curve number methodology for incremental losses (NRCS, 2007). The SCS curve number loss method should only be used for event simulation. Originally, the methodology was intended to calculate total infiltration during a storm. The software computes incremental precipitation during a storm by recalculating the infiltration volume at the end of each time interval. Infiltration during each time interval is the difference in volume at the end of two adjacent time intervals.

When SCS Curve Number is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Initial abstraction
  This entry field allows the user to enter an initial abstraction. The initial abstraction defines the amount of precipitation that must fall before surface excess results. However, it is not the same as an initial interception or loss since changing the initial abstraction changes the infiltration response later in the storm. If this field is left blank, it will be automatically calculated as 0.2 times the potential retention, which is calculated from the curve number.
• Curve number
  This entry field allows the user to enter a curve number.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious.

Infiltration Method: Smith Parlange

The Smith Parlange infiltration method approximates Richard's equation for infiltration into the soil. It assumes that the wetting front can be represented with an exponential scaling of the saturated conductivity. This linearization approach allows the infiltration computations to proceed quickly while maintaining a reasonable approximation of the wetting front. This method should only be used for event simulation.

When Smith Parlange is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Initial moisture content
  This field specifies the initial saturation of the soil at the beginning of a simulation. It should be specified in terms of volume ratio.
• Residual moisture content
  This field specifies the amount of water remaining in the soil after all drainage has ceased. It should be specified in terms of volume ratio.
• Saturated moisture content
  This field specifies the maximum water holding capacity in terms of volume ratio. It is often assumed to be the total porosity of the soil.
• Bubbling pressure
  This entry field specifies the bubbling pressure, also known as the wetting front suction. It is generally assumed to be a function of the soil texture.
• Pore size distribution
  This entry field specifies how the total pore space is distributed in different size classes. It is typically assumed to be a function of soil texture.
• Hydraulic conductivity
  The entry field specifies the hydraulic conductivity as the effective saturated conductivity. It can be estimated from field tests or approximated by knowing the soil texture.
• Impervious surface
  The field defines the percentage of the subbasin area that is impervious.
• Temperature gage
  This dropdown combo box lists temperature gage time series. The temperature gage is used to adjust for water density, water viscosity, and matric potential versus temperature. If no temperature gage is defined, then a temperature of 25° C (or 75° F) is assumed. The temperature gage must be defined in the Temperature Gage Data dialog box before selecting it.Clicking the […] button will display the Temperature Gage Data dialog box, which allows the user to define temperature gages.
  The user may enter the time-series values by manually typing them or using the clipboard to copy and paste. Alternatively, the user may load the time-series data from a HEC-DSS file.
• Beta zero
  This entry field is used to define the beta zero parameter. This parameter is used to correct the matric potential based on temperature, which is generally a function of soil texture.

Infiltration Method: Soil Moisture Accounting Loss

The soil moisture accounting loss method uses three soil layers to account for continuous changes in moisture content throughout the vertical profile of the soil. This method is intended to be used for continuous simulation. This method can also be used in combination with a canopy and surface storage method.

Three soil layers are used in this method to represent the dynamics of water movement in the soil. Layers include soil storage, upper groundwater, and lower groundwater. The groundwater layer is not designed to represent aquifer processes. It is intended to be used for representing shallow inter-flow processes. The soil layer is subdivided into an upper zone and a tension zone. Soil water only percolates from the upper zone while water in the tension zone resists percolation. Water in the upper groundwater layer percolates to the lower groundwater layer.

The soil moisture accounting loss method is designed to be used in combination with the linear reservoir baseflow method. When used in this way, water can move laterally out of upper and lower groundwater layers to enter baseflow. Water percolating out of the lower groundwater layer can be split between entering baseflow and leaving the land surface as aquifer recharge.

When Soil Moisture Accounting Loss is selected for the Infiltration (Loss) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Soil layer initial saturation
  This entry field is used to specify the initial condition of the soil as the percentage of the soil that is full of water at the beginning of the simulation.
• Upper/Lower groundwater layer initial saturation
  These entry fields are used to specify the initial condition of the upper and lower groundwater layers.
• Soil layer maximum infiltration rate
  This entry field is used to set the upper bound on infiltration from the surface storage into the soil. If a surface storage method is selected, the actual infiltration in a particular time interval is a linear function of the surface and soil storage. If no surface storage method is selected, water will always infiltrate at the maximum rate.
• Impervious surface
  This entry field is used to specify the percentage of the subbasin area which is impervious.
• Soil layer total storage
  This entry field specifies the total storage available in the soil layer. Provide a zero value if you wish to eliminate soil calculations and pass infiltrated water directly to groundwater.
• Soil layer tension storage
  This entry field specifies the amount of water storage in the soil that does not drain under the effects of gravity. Percolation from the soil layer to the upper groundwater layer will occur whenever the current soil storage exceeds the tension storage. Water in tension storage is only removed by evapotranspiration. Tension storage must be less than soil storage.
• Soil layer percolation rate
  This entry field specifies the upper bound on percolation from the soil storage into the upper groundwater. The actual percolation rate is a linear function of the current storage in the soil and the current storage in the upper groundwater.
• Upper groundwater layer total storage
  This entry field specifies the total storage in the upper groundwater layer. It may be zero if you wish to eliminate the upper groundwater layer and pass water percolated from the soil directly to the lower groundwater layer.
• Upper groundwater layer percolation rate
  This entry field specifies the upper bound on percolation from the upper groundwater layer into the lower groundwater layer. The actual percolation rate is a linear function of the current storage in the upper and lower groundwater layers.
• Upper groundwater layer time lag coefficient
  This entry field specifies the time lag on a linear reservoir whereby water in storage becomes lateral outflow. The lateral outflow is available to become baseflow.
• Lower groundwater layer total storage
  This entry field specifies the total storage in the lower groundwater layer. It may be zero if you wish to eliminate the lower groundwater layer and pass water percolated from the upper groundwater layer directly to deep percolation.
• Lower groundwater layer percolation rate
  This entry field specifies the upper bound on deep percolation out of the system. The actual percolation rate is a linear function of the current storage in the lower groundwater layer.
• Lower groundwater layer time lag coefficient
  This entry field specifies the time lag on a linear reservoir whereby water in storage becomes lateral outflow. It is usually larger than the Upper groundwater layer time lag coefficient. The lateral outflow is likewise available to become baseflow.

Infiltration Methods along with the Kinematic Wave Runoff Method

When Kinematic Wave is selected for the Runoff (Transform) Method, two surface area sections are shown in the Infiltration (Loss) Data panel for defining data of the infiltration method. These include one for impervious surfaces and the other for pervious surfaces within the subbasin.

Below is an example of the Infiltration (Loss) Data panel displayed for the SCS Curve Number method.

Pros and Cons of Infiltration Methods

The Green and Ampt infiltration method is essentially a simplification of comprehensive Richard’s equation for unsteady water flow in soil. This method should only be used for event simulation and not for a continuous simulation where there is an extended dry period(s) between precipitation events. It assumes the soil is initially at uniform moisture content, and infiltration occurs with so-called piston displacement. It is also assumed that the soil layer is infinitely deep, so reaching saturation can be defined purely by the Green and Ampt equation. The method automatically accounts for ponding at the soil-air interface.

The Compute Green Ampt command is used to compute the Green and Ampt infiltration parameters for each subbasin based upon the hydrologic soil data.

Follow the steps below to use the Compute Green Ampt command:

1. From the Watershed ribbon menu, select the Compute Green Ampt command.
   
2. The Compute Green Ampt dialog box will be displayed.
   

The following sections describe how to use the Compute Green Ampt command and interact with the above dialog box.

Selecting Subbasins

The Select Subbasins section includes a table that lists all the subbasins contained within the current scenario along with their initial content, saturated content, initial deficit, suction head, and hydraulic conductivity.

This section is used to manually select subbasins from the Map View. If a subbasin is already selected on the Map View before running this command, the same subbasin will be shown selected within the table.

Alternatively, click the [Pick] button to interactively select subbasins from the Map View. On clicking the [Pick] button, the Compute Green Ampt dialog box will temporarily disappear. A prompt will be displayed on the status bar instructing the user to select subbasins from the Map View. After selecting the subbasins, the user can press the [Enter] key or right-click and choose Done from the displayed context menu. The dialog box will be redisplayed, and the total number of selected subbasins will be displayed in the Total selected read-only entry.

Note that the user can select multiple subbasins from the Map View before running the Compute Green Ampt command by holding the [Ctrl] key while selecting the desired subbasins.

Defining Computational Data

The Define Computational Data section contains a Soil moisture method dropdown combo box option and three panels that are the GIS Polygon Layer, the NRCS Layer, and the Default Values. From these three panels, the user can compute Green Ampt infiltration parameters for the various methods available.

Soil Moisture Method

This dropdown combo box allows the user to select a method for specifying the initial soil moisture state. Two available options are:

• Initial Content
• Initial Deficit (default)

Based on the option selected, the content of the GIS Polygon Layer and the Default Values panels will be enabled and disabled.

GIS Polygon Layer

This panel is used for the user-defined soil map polygon data. The soil map data provides hydrologic soil group data for each defined polygon.

The following entries are provided in the data panel:

• Soil Map Layer
  This dropdown combo box allows the user to select the GIS polygon layer included within the project.
• Initial Moisture Content
  This dropdown combo box allows the user to select the GIS shapefile field that contains initial moisture content for each polygon area. Note that this dropdown entry is only available when Initial Content is selected for the Soil moisture method. Otherwise, this dropdown entry is unavailable (i.e., grayed out).
• Saturated Moisture Content
  This dropdown combo box allows the user to select the GIS shapefile field that contains saturated moisture content. Note that this dropdown entry is only available when Initial Content is selected for the Soil moisture method. Otherwise, this dropdown entry is unavailable (i.e., grayed out).
• Initial Moisture Deficit
  This dropdown combo box allows the user to select the GIS shapefile field that contains the initial soil moisture deficit. Note that this dropdown entry is only available when Initial Deficit is selected for the Soil moisture method. Otherwise, this dropdown entry is unavailable (i.e., grayed out).
• Wetting Front Suction Head
  This dropdown combo box allows the user to select the GIS shapefile field that contains wetting front suction.
• Saturated Hydraulic Conductivity
  This dropdown combo box allows the user to select the GIS shapefile field that contains hydraulic conductivity.
• Value type
  This dropdown combo box is used to determine how to apply the defined value to compute the Green Ampt infiltration parameters.
  
  If the user specifies a value in the above entry, then this dropdown combo box provides the following options for Initial moisture content, Saturated moisture content, and Initial moisture deficit:
• • Decimal Fraction
  • Percentage (default)

For Wetting front suction head:

• • in
  • mm

For Saturated hydraulic conductivity:

• • in/hr
  • mm/hr

NRCS Layer

The NRCS Layer panel uses data from the NRCS Soil Survey Database to determine the hydrologic soil group data for the watershed region. This web soil survey database is only available within the USA and does not work outside the USA.

Note that for countries other than the USA, the software provides similar soil survey databases. This data panel will change to represent the one that is available for the project area. The software will automatically download the data once the user has selected this option and the command is executed.

Initial Setup

This subpanel allows the user to define the setup to be used in computing the Green Ampt parameters using the NRCS soil survey database.

Initial soil moisture content estimate
This radio button selection allows the user to specify the initial soil moisture conditions at the beginning of the simulation in terms of volume ratio. The following options are provided:

• Dry - This radio button option is used for lands such as desert and rangeland.
• Normal - This radio button option is used for lands under normal soil moisture conditions. By default, this radio button option is selected.
• Wet or Saturated - This radio button option is used immediately after a storm or for irrigated agricultural land.
• Custom - This radio button option is used to enter a custom value. By default, the software uses a value of 0.80. However, the user can enter a different value ranging from 0.00 to 1.00. This value is applied to all subbasins—regardless of the soil content.

NRCS Map Layer

This subpanel is used to see the parameters computed from the NRCS soil textures determined from the soil classifications.

Default Values

This panel is used to define default values for computing the Green Ampt infiltration parameters.

The following input parameters are provided in the data panel:

• Initial moisture content
  This entry field allows the user to specify the initial soil moisture state in terms of moisture content. Note that this field is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
• Saturated moisture content
  This entry field specifies the maximum water holding capacity in terms of volume ratio. It is often assumed to be the total porosity of the soil. Note that this field is only available when Initial Content is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
• Initial moisture deficit
  This entry field allows the user to specify the initial soil moisture state in terms of a deficit. Note that this field is only available when Initial Deficit is selected for the Soil moisture method. Otherwise, this entry is unavailable (i.e., grayed out).
• Wetting front suction head
  This entry field is used to specify the wetting front suction that describes the attraction of water within the void spaces of the soil column. It is generally assumed to be a function of the soil texture. Clicking on the […] lookup button will display a Soil Suction lookup table dialog box.
• Hydraulic conductivity
  This entry field is used to define the rate at which surface water will enter the soil column when completely saturated. It can be estimated from field tests or approximated by knowing the soil texture. Clicking on the […] lookup button will display a Soil Saturated Hydraulic Conductivity lookup table dialog box.

Computing Green Ampt

When the data have been defined in the Compute Green Ampt dialog box, click the [Compute] button.

The software will then look at each subbasin selected and compute the Green Ampt infiltration parameters for each subbasin. The computed values will be shown in the Select subbasins editable column as shown below. The software also allows the user to modify these computed values.

Note that while the computation is running, the [Close] button changes to [Cancel] so that the user can abort applying the computed Green Ampt parameters values if desired..

Subbasins define the drainage area polygons that produce runoff to the other elements in the model. While a subbasin element conceptually represents infiltration, surface runoff, and subsurface processes interacting together, the actual subsurface calculations are performed by a baseflow method contained within the subbasin. There are many different baseflow methods provided with HEC-HMS. Some methods are designed primarily for simulating events, while others are intended for continuous simulation.

In GeoHECHMS, the baseflow method for a subbasin can be selected from the General Specifications section of the Subbasin Data dialog box. Each subbasin may use a different method, or several subbasins may use the same method.

Follow the steps below to select a baseflow method:

1. From the Input ribbon menu, select the Subbasin Data command.
   
2. The Subbasin Data dialog box will be displayed.
   
3. From the Subbasin ID dropdown combo box, select the subbasin to assign a baseflow method.
4. From the Baseflow method dropdown combo box, select the baseflow method.
   
5. To enter the parameters for the selected baseflow method, select the Baseflow Data option from the Subbasin Specifications dropdown combo box. Note that identical parameters between different baseflow methods are shared so that the user does not have to enter the data twice when switching between methods.
   
6. The Baseflow Data panel will be displayed. Note that the Baseflow Data panel content changes based upon the baseflow method selected in the General Specifications section.
   

The following sections describe the different baseflow methods and how to enter the parameters for each method in the Baseflow Data panel.

Baseflow Method: None

When None is selected for the baseflow method, the Baseflow Data dropdown combo box entry is disabled (i.e., grayed out).

If None is selected for the baseflow method, the subbasin will not compute baseflow, and the outflow will only include direct runoff from the transform method.

Baseflow Method: Bounded Recession

The bounded recession baseflow method is intended primarily for real-time forecasting operations. The method is very similar to the recession method (explained below) and does not conserve mass in the subbasin. The principal difference is that the bounded recession method allows for the specification of monthly baseflow limits. The baseflow is computed according to the recession methodology, and then the monthly limits are imposed. Though there are many similarities with the recession method, one important difference is that this method does not reset the baseflow after a storm event.

When Bounded Recession is selected for the baseflow method, the following data panel will be displayed:

The following input parameters are provided in the data panel:

1. Initial type
   This dropdown combo box allows the user to select one of the following options to specify the initial baseflow at the beginning of a simulation:
   • Discharge: When this option is selected, the user must specify the initial baseflow as a discharge in units of volume per time (cfs). Selecting this option is particularly effective at determining the channel’s initial flow when there is observed streamflow data at the subbasin outlet.
   • Discharge per Area (default): When this option is selected, the user must specify the initial baseflow as a discharge in units of volume per area per time (cfs/mi²). Selecting this option is particularly effective when general guidelines for watershed yield must be used to estimate the initial flow.
2. Initial discharge
   This entry field is used to specify the amount of initial baseflow at the beginning of the simulation.
3. Recession constant
   This entry field specifies the rate at which baseflow recedes between storm events. It is defined as the ratio of baseflow at the current time to the baseflow one day earlier.

Furthermore, baseflow values for the months of January through December must be put in the table. These values are used to limit the computed baseflow.

Baseflow Method: Linear Reservoir

The linear reservoir baseflow method, as its name implies, uses a linear reservoir to model the recession of baseflow after a storm event. It is the only baseflow method that conserves mass within the subbasin. Infiltration or percolation computed by the infiltration method is provided as the inflow to the linear reservoirs. This method can use one, two, or three reservoirs. Partition fractions are used to split the inflow to each of the reservoirs. The inflow is multiplied by the partition fraction to determine the amount of inflow going to each reservoir. The sum of the partition fractions must be less than or equal to one. If the sum of the fractions is less than one, the remaining percolated water is considered as aquifer recharge. If the sum of the fractions is exactly equal to one, then all percolation will become baseflow, and there will be no aquifer recharge.

All losses from the SCS Curve Number, Exponential, Initial and Constant, Green and Ampt, and Smith-Parlange infiltration methods are routed to the linear reservoir baseflow model. Only percolation losses (during saturated conditions) in the Soil Moisture Accounting, Layered Green and Ampt, and Deficit and Constant infiltration methods are routed to the linear reservoir baseflow model. Refer to this article in our knowledge base to learn more about the infiltration methods.

When using the linear reservoir method with the Soil Moisture Accounting method, the number of baseflow reservoirs should be consistent with the number of groundwater layers in the infiltration method. The lateral outflow from the upper and lower groundwater layers is provided as the inflow to baseflow reservoirs 1 and 2. The percolation out of the lower groundwater layer is provided as inflow to baseflow reservoir 3. Partition fractions are not used for baseflow reservoirs 1 and 2 because their inflow is determined by the respective lateral outflow. A partition fraction should be used with the percolation to define the split between aquifer recharge and inflow to baseflow reservoir 3.

When Linear Reservoir is selected for the baseflow method, the following data panel will be displayed:

The following input parameters are provided in the data panel:

1. Reservoirs
   This spin control button is used to specify the number of reservoirs (number of groundwater layers). The minimum is one, and the maximum is three. Separate initial conditions and parameters must be specified for each reservoir. The Reservoir 2/3 Specifications sections are enabled or disabled based on the number of reservoirs specified.
2. Initial type
   This dropdown combo box allows the user to select one of the following options to specify the initial baseflow at the beginning of the simulation:
   • Discharge
   • Discharge per Area (default)
3. GW 1/2/3 initial
   These entry fields are used to specify the amount of initial baseflow for each of the reservoirs.
4. GW 1/2/3 fraction
   These entry fields determine how water from the infiltration method is distributed to the reservoirs. Each fraction must be greater than zero and less than or equal to one. When the sum of the fractions is one, there will be no aquifer recharge. When the sum of the fractions is less than one, the remainder of the percolation becomes aquifer recharge.
5. GW 1/2/3 coefficient
   These entry fields define the time constant for each reservoir (linear reservoir coefficient). Since this coefficient is measured in hours, it gives a sense of the response time for a component of subsurface flow within a subbasin.
6. GW 1/2/3 steps
   These spin control buttons are used to define the number of routing steps to subdivide the routing through each reservoir. The routing steps relate to the amount of attenuation during the routing. Minimum attenuation is achieved when only one routing step is defined. Baseflow attenuation increases as the number of steps increases.

Baseflow Method: Monthly Constant

The monthly constant baseflow method allows the specification of a constant baseflow for each month of the year. It does not conserve mass within the subbasin. It is intended primarily for continuous simulation in subbasins where a constant flow for each month approximates the baseflow.

When Monthly Constant is selected for the baseflow method, the following data panel will be displayed:

Baseflow values for January through December must be entered into the table. These values are used to limit the computed baseflow.

Baseflow Method: Nonlinear Boussinesq

The nonlinear Boussinesq baseflow method is designed to approximate the typical behavior observed in subbasins when channel flow recedes after an event. This method is similar to the recession baseflow method but can be parameterized using observable field data. This method is intended primarily for event simulation. However, it can also be used for continuous simulation because of its ability to reset automatically after each storm event. It does not conserve mass within the subbasin.

When Nonlinear Boussinesq is selected for the baseflow method, the following data panel will be displayed:

The following input parameters are provided in the data panel:

1. Initial discharge type
   This dropdown combo box allows the user to select one of the following options to specify the initial baseflow at the beginning of the simulation.
   • Discharge
   • Discharge per Area (default)
2. Initial discharge
   This entry field is used to specify the amount of initial baseflow at the beginning of the simulation.
3. Threshold type
   This dropdown combo box allows the user to select one of the following options to reset the baseflow during a storm event:
   • Ratio to Peak (default)
   • Threshold Discharge
4. Threshold ratio
   This entry is only enabled if the Ratio to Peak option is selected for the Threshold type. Otherwise, this entry is disabled (i.e., grayed out). It specifies the flow ratio to the peak. The baseflow is reset when the current flow divided by the peak flow falls to the specified value. For example, if a ratio of 0.2 is provided, the baseflow will reset on the receding limb of an event hydrograph when the flow has decreased to 20% of the event peak flow.
5. Threshold discharge
   This entry field is only enabled if the Threshold discharge is selected for the Threshold type. Otherwise, this entry is disabled (i.e., grayed out). It specifies the threshold baseflow value. The baseflow is always reset when the receding limb of the hydrograph falls to the specified flow value, regardless of the peak flow during the previous storm event.
6. Subsurface flow length
   This entry field is used to specify the characteristic subsurface flow length. This could be estimated as the mean distance from the subbasin boundary to the stream. Clicking the […] button allows the user to measure the subsurface flow length from the Map View.
7. Hydraulic conductivity
   This entry field is used to specify the conductivity of the soil. This could be estimated from field tests or from the soil texture. Clicking the […] lookup button will display the Soil Saturated Hydraulic Conductivity lookup dialog box.
8. Porosity
   This entry field is used to specify the drainable porosity in terms of volume ratio. The upper limit would be the total porosity minus the residual porosity. The actual drainable porosity depends on local conditions.

Baseflow Method: Recession

The recession baseflow method is designed to approximate the typical behavior observed in subbasins when channel flow recedes exponentially after an event. This method is intended primarily for event simulation. However, it does have the ability to automatically reset after each storm event and consequently may be used for continuous simulation. It does not conserve mass within the subbasin.

When Recession is selected for the baseflow method, the following data panel will be displayed:

The following input parameters are provided in the data panel:

1. Initial discharge type
   This dropdown combo box allows the user to select one of the following options to specify the initial baseflow flow at the beginning of a simulation.
   • Discharge
   • Discharge per Area(default)
2. Initial discharge
   This entry field specifies the amount of initial baseflow at the beginning of a simulation.
3. Recession constant
   This entry field describes the rate at which baseflow recedes between storm events. It is defined as the ratio of baseflow at the current time, to the baseflow one day earlier.
4. Threshold type
   This dropdown combo box allows the user to select from one of the following options to reset the baseflow during a storm event.
   • Ratio to Peak (default)
   • Threshold Discharge
5. Threshold ratio
   This entry is only enabled if the Ratio to Peak option is selected for the Threshold type. Otherwise, this entry is disabled (i.e., grayed out). It specifies the flow ratio to the peak.
6. Threshold discharge
   This entry field is only enabled if the Threshold Discharge is selected for the Threshold type. Otherwise, this entry is disabled (i.e., grayed out). It specifies the threshold baseflow value. The baseflow is always reset when the receding limb of the hydrograph falls to a specified flow value, regardless of the peak flow during the previous storm event.

Pros and Cons of Baseflow Methods

In SCS Curve Number hydrology, double counting occurs when the same impervious surface is represented twice in a subbasin: once inside the curve number selected by the user and again as a separate percent impervious value. The model has no way of knowing that the rooftops, roads, and sidewalks have already been baked into the CN, so it adds an additional impervious calculation on top of it. The result is consistently overestimated peak flows, oversized stormwater infrastructure, and overly conservative floodplain mapping.

This article explains how to avoid double counting impervious areas during hydrologic analysis, ensuring that impervious surfaces are represented only once.

Why Double Counting Matters

Engineers rely on hydrologic models to size storm sewers, culverts, detention basins, channels, and floodplain limits. Each of those design decisions is sensitive to peak discharge. If imperviousness is counted twice, the modeled peak grows for reasons that have nothing to do with the watershed; the impact compounds as the design moves downstream.

Common consequences of double counting include:

• Inflated peak discharges for the design storm, leading to oversized inlets, pipes, and culverts.
• Larger detention requirements than the project necessitates, increasing land use requirements and construction costs.
• Overly conservative floodplain limits that can affect insurance ratings, lot layouts, and permitting.
• Calibration drift where the user attempts to match observed flows by tweaking unrelated parameters such as time of concentration or initial abstraction, masking the underlying input error.

Curve Number and Impervious Surface

The SCS curve number (CN) is a dimensionless index that estimates direct runoff from a rainfall depth based on land use, soil type, and antecedent moisture conditions. Standard CN tables, such as those in the NRCS TR-55 manual and NEH-630 Chapter 9 already include the effect of typical impervious cover for many urban and developed land use categories.

The impervious surface percentage defines the fraction of a subbasin where rainfall does not infiltrate, such as roofs, paved roads, parking lots, and sidewalks. When entered separately, it tells the model to compute runoff from that fraction of the subbasin using a fully impervious response and to combine it with the runoff from the pervious portion.

Both inputs describe imperviousness, but they describe it in two different ways. The trouble starts when both are used to describe the same impervious cover at the same time.

Where This Issue Can Occur

When the user models runoff with the SCS Curve Number method, both the curve number and the impervious surface percentage are entered together in the Subbasin Data dialog box. The dialog box accepts both inputs without warning, which is why it is up to the user to make sure they are consistent. Refer to this article in our knowledge base to learn more about the Subbasin Data command.

How to Avoid Double Counting

The fix is to make sure imperviousness is represented in exactly one place in each subbasin. There are two reliable ways to do this. Both produce technically correct results. The right choice depends on how the curve number was derived.

Method 1 — Use a Pre-Weighted Curve Number

If the selected CN value already includes the effect of impervious cover for the land use (for example, a value taken from a standard table for “1/4-acre urban residential” or “commercial and business areas”), then the Impervious surface percentage in the Subbasin Data dialog box should be set to 0%. The CN already accounts for the imperviousness; entering it again would double count it.

The standard NRCS TR-55 table shown below is a good example of pre-weighted values. Each developed land use has an “average percent impervious area” included in the listed CN.

Method 2 — Separate Pervious CN with Explicit Impervious Percentage

If the CN represents only the pervious portion of the subbasin (for example, a value for open space, meadow, woods, or row crops), then enter the actual impervious percentage in the Impervious surface entry field. The software then computes runoff from the pervious area using the entered CN, computes runoff from the impervious area using a fully impervious response and combines the two contributions to give total subbasin runoff.

This method is useful when:

• The user has measured impervious cover from aerial mapping, GIS data, or CivilGEO’s built-in imperviousness coverage and wants the model to reflect the actual existing site conditions.
• The user is evaluating future build-out where the impervious fraction will change but the underlying pervious land use will not.
• The user is comparing existing vs proposed scenarios and wants imperviousness to be the single variable that changes between runs.

Quick Decision Guide

Use the table below as a quick check before running the model. The goal is to make sure the CN value and the impervious surface input are consistent and do not represent the same impervious area twice.

unknown node

Double Counting Example Scenario — New Residential Subdivision

A user is sizing a detention basin for a proposed residential subdivision on Hydrologic Soil Group C. The subdivision has roughly 30% impervious cover from streets, driveways, rooftops, and sidewalks. The subbasin is set up with the SCS Curve Number loss method.

Incorrect Setup — Double Counted

The user picks the CN for “1/4-acre urban residential” from a standard table, which already accounts for typical urban imperviousness, and enters an additional 30% impervious value. The same imperviousness is now represented twice. The computed peak discharge is artificially inflated, and the detention basin is sized for a flow that does not exist on the actual site.

Correct Method 1 — Pre-Weighted CN

The user picks the CN for “1/4-acre urban residential” and sets the impervious value to 0%. Imperviousness is represented once inside the CN, and the computed peak reflects the developed condition consistent with the CN table.

Correct Method 2 — Pervious CN with Explicit Imperviousness

The user picks a pervious-only CN (for example, “open space, good condition” on HSG C) and enters 30% in impervious value. The software computes runoff from the 70% pervious portion using the CN and from the 30% impervious portion as fully impervious, then combines the two. Imperviousness is again represented once.

Conclusion

Double counting impervious areas is one of the most common sources of overestimated runoff in SCS Curve Number modeling. Before computing runoff for a subbasin, the user should confirm one of two things: either the curve number already represents the developed land use and the impervious surface field is set to 0%, or the curve number represents only the pervious land cover, and the impervious surface field carries the actual percent imperviousness.

When impervious cover is represented correctly, the model produces peak discharges that are consistent with the source data, detention sizing that reflects the real site, and floodplain limits that support agency review.

Subbasins are represented by drainage area polygons that produce the runoff that drain into other elements in the model. Runoff (Transform) methods are used to transform the excess precipitation (after the losses have been subtracted) into surface runoff (direct runoff). Several runoff methods are available to use including unit hydrograph methods and kinematic wave implementation.

In GeoHECHMS, the runoff method for a subbasin can be selected from the General Specifications section of the Subbasin Data dialog box.

Follow the steps given below to select a runoff method:

1. From the Input ribbon menu, select the Subbasin Data command.
2. The Subbasin Data dialog box will be displayed.
3. From the Subbasin ID dropdown combo box, select the subbasin that will be assigned a runoff method.
4. From the Runoff (transform) method dropdown combo box, select the runoff method.
5. To enter the data for the selected runoff method, select the Runoff (Transform) Data option from the Subbasin Specifications dropdown combo box.
6. The Runoff (Transform) Data panel will be displayed. Note that the Runoff (Transform) Data panel content changes based upon the runoff (transform) method selected in the General Specifications section.

The following sections describe the different runoff methods and how to enter the parameters for each method in the Runoff (Transform) Data panel.

Runoff Method: None

When None is selected for the Runoff (Transform) Method, then the Runoff (Transform) Data dropdown combo box entry is disabled (i.e., grayed out).

Runoff Method: Clark Unit Hydrograph

The Clark Unit Hydrograph is a synthetic unit hydrograph method. This means that the user is not required to develop a unit hydrograph through analysis of past observed hydrographs. Instead, a time versus area curve (time-area curve) is used to develop the translation hydrograph resulting from a burst of precipitation. The resulting translation hydrograph is routed through a linear reservoir to account for storage attenuation effects across the subbasin.

When Clark Unit Hydrograph is selected for the Runoff (Transform) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

Unit Hydrograph Method

The Unit hydrograph method dropdown combo box entry allows the user to select the method for the Clark unit hydrograph. There are two options available: Standard Parameters and Variable Parameters. Based on the option selected, the different entry fields become either enabled or disabled in the data panel.

Standard Parameters

On selecting this option, the following entries are available to the user:

Time of Concentration

The Time of concentration entry field allows the user to define the maximum travel time in the subbasin.

Storage Coefficient

The Storage coefficient entry field allows the user to enter the storage coefficient, which is used in the linear reservoir that accounts for storage effects.

TOC vs Drainage Area Method

The TOC vs drainage area method entry field allows the user to define the time area method. There are two options available:

1. Default: This option provides a default time area curve that represents the subbasin using an elliptical shape.
2. Percentage Curve: This option provides a user-specified time-area curve. The relationship must be defined as a percentage curve in the paired data manager before it can be used in the subbasin element.

TOC vs Drainage Area Percentage Curve

The TOC vs drainage area percentage curve dropdown combo box allows the user to select an already defined percentage curve. Note that this dropdown combo box is only enabled when the Percentage Curve option is selected for the TOC vs Drainage Area Method entry. Otherwise, this dropdown combo box is disabled (e.g., grayed out). Clicking on the […] button will display a Percentage Curve Data dialog box, which allows the user to define a new user-defined percentage curve.

Variable Parameters

On selecting this option, the following additional entries (shown in the red rectangle box) get enabled in the data panel.

The user must enter the data for the Time of concentration, Storage coefficient, TOC vs drainage area method and the following additional entries:

Excess (index) Precipitation Rate

The Excess (index) precipitation rate entry field allows the user to enter the excess (index) precipitation rate. This value represents the excess precipitation rate, which is used to relate the Time of Concentration and Storage Coefficient against the Concentration Curve and Storage Curve for the variable unit hydrograph method.

Time of Concentration Percentage Curve

The Time of concentration percentage curve dropdown combo box allows the user to select an already defined percentage curve. Clicking on the […] button will display a Time of Concentration Percentage Curve Data dialog box, which allows the user to define a new user-defined percentage curve.

Storage Coefficient Percentage Curve

The Storage coefficient percentage curve dropdown combo box allows the user to select an already defined percentage curve. Clicking on the […] button will display the Storage Coefficient Percentage Curve Data dialog box, which allows the user to define a new user-defined percentage curve.

Runoff Method: Kinematic Wave

The Kinematic Wave method is designed principally for representing urban areas, although it can be used for undeveloped regions as well. It is a conceptual model that includes one or two representative surface areas. Typically, one surface area is used for impervious surfaces and one for pervious surfaces. Separate canopy, surface, and loss rate information is required for each surface area and is entered separately. The kinematic wave equations are used to calculate runoff for each surface area and then a weighted composite runoff is calculated using the representative percentage for each surface area.

The composite runoff is directed to a subcollector, which is used to represent primary collection in the stormwater management system. The most common collection system is represented by gutters along streets. In areas without gutters, unlined ditches next to the street may perform a similar stormwater collection function. The composite runoff is applied to the subcollector as a uniform lateral boundary condition, with scaling based on the representative area of a typical subcollector. Water is routed through a representative subcollector using the kinematic wave equations. There is no seepage from the subcollector so it may be difficult to use for representation of vegetated swales and other types of best management practices.

The outflow from the representative subcollector is directed to a collector, which is used to represent the next step up in the stormwater management system. Conceptually, the collector receives inflow from multiple gutters or ditches. The outflow from the subcollector is scaled up using the representative area of a typical collector and applied to the collector as a uniform lateral boundary condition. Water is routed through a representative collector using the kinematic wave equations. Again, there is no seepage from the collector.

The outflow from the representative collector is directed to a channel. The channel may be used to represent the final step in the stormwater management system, or alternatively, may be used to represent the river passing through the subbasin. Conceptually, the channel receives inflow from multiple collectors. The outflow from the representative collector is scaled up using the area of a typical collector and the area of the subbasin. The scaled flow is applied to the channel as a uniform lateral boundary condition. Water is routed through the channel to become the outflow from the subbasin.

When Kinematic Wave is selected for the Runoff (Transform) Method, the following data panel will be displayed.

Surface Area #1

The following input parameters are provided for the Surface Area #1 panel.

Total Length

The Total length entry field allows the user to enter the length of the surface area. For impervious areas, this should be the average flow length from the point where precipitation falls, to where the runoff first enters a collection gutter or channel. For pervious areas, this should likewise be the average flow length. When using the two surface areas, it is helpful to adopt a convention for which the surface area number (one or two) represents the pervious area. Clicking on the […] Measure button allows the user to select the length from the subbasin surface area to the collection network within the subbasin from the Map View.

Average Slope (V:H)

The Average slope (V:H) entry field allows the user to enter the average basin slope along the flow line from the point where precipitation falls to where the runoff first enters a gutter or channel. Clicking on the […] Measure button allows the user to select the basin slope from the subbasin outlet along the main watercourse, to a point opposite the subbasin centroid on the Map View.

Overland Flow Roughness

The Overland flow roughness entry field allows the user to define Manning's roughness coefficient for the overland flow surface areas. Clicking on the […] lookup button will display an Overland Flow Roughness lookup table dialog box.

Subbasin Area

The Subbasin area entry field allows the user to enter the percentage of the subbasin area occupied by each surface area. If the user wants to use one surface area, then enter 100 for the first surface area and 0 for the second. Note that as the user changes this value, the Subbasin area spin control in the Surface Area #2 panel changes accordingly. The two spin controls always add up to 100%. So, setting this value to 20% will set the spin control in the Surface Area #2 panel to 80%.

Routing Steps

The Routing steps entry field allows the user to enter the number of routing steps. The software uses the routing steps as a prompt to determine the correct distance step to use during runoff calculations. By default, the software uses a value of 5. However, the user can enter a different value ranging from 5 to 99.

Surface Area #2

Note that this panel is the same as the Surface Area #1 panel. If the user has defined the Subbasin area in the Surface Area #1 panel as 100%, then the fields provided in the Surface Area #2 panel are disabled (i.e., grayed out).

Subcollector

The composite runoff is directed to a subcollector. The following input parameters are provided for the Subcollector panel.

Additional Drainage Area

The Additional drainage area entry field allows the user to enter the area of the subcollector. This entry is used to determine how to apply the composite outflow from the subcollector to the collector channel. Clicking on the […] Measure button allows the user to draw an outline of the collector area, which prompts the software to measure the drainage area from the Map View.

Total Length

The Total length entry field allows the user to enter the flow length. The length should be the average distance from the starting point of the subcollector to the point at which it will enter a collector.

Average Slope (V:H)

The Average slope (V:H) entry field allows the user to enter the average slope along the average flow length.

Manning’s Roughness

The Manning’s roughness entry field allows the user to enter the roughness coefficient, which should be the average value for the entire subcollector.

Number Subreaches

The Number subreaches entry field allows the user to enter the number of subreaches, a value which the software uses to determine the correct distance step to use during routing calculations.

Element Shape

The Element shape dropdown combo box allows the user to specify the cross section shape. The dropdown combo box lists circular pipe, deep rectangle, rectangle, trapezoid, and triangle shapes.

Based upon the shape selected, additional information will be needed to describe the dimensions of the cross section shape. This information may include a diameter (circular pipe), channel width (deep rectangle, rectangle), bottom width (trapezoid), or side slope (trapezoid, triangle).

Collector

The outflow from the subcollector enters a collector as lateral inflow. The collector channel is intended to represent small ditches or open channels that are part of an engineered stormwater management system. The parameters for the collector are exactly the same as for the subcollector.

The Length, Slope, Manning's roughness, Number subreaches, and Element shape all have the same definitions. The drainage area served by a typical collector must be entered. This value is used to apply lateral inflow from the collector to the main channel.

Channel

The outflow from the representative collector is directed to a channel. The following input parameters are provided in the Channel panel.

Routing Method

The Routing method dropdown combo box allows the user to select the routing method. There are two options available: Kinematic Wave and Muskingum Cunge. The runoff on the two surface areas is always routed using the kinematic wave method. However, the user may choose to use Muskingum-Cunge routing in the subcollector, collector, and main channel. All three channels use the same method.

Total Length

The Total length entry field allows the user to specify the flow length within the subbasin to the outlet. Clicking on the […] Measure button allows the user to select length within the subbasin from the Map View.

Average Slope (V:H)

The Average slope (V:H) entry field allows the user to enter the average slope for the whole channel. Clicking on the […] Measure button allows the user to select the basin slope from the subbasin outlet along the main watercourse, to a point opposite the subbasin centroid on the Map View.

Manning’s Roughness

The Manning’s roughness entry field allows the user to enter the Manning’s roughness value for the whole channel. Clicking on the […] lookup button will display a Manning’s Roughness lookup table dialog box.

Number Subreaches

The Number subreaches entry field allows the user to enter the number of subreaches, which is used to determine the distance step to use during routing calculations. By default, the software uses a value of 5. However, the user can enter a different value ranging from 2 to 99. Note that when the Muskingum Cunge routing method is selected, then this entry is disabled (i.e., grayed out).

Element Shape

The Element shape dropdown combo box allows the user to specify the cross section shape. If the Kinematic Wave routing method is selected, the dropdown combo box lists Circular Pipe, Deep Rectangle, Rectangle, Trapezoid, and Triangle options. When the Muskingum-Cunge routing method is selected, the dropdown combo box lists 8-Point Cross Section, Circular Pipe, Rectangle, Trapezoid, and Triangle.

Based upon the shape selected, additional information will be needed to describe the dimensions of the cross section shape. This information may include a diameter (circular pipe), channel width (deep rectangle, rectangle), bottom width (trapezoid), side slope (trapezoid, triangle), or left and right overbank Manning’s n (8-point cross section shape).

Trapezoid Bottom Width

The Trapezoid bottom width entry field allows the user to enter the channel bottom width.

Side Slope (V:H)

The Side slope (V:H) entry allows the user to enter the channel side slope. The side slope is dimensionless and entered as the units of horizontal distance per one unit of vertical distance.

Runoff Method: SCS Unit Hydrograph

The Soil Conservation Service (SCS) unit hydrograph method defines a curvilinear unit hydrograph by first setting the percentage of the unit runoff that occurs before the peak flow (NRCS, 2007). A triangular unit hydrograph can then be fit to the curvilinear unit hydrograph so that the total time base of the unit hydrograph can be calculated. The standard unit hydrograph is defined with 37.5% of unit runoff occurring before the peak flow. This definition corresponds to a peak rate factor of 484, which incorporates the percentage of unit runoff before the peak, calculated total time base, and unit conversions when applying the equations within the US Customary unit system.

When SCS Unit Hydrograph is selected for the Runoff (Transform) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

Hydrograph Peaking Factor

The Hydrograph peaking factor dropdown combo box allows the user to select the peaking factor, which essentially controls the volume of water on the rising and recession limbs. By default, the Standard (PRF=484) option is selected.

Lag Time

The Lag time entry field allows the user to enter the lag time, which is defined as the length of time between the centroid of precipitation mass and the peak flow of the resulting hydrograph.

Runoff Method: Snyder Unit Hydrograph

The Snyder Unit Hydrograph method is a synthetic unit hydrograph method. The original methodology only supported computing the peak flow as the result of a unit of precipitation. Later, equations were developed to estimate the time base of the hydrograph and the width at 50% of the peak flow. Because the Snyder method does not compute all ordinates of the hydrograph, a Clark hydrograph is constructed in such a way that the Snyder properties are maintained.

When Snyder Unit Hydrograph is selected for the Runoff (Transform) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

Lag Time Method

The Lag time method dropdown combo box allows the user to select the lag time method. There are three options available: Standard, Ft Worth District, and Tulsa District. Based on the option selected, the content of this panel changes to define additional data.

Standard

On selecting this option, the following entries are available to the user:

Lag Time

The Lag time entry field allows the user to enter the lag time.

Peaking Coefficient

The Peaking coefficient entry field allows the user to enter the peaking coefficient, which is used to measure the steepness of the hydrograph that results from a unit of precipitation.

Ft Worth District

On selecting this option, the following input parameters will be displayed.

Total Length

The Total length entry field allows the user to enter the length from the point of the outlet of the subbasin, along the main watercourse to the most hydraulically remote point on the subbasin boundary. Clicking on the […] Measure button allows the user to select the length from the Map View.

Centroid Length

The Centroid length entry field allows the user to enter the length from the outlet of the subbasin, along the main watercourse, to a point opposite the subbasin centroid. Clicking on the […] Measure button allows the user to select the length from the Map View.

Weighted Slope (V:H)

The Weighted slope (V:H) entry field allows the user to enter the weighted slope. The weighted slope is the slope of the main watercourse between points located at 10 percent and 85 percent of the length of the main watercourse, measured from the subbasin outlet. Clicking on the […] Measure button allows the user to select the basin slope from the subbasin outlet along the main watercourse, to a point opposite the subbasin centroid on the Map View.

Urbanization

The Urbanization spin control allows the user to enter the urbanization percentage. This percentage is estimated as the percentage of the subbasin where the drainage system has been improved to carry runoff more efficiently to the outlet. By default, the software uses a value of 25. However, the user can enter a different value ranging from 0 to 100.

Sand

The Sand spin control allows the user to enter the sand percentage. Zero percent indicates essentially all-clay soils with characteristically low infiltration rates. Conversely, 100 percent indicates essentially all-sandy soils with characteristically high infiltration rates. By default, the software uses a value of 25. However, the user can enter a different value ranging from 0 to 100.

Peaking Coefficient

The Peaking coefficient entry field is estimated by another method and entered directly. The Ft Worth District method does not include estimation of the peaking coefficient.

Tulsa District

On selecting this option, the following input parameters will be displayed.

Note that all the input parameters of this method are similar to the Ft Worth District method, except for the following entry:

Channelization

The Channelization spin control allows the user to enter the percentage of channelization. This percentage is estimated as the percentage of the subbasin where the drainage system has been improved to carry runoff more efficiently to the outlet. By default, the software uses a value of 25. However, the user can enter a different value ranging from 0 to 100.

Runoff Method: User-Specified S-Graph

The User-Specified S-Graph method is not synthetic. It uses a special summation unit hydrograph (s-graph) to represent the response of a subbasin to a unit of precipitation. The s-graph is defined with the percentage of the time lag as the independent variable (x-axis), and the percentage of the cumulative runoff volume as the dependent variable (y-axis). Computationally, the s-graph is scaled by a specified time lag and then successive differences are taken along the curve to compute a unit hydrograph directly from the scaled s-graph.

When User-Specified S-Graph is selected for the Runoff (Transform) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

Lag Time Method

The Lag time method dropdown combo box allows the user to select the lag time method. There are two options available: Standard and Regression. Based on the option selected, the content of this panel changes to define additional data.

Standard

On selecting this option, the following entries are available to the user:

Summation Unit Hydrograph

The Summation unit hydrograph dropdown combo box allows the user to select an already defined summation unit hydrograph (s-graph). Clicking on the […] button will display a Summation Unit Hydrograph Data dialog box, which allows the user to define a new user-defined summation unit hydrograph.

Lag Time

The Lag time entry field allows the user to enter the lag time.

Regression

On selecting this option, the following input parameters will be displayed.

Total Length

The Total length entry field allows the user to enter the length of the longest flow path in the subbasin. Clicking on the […] Measure button allows the user to select the length from the Map View.

Centroid Length

The Centroid length entry field allows the user to enter the length along the stream from the outlet to a point opposite the centroid. Clicking on the […] Measure button allows the user to select the length from the Map View.

Weighted Slope (V:H)

The Weighted slope (V:H) entry field allows the user to enter the slope of the stream. Clicking on the […] Measure button allows the user to select the basin slope from the subbasin outlet along the main watercourse, to a point opposite the subbasin centroid on the Map View.

Hydraulic Efficiency Coefficient

The Hydraulic efficiency coefficient entry field allows the user to enter the hydraulic efficiency of the stream channel.

M Exponent

The M exponent entry field allows the user to enter the M exponent value. This value should be determined through a regional regression study of watershed properties and observed lag time.

P Exponent

The P exponent entry field allows the user to enter the P exponent value. This value should be determined through a regional regression study of watershed properties and observed lag time.

Runoff Method: User-Specified Unit Hydrograph

The User-Specified Unit Hydrograph method is not synthetic. Consequently, a separate unit hydrograph must be developed for each subbasin. Ideally, these unit hydrographs are developed from multiple storm observations when precipitation and flow have been measured at similar time intervals. The unit hydrograph developed in this manner has a so-called duration that is equal to the duration of excess precipitation. For example, a 3-hour duration unit hydrograph is generated by a precipitation event with 3 hours of excess precipitation.

When User-Specified Unit Hydrograph is selected for the Runoff (Transform) Method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

Unit Hydrograph

The Unit hydrograph dropdown list allows the user to select the unit hydrograph to use. The unit hydrograph must be defined in the Unit Hydrograph Data dialog box before selecting it. Clicking on the [...] button will display the Unit Hydrograph Data dialog box, which allows the user to define new user-defined unit hydrograph flow values.

Number of Passes

The Number of passes spin control is used to define the number of passes. The number of passes is used to correct for minor inflection points in the S-curve that is used to convert the unit hydrograph from the input duration to the duration required as per the computational interval.

Pros and Cons of Runoff (Transform) Methods

The Soil Conservation Service (SCS) unit hydrograph method defines a curvilinear unit hydrograph by setting the percentage of the unit runoff that occurs before the peak flow. The unit hydrograph impacts the shape of the runoff hydrograph generated by the model, particularly the peak rate of discharge.

The standard unit hydrograph is defined as 37.5% of the unit runoff occurring before the peak flow. This definition corresponds to a peak rate factor (PRF) of 484, which incorporates the percentage of unit runoff before the peak.

However, the percentage of unit runoff occurring before the peak flow is not uniform across all watersheds, as it depends on flow length, ground slope, and other watershed properties.

For stormwater projects, the user can follow the steps below to select a hydrograph peak rate factor:

1. Under the File ribbon menu, select the Options backstage page.
2. The Options backstage page will be displayed. From either the Project Options or Application Options, click the SCS hydrograph peaking factor dropdown combo box under the General Preference section.

The SCS hydrograph peaking factor option allows the user to select the peaking factor, which controls the runoff on the rising and recession limbs of the hydrograph. By default, the Standard (PRF = 484) option is selected. The user can pick any other peak rate factor value, and the software will automatically construct the unit hydrograph and display it as a graphical plot in the Subbasin Data dialog box or Plot Hydrographs dialog box.

The following graphical representation shows a comparison between two dimensionless unit hydrographs with different peak rate factor values.

The following table describes the typical hydrograph peaking factors available in CivilGEO Software:

Note that flat watersheds typically have a lower PRF that may be as small as 100. In contrast, steeper watersheds have a larger PRF that may range up to 600.

The following table provides a description of typical hydrograph peaking factors along with their limb ratios (ratio of the recession limb length to rising limb length):

The Subbasin TOC Data command allows the user to compute the time of concentration (TOC) for one or more subbasins using the Federal Aviation Administration (FAA) method. The user can select the TOC/Lag time method from the Subbasin - TOC/Lag Time Method dropdown combo box in the General Preferences section of the Options backstage page. Refer to this article in our knowledge base to learn more about TOC methods.

The FAA TOC method computes TOC using the following equation:

Where:

• L = Longest flow path in the subbasin
• S = Flow path average slope
• C = Rational Method runoff coefficient

Follow the steps below on how to compute time of concentration using the FAA TOC method:

1. From the Watershed ribbon menu, select the Subbasin TOC Data command.
   
   Alternatively, from the Input ribbon menu, click the Drainage Subbasins dropdown menu and select the Subbasin TOC Data command.
   
2. The Subbasin TOC Data dialog box will be displayed, as shown below.
   

The Subbasin TOC Data dialog box contains three tabs as described below:

• Compute TOC
• Convert CN to Runoff Coefficient
• Rainfall Intensity Lookup

Compute TOC

In the Compute TOC panel, the Select Subbasins section allows the user to select single or multiple subbasins defined in the current scenario for computing the time of concentration using the FAA method.

The user can select the subbasin(s) using any of the following methods:

• By checking the checkboxes corresponding to each subbasin in the Select Subbasins section.
• By clicking the [Pick] button. In this selection method, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the subbasin(s) from the Map View. After selecting the subbasin(s), press the [Enter] key or right-click and choose Done from the displayed context menu. The Subbasin TOC Data dialog box will be redisplayed.

After selecting the subbasin(s), the total number of selected subbasins will be displayed in the Total selected read-only field, as shown below.

If the US unit system is selected on the Options backstage page, the US units will be displayed in the Select Subbasins section. The user can also change the unit system to a metric (SI) unit, if necessary. Refer to this article in our knowledge base to learn more about setting the model units to metric (SI) or US units.

The table in the Select Subbasins section contains the following columns:

• Subbasin
  This read-only column lists all subbasin IDs contained in the current scenario.

• Flow Path Slope (ft/ft)
  This read-only column lists the computed flow path slopes using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Flow Path Length (ft)
  This read-only column lists the computed flow path lengths using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Runoff Coefficient
  This editable column lists the computed runoff coefficients. Clicking the […] lookup button displays a Runoff Coefficients dialog box, which allows the user to select the runoff coefficient.
  

• Computed TOC (minutes)
  This editable column lists the computed TOC value. By default, this column lists the TOC value defined in the Subbasin Data dialog box. To learn more about the Subbasin Data command, refer to this article in our knowledge base.

Convert CN to Runoff Coefficient

This panel is used to convert computed curve number (CN) values into equivalent runoff coefficient values for the selected subbasin(s).

Note
Subbasins selected in the Compute TOC panel will also appear as selected in this panel.

The table in the Select Subbasins section contains the following columns:

• Subbasin
  This read-only column lists all subbasin IDs contained in the current scenario.

• Flow Path Slope (ft/ft)
  This read-only column lists the computed flow path slopes using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Curve Number
  This editable column lists the computed curve numbers using the Compute CN command. To learn more about the Compute CN command, refer to this article in our knowledge base.

• Impervious Area (%)
  This editable column lists the computed impervious areas using the Compute % Impervious command. To learn more about the Compute % Impervious command, refer to this article in our knowledge base.

• Runoff Coefficient
  This editable column lists the runoff coefficient that is computed when the user clicks the [Compute] button.

Clicking on the [Compute] button allows the software to compute the runoff coefficient for the selected subbasin(s) and display the result in the Runoff Coefficient column.

Notes

• If SCS Curve Number is selected as the EPA SWMM infiltration method in the Scenario Manager dialog box and no curve number is defined for the subbasin, the following informational dialog box will be displayed:
  
• If a method other than SCS Curve Number is selected as the EPA SWMM infiltration method in the Scenario Manager dialog box and no curve number is defined for the subbasin, the following informational dialog box will be displayed:
  
• If no rainfall intensity is defined in the Rainfall Intensity Lookup panel, the following informational dialog box will be displayed:
  

Rainfall Intensity Lookup

This panel is used to retrieve the rainfall intensity value required to convert a previously computed curve number (CN) into an equivalent runoff coefficient.

Location Selection

This section allows the user to select the location from which the rainfall intensity value is to be retrieved.

The following options are available in this section:

• Centered on current view extents
  If this radio button option is selected, the software automatically selects the central location of the region displayed in the current Map View extents for retrieval of rainfall intensity value.

• Centered on subbasins
  If this radio button option is selected, the software automatically selects the central location of the subbasin model defined in the current scenario. If no subbasins are defined, this option will be disabled (i.e., grayed out).

• Select location
  This radio button option allows the user to pick the required location from the Map View interactively by using the [Pick] button. The address of the selected location will be displayed in the read-only field next to this option. The latitude and longitude values for the selected location will be displayed in the Latitude & longitude fields. The user can click the [Clear] button to clear the selected location and redo the entire process.
  

If the user selects either the Centered on current view extents or the Centered on subbasins options, the software will display the information of the selected location after successfully retrieving the rainfall intensity value.

Perform Rainfall Intensity Lookup

This section allows the user to retrieve the rainfall intensity value for the selected location.

The following entries are available in this section:

• Rainfall data source
  This dropdown combo box entry lists the rainfall data sources that are available for the selected location. The following rainfall data sources are available:

• • Austria – Bundesministerium Wasserwirtschaft eHYD Data Service
  • Canada – Environment Canada & Ontario Ministry of Transportation
  • Canada – Ontario Ministry of Transportation
  • Germany – Deutscher Wetterdienst – Klima und Umwelt
  • USA – ISWS (Illinois) Bulletin 75 (PFDS) Precipitation Frequency Data Server
  • USA – NOAA Atlas 14 (PFDS) Precipitation Frequency Data Server
  • USA – NOAA Atlas 2 (PFDS) Precipitation Frequency Data Server
  • USA – NRCC (Northeast Regional Climate Center) PFDS
  • USA – SUDAS (lowa) Bulletin 71 (PFDS) Precipitation Frequency Data Server

Note: The specific rainfall data source entry will be displayed based on the selected location.

• Return interval
  This dropdown combo box allows the user to select the return intervals that are provided by the selected rainfall data source. By default, the 2 Year return interval is selected.

• Rainfall intensity
  This entry field contains the rainfall intensity value that is returned for the selected location, rainfall data source, and return interval. Clicking the [Retrieve] button allows the software to retrieve the rainfall intensity value and display the results in the entry field.

Computing TOC

Once all the data have been defined in the Subbasin TOC Data dialog box, click the [Compute] button. The software will then compute the time of concentration for the selected subbasin(s) and display the results in the Computed TOC column.

The Subbasin TOC Data command allows the user to subdivide the subbasin flow path into individual segments in order to assign the appropriate SCS TR-55 runoff flow types to compute the SCS TR-55 Time of Concentration (TOC) / Lag Time. The defined flow path and corresponding computed Time of Concentration / Lag Time is then stored for each subbasin and used in the HEC-HMS runoff computations in the following runoff methods:

To use the Subbasin TOC Data command, follow the steps below:

1. From the Watershed ribbon menu, select the Subbasin TOC Data command.
2. The Subbasin TOC Data dialog box will be displayed.
   

The following sections describe how to compute the subbasin flow path TOC / Lag Time and interact with the above dialog box.

Selecting Subbasin Flow Path

Using the dropdown combo boxes and corresponding [Pick] buttons provided under the Select Subbasin Flow Path section, the user can select the required subbasin and associated flow path for purposes of computing the SCS TR-55 Time of Concentration (TOC) / Lag Time.

The following entries are provided in the Select Subbasin Flow Path section:

Subbasin ID

The Subbasin ID dropdown combo box allows the user to select the subbasin. If only one subbasin is present in the current scenario, then that subbasin will be automatically selected.

Note that if no subbasin is already selected, the dropdown combo box status will appear as “Not Selected”.

Alternatively, the user can click the [Pick] button to interactively select the subbasin from the Map View. On clicking the [Pick] button, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can then click on the required subbasin on the Map View to select it. The Subbasin TOC Data dialog box will be redisplayed, and the Subbasin ID dropdown combo box will automatically update to show the ID of the selected subbasin.

Flow Path ID

The Flow path ID dropdown combo box allows the user to select the subbasin flow path. If only one flow path is defined for the selected subbasin, then that flow path will be automatically selected.

Note that if no flow path is already selected, the dropdown combo box status will appear as “Not Selected”.

Alternatively, the user can click the [Pick] button to interactively select the flow path from the Map View. On clicking the [Pick] button, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can click on the required flow path to select it. The Subbasin TOC Data dialog box will be redisplayed, and the Flow path ID dropdown combo box will automatically update to show the ID of the selected flow path.

Terrain

The Terrain dropdown combo box allows the user to select the terrain surface associated with the selected subbasin. If only one terrain surface is present in the project, then that terrain surface will be automatically selected. Note that if there is no terrain surface corresponding to the selected subbasin, the dropdown combo box status will appear as “Not Selected”.

Flow Path Profile Plot

After the subbasin and associated flow path have been selected, the software will display a profile view of the terrain surface along the flow path line, starting at the highest endpoint of the flow path line at the left side end of the profile. This profile view allows the user to subdivide the flow path into flow segments, where each segment corresponds to a specific flow type [i.e., sheet flow, shallow concentrated flow, channel flow, pipe flow, and zero travel time (ponds)].

The user can extract the flow path profile for different terrain surfaces by selecting the terrain surface from the Terrain dropdown combo box and clicking the [Extract] button. The new flow path profile for the selected terrain surface will be displayed in the profile plot.

Editing Flow Segments on Profile Plot

The user can grab the joint between any two segments and drag the joint to the left or right. User adjustments will be reflected in the flow path profile line. As the joint is moved, the length and slope will automatically update in the data table shown in the Define SCS TR-55 TOC Data section. In addition, the length of each segment is shown above the segment so that the user can see how the edits are impacting segment length.

Adding/Merging Flow Segments on Profile Plot

The user can also add new segments or merge existing segments of the flow path using the Add Segment or Merge Segments option from the profile plot context menu.

To add a flow segment, the user can right-click at a location on the profile line and select the Add Segment option from the displayed context menu. The software will insert a new segment breakpoint at the cursor location. After adding segments, the user can define the runoff flow type for each segment using the data table provided under the Define SCS TR-55 TOC Data section.

To merge two flow path segments, the user can select the segment breakpoint between the two segments on the profile plot. The selected segment breakpoint will be highlighted. The user can then right-click and select the Merge Segments option from the displayed context menu.

The software will merge the segments adjacent to the segment break point. However, the flow line (and profile line) will not shorten. The two bracketing flow segments surrounding the segment breakpoint will fill-in the gap.

Editing Flow Path Segments in Data Table

The flow path segments can also be edited using the data table entries provided under the Define SCS TR-55 TOC Data section.

If the Maintain total flow length (adjacent segments will automatically adjust their lengths) checkbox entry is checked, then:

• The total length of the flow path will not change. Note that the total length of the flow path is displayed in the Total flow path length read-only field.
• The slope values are editable and can be changed for the flow segments.
• If the user manually edits the segment length values in the table, the segment length below the current entry will change accordingly to reflect the manual edit.
• If the user enters a segment length value larger than feasible relative to the segment length below it, the following informational dialog box will be displayed, and the entry will be discarded.
  

The user can freely edit the segmented flow path lengths and slopes. The profile plot will adjust accordingly, ignoring the profile that was extracted from the selected terrain surface.

Data Table Entries

The following entries are provided in the data table:

Runoff Flow Type

The Runoff Flow Type dropdown combo box is used to define the runoff flow type for each segment. It lists five options to define segment flow type: Channel Flow, Pipe Flow, Shallow Concentrated Flow, Sheet Flow, and Zero Travel Time (Ponds).

• Channel Flow: It is assumed to begin where surveyed cross section information has been obtained (where channels are visible on aerial photographs or blue lines that indicate streams appear on USGS quadrangle sheets).
• Pipe Flow: It is a flow through a pipe network.
• Shallow Concentrated Flow: Sheet flow usually becomes shallow concentrated flow after a distance of 300 feet.
• Sheet Flow: It is a flow over plane surfaces and usually occurs in the headwater of streams.
• Zero Travel Time (Ponds): It accounts for flow segments through a reservoir, lake, pond, or other structure that has a zero travel time.

Flow Length (ft)

The Flow Length (ft) entry is used to alter the length of flow segments. This entry accepts only positive, non-zero integers.

Slope (ft/ft)

The Slope (ft/ft) editable entry represents the slope of the flow segment line on the selected terrain surface. This entry accepts only positive, non-zero floating numbers. The user can click the adjacent [Recalc] button to recompute the segment slope for the defined flow length and elevation terrain surface. Similarly, the [Recalc All] button allows the user to recompute the slope value for the flow path segments available in the scenario.

Hydraulic Radius (ft)

The Hydraulic Radius (ft) entry is used in the Channel Flow and Pipe Flow equations while computing the Time of Concentration (TOC) values. This entry is only enabled for the Channel Flow and Pipe Flow rows, and its default value is 0.45 ft.

Surface Roughness/Type

The Surface Roughness/Type entry automatically changes depending upon the runoff flow type selected.

Shallow Concentrated Flow

When Shallow Concentrated Flow is selected as the Runoff Flow Type, a dropdown combo box will be displayed with the following entries:

• Agricultural Crops
• Bare Ground
• Forest With Underbush
• Forest Without Underbush
• Long Grass
• Paved
• Short Grass
• Unpaved
  

The TOC for the Shallow Concentrated Flow is computed using the following equation.

The Shallow Concentrated Flow computational procedure uses the same mathematical formula as used in the NRCS Upland Method (USDA NRCS NEH National Engineering Handbook, Chapter 15, Travel Time, Time of Concentration and Lag). The basic difference between the two methods is that the Shallow Concentrated Flow utilizes two surface types (i.e., paved and unpaved). Due to the similarity between these two methods, the software includes the capabilities in the NRCS Upland Method to provide additional flexibility in computing the TOC for the Shallow Concentrated Flow.

Note that the surface roughness values used to calculate the TOC for the Shallow Concentrated Flow are shown in the below table.

Channel Flow

When Channel Flow is selected as the Runoff Flow Type, an entry field is provided to enter the roughness value. The user can click the […] lookup button to display the Manning’s Roughness lookup table dialog box, as shown below.

Pipe Flow

When Pipe Flow is selected as the Runoff Flow Type, an entry field is provided to enter the roughness value. The user can click the […] lookup button to display the Channel and Pipe Manning’s Roughness lookup table dialog box, as shown below.

A default value of 0.29 ft is used for Hydraulic Radius. This value is for a 12 in pipe with maximum discharge. For maximum discharge, the depth must be 0.939 times the diameter, or the hydraulic radius must be 0.286 times the diameter.

Sheet Flow

When Sheet Flow is selected as the Runoff Flow Type, an entry field is provided to enter the roughness value. The user can click the […] lookup button to display the Overland Flow Roughness lookup table dialog box, as shown below.

Zero Travel Time (Ponds)

When Zero Travel Time (Ponds) is selected as the Runoff Flow Type, the software does not require any roughness value or surface type. It accounts for flow segments that pass through a reservoir, lake, pond, or other structure. This travel time is normally very small and can be assumed as zero.

Velocity (ft/s)

The Velocity (ft/s) is a read-only entry that displays the computed flow velocity for that row.

TOC (minutes)

The TOC (minutes) is a read-only entry that displays the computed time of concentration for that row.

Total TOC (minutes)

The Total TOC (minutes) is a read-only entry that displays the summation of all the individual flow segment TOC values.

Lag time (minutes)

The Lag time (minutes) is a read-only entry that displays the corresponding lag time based upon the total TOC.

Editing Flow Path Segments on Map View

The user can click the [Edit TOC Segments] button to add, edit, or delete flow path segments on the Map View.

On clicking the [Edit TOC Segments] button, the Subbasin TOC Data dialog box will temporarily disappear. The user can then select the flow segment nodes and drag them along the flow path line on the Map View.

Adding/Deleting Flow Path Segments on Map View

If necessary, the user can add or delete a flow segment while editing the flow path on the Map View. To add or delete a flow segment from a TOC flow path, the user can right-click at a location where a new flow segment node is to be inserted or select an existing TOC flow segment that is to be deleted. Then select the Add Segment or Delete Segment option from the displayed context menu.

On selecting the Add Segment option, the software will insert a node at the selected location and subdivide the previous flow segment into two separate segments. The two flow segments will have the same runoff flow type as the previous flow segment. The inserted node will be selected. The user can click and drag the node along the TOC flow path to adjust the length of the two adjacent flow segments.

On selecting the Delete Segment option, the software will remove the selected TOC flow segment. The remaining flow segments will fill-in (backfill) any removed flow segments. When removing a flow segment, the next downstream flow segment will fill-in the removed flow segment (by convention) where possible. If the most downstream flow segment is removed, then the next upstream flow segment will fill-in the removed flow segment.

Note that if there is only one flow segment on the flow path, the user cannot remove the flow segment, and the Delete Segment option is grayed out (unavailable).

After editing the flow paths, press the [Enter] key or right-click and select Done from the displayed context menu. The Subbasin TOC Data dialog box will be redisplayed.

SCS TR-55 Time of Concentration Computation

The software requires a 2-year-24-hour rainfall depth value to calculate TOC for sheet flow. For the regions falling within the United States, the software automatically retrieves and displays the 2-year-24-hour rainfall depth value in the 2yr-24hr rainfall depth entry field.

The Precipitation data source dropdown entry is used to select the precipitation data source for regions such as Austria, Germany, Ontario (Canada), and the USA. The precipitation data source is used for computing the 2yr-24hr rainfall depth data for the selected location.

The available precipitation data source options are:

• Austria – Bundesministerium Wasserwirtschaft eHYD Data Service
• Canada – Environment Canada & Ontario Ministry of Transportation
• Canada – Ontario Ministry of Transportation
• Germany – Deutscher Wetterdienst – Klima und Umwelt
• USA – NOAA Atlas 2 (PFDS) Precipitation Frequency Data Server
• USA – NOAA Atlas 14 (PFDS) Precipitation Frequency Data Server (default)
• USA – ISWS (Illinois) Bulletin 75 (PFDS) Precipitation Frequency Data Server
• USA – SUDAS (lowa) Bulletin 71 (PFDS) Precipitation Frequency Data Server
• USA – NRCC (Northeast Regional Climate Center) PFDS

Note: The specific precipitation data source entry will be displayed based on the selected location.

The 2yr-24hr rainfall depth entry field is used to define the 2-year-24-hour rainfall depth value. Clicking the [Retrieve] button causes the software to automatically retrieve the 2-year-24-hour rainfall depth value for the regions falling within the United States.

For regions falling outside the precipitation data coverage area, the user needs to manually enter the rainfall depth value in the 2yr-24hr rainfall depth entry field.

To compute the Time of Concentration (TOC) / Lag Time for the selected flow path, the user can click the [Compute] button. On clicking the [Compute] button:

• The TOC for individual segments will be displayed in the corresponding TOC (minutes) read-only field.
• The TOC and Lag Time of the complete flow path will be displayed in the Total TOC (minutes) and Lag time (minutes) read-only field, respectively.
• All HEC-HMS stormwater runoff methods that require the SCS TR-55 TOC Time of Concentration (TOC) / Lag Time will have the data populated into the appropriate fields.
  

Furthermore, the user can click the [Compute All] button to compute the TOC /Lag Time for all subbasins at once. On clicking the [Compute All] button, the software will go through each subbasin and compute the TOC / Lag Time. If a subbasin has multiple flow paths, the software will compute the TOC / Lag Time for each flow path. However, the flow path with the largest TOC /Lag Time will be assigned to the subbasin. Click the [Restore] button to restore the TOC flow segments back to their original segments and computed values.

The Subbasin TOC Data command allows the user to compute the time of concentration (TOC) for one or more subbasins using the SCS Watershed Lag method. The user can select the TOC/Lag time method from the Subbasin - TOC/Lag Time Method dropdown combo box in the General Preferences section of the Options backstage page.

The SCS Watershed Lag method computes TOC using the following equation:

Where:

L = Longest flow path in the subbasin

S = Average subbasin slope

RT = Maximum potential retention

Also,

Here, CN = Curve number

Follow the steps below that describe how to compute time of concentration using the SCS Watershed Lag TOC method:

1. From the Watershed ribbon menu, select the Subbasin TOC Data command.
   
   Alternatively, from the Input ribbon menu, click the Drainage Subbasins dropdown menu and select the Subbasin TOC Data command.
   
2. The Subbasin TOC Data dialog box will be displayed, as shown below.
   

The following sections describe how to use the Subbasin TOC Data command and interact with the above dialog box.

Selecting Subbasins

The Select Subbasins section allows the user to select single or multiple subbasins defined in the current scenario to compute the time of concentration using the SCS Watershed Lag method.

The user can select the subbasin(s) using any of the following methods:

• By checking the checkboxes corresponding to each subbasin in the Select Subbasins section.
• By clicking the [Pick] button. In this selection method, the Subbasin TOC Data dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the subbasin(s) from the Map View. After selecting the subbasin(s), press the [Enter] key or right-click and choose Done from the displayed context menu. The Subbasin TOC Data dialog box will be redisplayed.

After selecting the subbasin(s), the total number of selected subbasins will be displayed in the Total selected read-only field, as shown below.

Note that if the US unit system is selected on the Options backstage page, the US units will be displayed in the Select Subbasins section. The user can also change the unit system to a metric (SI) unit, if necessary. Refer to this article in our knowledge base to learn more about setting the model units to metric (SI) or US units.

The table in the Select Subbasins section contains the following data column entries:

• Subbasin
  This read-only entry lists all subbasin IDs contained in the current scenario.

• Flow Path Length (ft)
  This read-only entry lists the computed flow path lengths using the Automated Flow Paths command. To learn more about the Automated Flow Paths command, refer to this article in our knowledge base.

• Subbasin Slope (ft/ft)
  This editable entry lists the computed subbasin slopes using the Compute Slopes command. To learn more about the Compute Slopes command, refer to this article in our knowledge base.

• Curve Number
  This editable entry lists the computed curve numbers using the Compute CN command. Clicking the […] lookup button displays a SCS Curve Number dialog box, which allows the user to select the curve number for urban areas. To learn more about the Compute CN command, refer to this article in our knowledge base.
  

• Computed TOC (minutes)
  This editable entry lists the computed TOC value. By default, this entry lists the TOC value defined in the Subbasin Data dialog box.

Computing TOC

Once all the data have been defined, click the [Compute] button. The software will then compute the time of concentration for the selected subbasin(s) and display the result in the Computed TOC data column entry.

The Subbasin TOC Data Table Edit command allows the user to view and edit all subbasin TOC data in a single editable table. Note that the table will display TOC data only after it has been calculated using the Subbasin TOC Data command. Refer to this article in our knowledge base to learn more about Subbasin TOC Data command.

Follow the steps below to use the Subbasin TOC Data Table Edit command:

1. From the Input ribbon menu, click on the Drainage Subbasins dropdown menu and then select the Subbasin TOC Data Table Edit command.
   
2. The Subbasin TOC Data Table Edit dialog box will be displayed.
   

The following sections describe the Subbasin TOC Data Table Edit dialog box.

Subbasin TOC Data Table

The following data entries are provided in the Subbasin TOC Data editable table.

Subbasin ID

This column lists all subbasins contained in the current scenario. The user can use the filter option from the column header to limit the subbasins displayed in the table.

TOC Segment

This column sequentially numbers the TOC segment from the most upstream to the most downstream segments.

Runoff Flow Type

This column having a dropdown combo box lists the available runoff flow types, as listed below.

• Channel Flow
• Pipe Flow
• Shallow Concentrated Flow (default for newly inserted rows)
• Sheet Flow
• Zero Travel Time (Ponds)
  

Flow Length

This column lists the length of the TOC segment. Clicking the […] button under this column causes the dialog box to temporarily disappear, allowing the user to measure the flow path segment length from the Map View.

Slope

This column lists the slope of the TOC segment. If the user has specified a Terrain elevation surface, then the user can measure the Flow Length from the Map View and the software will automatically compute the TOC segment slope.

If no Flow Length is defined, then this entry will be blank. If a Flow Length is specified and no Terrain Elevation Surface is defined, then the default value of slope will be 0.0300.

The user can click the [Recalc] button to recompute the segment slope for the defined flow length and elevation terrain surface. Similarly, the [Recal All] button allows the user to recompute the slope value for all the flow path segments available in the scenario at once.

Hydraulic Radius

This column will be enabled if Channel Flow or Pipe Flow is selected as the Runoff Flow Type. The default value of the Hydraulic Radius is 0.45 ft for the Channel Flow and 0.29 ft for the Pipe Flow.

Surface Roughness/Type

This column changes its behavior depending upon the Runoff Flow Type selected. When Sheet Flow is selected as the Runoff Flow Type, then an entry field with a lookup […] button will be displayed in the corresponding Surface Roughness/Type column field. If Sheet Flow is selected as the Runoff Flow Type, the default value for the corresponding Surface Roughness/Type column will be 0.40000.

Clicking the […] button under the Surface Roughness/Type column displays the Overland Flow Roughness lookup dialog box.

When Shallow Concentrated Flow is selected as the Runoff Flow Type, then a dropdown combo box will be displayed in the corresponding Surface Roughness/Type column field with the following entries.

• Unpaved (default entry)
• Agricultural Crops
• Bare Ground
• Forest With Underbrush
• Forest Without Underbrush
• Long Grass
• Paved
• Short Grass
  

When Channel Flow is selected as the Runoff Flow Type, then an entry field with a lookup […] button will be displayed in the corresponding Surface Roughness/Type column field. If Channel Flow is selected as the Runoff Flow Type, the default value for the corresponding Surface Roughness/Type column will be 0.04500.

Clicking the […] button under the Surface Roughness/Type column displays the Manning’s Roughness lookup dialog box.

When Pipe Flow is selected as the Runoff Flow Type, then an entry field with a lookup […] button will be displayed in the corresponding Surface Roughness/Type column field. If Pipe Flow is selected as the Runoff Flow Type, the default value for the corresponding Surface Roughness/Type column will be 0.01300.

Clicking the […] button under the Surface Roughness/Type column displays the Channel and Pipe Manning’s Roughness lookup dialog box.

When Zero Travel Time (Ponds) is selected as the Runoff Flow Type, then the Surface Roughness/Type column field becomes read-only. This option is used for flow segments that pass through a reservoir, lake, pond, or other structure where travel time is considered negligible or zero.

Velocity

This column lists the computed flow velocity.

TOC

This column lists the computed Time of Concentration (TOC) value.

Terrain Elevation Surface

The Terrain elevation surface dropdown entry is used to select the terrain surface available in the project. The dropdown entry by default is set to the current scenario’s terrain surface.

Precipitation Data Source

The Precipitation data source dropdown entry is used to select the precipitation data source for regions such as Austria, Germany, Ontario (Canada), and the USA. The precipitation data source is used for computing the 2yr-24hr rainfall depth data for the selected location.

The available precipitation data source options are:

• Austria – Bundesministerium Wasserwirtschaft eHYD Data Service
• Canada – Environment Canada & Ontario Ministry of Transportation
• Canada – Ontario Ministry of Transportation
• Germany – Deutscher Wetterdienst – Klima und Umwelt
• USA – NOAA Atlas 2 (PFDS) Precipitation Frequency Data Server
• USA – NOAA Atlas 14 (PFDS) Precipitation Frequency Data Server (default)
• USA – ISWS (Illinois) Bulletin 75 (PFDS) Precipitation Frequency Data Server
• USA – SUDAS (Iowa) Bulletin 71 (PFDS) Precipitation Frequency Data Server
• USA – NRCC (Northeast Regional Climate Center) PFDS

Note: The specific precipitation data source entry will be displayed based on the selected location.

2yr-24hr Rainfall Depth

The 2yr-24hr rainfall depth entry stores the 2-year 24-hour rainfall depth for the sheet flow computations. Clicking the [Retrieve] button retrieves the rainfall data for the selected location.

After defining or editing the required table value(s), click the [Compute] button to calculate the results of the Subbasin TOC Data table.

Exporting Subbasin TOC Data Table

After computing the subbasin TOC data, click the [Export] button to export the subbasin TOC tabular data into a spreadsheet. Alternatively, right-click on any of the column headers or editable table cells and select Export Table to Excel or Export Table to PDF command from the context menu to export the data into the required format.

When the software imports a model, it automatically places the reaches on the Map View. However, if the original model was not spatially georeferenced, the reaches will not align with any loaded background base map. While the software can operate without any issues in this situation, the user may prefer that the reaches be georeferenced to the background base map. Therefore, it may become necessary to georeference the imported reaches.

The Georeference Reaches command in GeoHECHMS software is used to manually georeference each of the reaches to the background base map displayed in the Map View. The process of georeferencing a reach to the Map View can be a trial-and-error process, especially when the exact location of the original reach is not known. Using the Georeference Reaches command, this process is accelerated.

Follow the steps below to georeference an existing reach:

1. From the Input ribbon menu, click on the Routing Reaches dropdown menu and then select the Georeference Reaches command.
   
   
2. The Georeference Reaches dialog box will be displayed.
   
   

The following sections describe how to georeference an existing reach and interact with the above dialog box.

Selecting Reach to Georeference

The Select Reach to Georeference section allows the user to select the reach to be georeferenced.

Note that if a reach has been preselected from the Map View before running this command, then the selected reach will be displayed in the Reach ID dropdown combo box.

If the model contains a single reach, then it will automatically be selected in the Reach ID dropdown combo box. If the model contains multiple reaches, then the user can select the desired reach from the Reach ID dropdown combo box.

Alternatively, the user can click the [Pick] button to select the reach from the Map View. Clicking on the [Pick] button causes the dialog box to temporarily disappear, and the user will be prompted to select the reach from the Map View. After selecting a reach, the dialog box will be redisplayed with the reach shown as selected. The selected reach will also be highlighted on the Map View.

Note that the user can click the [Clear] button to cancel all the previous selections and redo the entire process.

After selecting the reach, the user can select one of the following georeference options:

• Snap to Alignment Polyline
• Slide Along Alignment Polyline
• Draw on Map View

Snap to Alignment Polyline

If an existing alignment polyline for the selected reach exists on the Map View, the Snap to Alignment Polyline section can be used to snap the reach to the alignment polyline. Selecting the Snap to Alignment Polyline radio button option enables this section.

This section contains the following options:

• Pick near upstream end
  Clicking the [Pick] button adjacent to this read-only entry allows the user to pick a point from the Map View near the upstream end of the alignment polyline. On clicking the [Pick] button, the dialog box will temporarily disappear, and the user will be prompted to select an upstream point on the alignment polyline. After selecting the upstream point, the dialog box will be redisplayed and the Pick near upstream end entry changes from Not Selected to Selected. In addition, a temporary highlighted point is shown on the alignment polyline to display the selected location. To abort the selection process, press the [Esc] key or right-click and choose Cancel from the displayed context menu.
• Pick near downstream end
  Clicking the [Pick] button adjacent to this read-only entry allows the user to pick a point from the Map View near the downstream end of the alignment polyline. On clicking the [Pick] button, the dialog box will temporarily disappear, and the user will be prompted to select a downstream point on the alignment polyline. After selecting the downstream point, the dialog box will be redisplayed and the Pick near downstream end entry will change from Not Selected to Selected. In addition, a temporary highlighted point is shown on the alignment polyline to display the selected location. To abort the selection process, press the [Esc] key or right-click and choose Cancel from the displayed context menu.

After selecting the upstream and downstream end of the reach, click the [Snap] button and the reach will get snapped to the alignment polyline.

Note that the [Snap] button can only be enabled when the upstream and downstream ends of the reach have been selected. In addition, a preview of the snapped reach is shown on the alignment polyline, which allows the user to reselect the downstream and upstream points or cancel the selection process.

The user can turn on the Scale to fit checkbox option to scale the reach to fit within the alignment polyline.

If the upstream and downstream points do not overlay the same continuous alignment polyline, then the following informational message will be displayed.

Slide Along Alignment Polyline

If the reach has been snapped to an alignment polyline on the Map View, but is not precisely located where it should be, the Slide Along Alignment Polyline section can be used to slide the reach along the alignment polyline. Selecting the Slide Along Alignment Polyline radio button option enables this section.

Clicking the [Pick] button allows the user to select an alignment polyline from the Map View. On clicking the [Pick] button, the dialog box will temporarily disappear, and the user will be prompted to select an alignment polyline. After selecting the alignment polyline, a Snap Entity dialog box will be displayed. Click the [Yes] button to snap the entity on the selected polyline. Otherwise, click the [No] button to cancel the selection and redo the entire process.

After selecting the alignment polyline, the Georeference Reaches dialog box will be redisplayed and the Select alignment polyline element read-only entry will change from Not Selected to Selected.

Click the [Slide] button to manually slide the reach along the underlying alignment polyline. Clicking the [Slide] button makes the dialog box temporarily disappear and prompts the user to select the reach and drag it along the underlying alignment polyline. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu. To abort the slide operation, press the [Esc] key and the moved reach will return to its original location.

While sliding the reach along the alignment polyline, the user would not be allowed to move the reach beyond the end of the alignment polyline. The Recompute adjacent node element elevations checkbox option allows the user to recompute the elevation of the adjacent node type elements after georeferencing the reach.

Note that if the endpoints of the reach do not overlay the same continuous polyline, then the following informational message will be displayed.

Draw on Map View

This section allows the user to draw the reach on the Map View. Selecting the Draw on Map View radio button option enables this section.

Clicking the [Draw] button temporarily removes the dialog box and prompts the user to draw the reach on the Map View. Once the reach is drawn, press the [Enter] key or right-click and select Done from the displayed context menu. The reach will then automatically snap to the drawn alignment polyline.

The user can turn on the Create curvilinear polyline checkbox option to draw the polyline using a curvilinear segment. Refer to this article in our knowledge base to learn more about element digitizing on the Map View.

The user can turn on the Scale to fit checkbox option to scale the reach to fit within the alignment polyline.

In GeoHECHMS, the Merge Reaches command is used to merge two or more connected reaches (end to end along an alignment) into a single reach. This article describes how to use the Merge Reaches command.

Follow the steps below to merge two or more reaches:

1. From the Input ribbon menu, click the Routing Reaches dropdown menu and select the Merge Reaches command.
   
   
2. The Merge Reaches dialog box will be displayed.
   
   
3. Click the [Pick] button to select the reaches to be merged.
   
   
4. The Merge Reaches dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next.
5. Select reaches that are connected along a single alignment to merge from the Map View.
6. When finished, press the [Enter] key or right-click and select Done from the displayed context menu.
7. The Merge Reaches dialog box will be redisplayed, and the number of selected reaches will be displayed in the Pick Reaches read-only field, as shown below.
   
   Note that if the reaches have been preselected prior to running this command, the same number of selected reaches will be displayed in the Pick Reaches read-only field.
8. Click the [OK] button and the software will then merge the selected reaches into a single reach.
   
   
9. If the user has selected reaches that are not connected along a single alignment, the following informational dialog box will be displayed on clicking the [OK] button.
   
   

The Automated Draw Reaches command allows the user to automatically draw reaches from the user selected locations on the Map View.

Follow the steps below to use the Automated Draw Reaches command:

1. From the Input ribbon menu, select the Routing Reaches dropdown menu and then choose the Automated Draw Reaches command.
   
2. The Automated Draw Reaches dialog box will be displayed.
   

The following sections describe the Automated Draw Reaches command and how to interact with the above dialog box.

Selecting Reach Starting Point

The Select Reach Starting Point section is used to select a starting point on the Map View for drawing the reach.

To select a starting point, follow the steps below:

1. Click on the [Pick] button, and the dialog box will temporarily disappear.
   
2. The status bar (shown under the Map View) will prompt you to select a point on the Map View. Select the starting point on the Map View to draw the reach.
3. After selecting the starting point, the user is immediately returned to the Automated Draw Reaches dialog box, and the Select point read-only field will be changed from Not Selected to Selected.
   

Reach Draw Direction

This section is used to define the direction of the reach polyline to be drawn based on the selected starting point.

The following options are provided:

• Downstream flow direction
  This radio button option is used to define the flow direction of the reach polyline downstream from the starting point. Note that this option is selected by default when the dialog box is displayed.
• Upstream flow direction
  This radio button option is used to define the flow direction of the reach polyline upstream from the starting point.

Reach Specifications

This section is used to specify a reach ID for each drawn reach. The user can assign these IDs either manually or automatically using some predefined formats.

Follow the steps below to assign reach IDs to the reaches:

1. If a reach has been drawn while the Reach ID option was enabled, the user can manually enter the reach ID in the corresponding field as shown below.
   
2. Alternatively, the user can enable the Auto-name reach ID radio button option to automatically name newly drawn reaches as per the user’s predefined naming formats as shown below.
   

The different reach naming formats present in the Auto-name reach ID option are as follows:

• Reach ID prefix: This option allows a prefix to be added to the start of the reach ID.
• Reach ID digits: This option permits the specification of a set number of digits to use for the reach ID. For example, using 3 digits causes the reach ID to be of the format 001, 002, etc.
• Next available reach ID: This entry defines the next element ID number to be used.
• Reach ID increment: This entry defines the increment to use when numbering elements. The default value is 1.
• Reach ID suffix: This option allows a suffix to be added to the end of the reach ID.
• Reach ID preview: This entry provides a preview of the reach naming specifications defined above.

General Options

This section is used to define the general options for drawing the reach.

• Terrain surface
  The dropdown combo box lists the terrain surfaces associated with the project which are used to define the reach polyline. By default, the current scenario’s terrain surface is selected. If there is no terrain surface defined for the selected scenario, then this entry is blank.
• Reach Maximum length
  This optional checkbox option is used to define the maximum length that the reach polyline will be drawn. If left blank, the polyline will extend to the limits of the underlying flow direction grid.
  
  Alternatively, click the [Pick] button to draw a polyline representing the maximum reach length. On clicking the [Pick] button, the dialog box will temporarily disappear. A prompt will be displayed on the status bar instructing the user to draw an approximate reach polyline on the Map View. When starting to draw a reach polyline by holding down the [Shift] key, the drawing is done along an ortho line. Similarly, if the user holds down the [Ctrl] key while drawing, the drawing is done along a curvilinear line. In addition, the status bar shows the accumulated length while drawing the reach polyline. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu. The drawn polyline will be removed, and the Automated Draw Reaches dialog box will be redisplayed with the measured distance, as shown below.
  
  Note that the [Pick] button will be disabled (i.e., grayed out) up until the point that the user selects a starting point.

Computing Flow Direction

When all the options have been properly defined, click the [Compute] button to compute the flow direction grid for the selected terrain surface.

While the computation is running, the [Compute] button will change to [Cancel] button, which allows the user to abort the command if needed. In addition, a progress bar at the bottom will show the progress of the computation along with status messages.

After computing the flow direction grid, the software will internally assign the created polyline as a reach on the Map View.

In GeoHECHMS, the Reverse Reaches command is used to reverse the reach flow direction.

Follow the steps below to use the Reverse Reaches command:

1. From the Input ribbon menu, click the Routing Reaches dropdown menu and select the Reverse Reaches command.
2. The Reverse Reaches dialog box will be displayed.
3. The Select Reaches section lists all the available reach(s) of the scenario. The user can select the desired reach using the check box option. Alternatively, the user can click the [Pick] button to manually select the reach from the Map View and then press [Enter] or can right-click and select Done from the context menu to complete the selection. (The number of selected reach(s) will be displayed in the Total Selected field as shown below.)
4. After selecting the reach(s), click on the [Reverse] button and the software will reverse the reach flow direction as shown below.
5. Alternatively, the user can select the reach directly from the Map View and reverse its flow direction using the Reverse Reach command from the right-click context menu.

A reach is a section of a stream or river along which similar hydrologic routing conditions exist, such as discharge, depth, area, and slope.

In GeoHECHMS, reaches can be defined by either drawing or assigning polyline(s) on the Map View using the following commands:

• Draw Reaches
• Assign Reaches

Drawing/Assigning Reaches

The Draw/Assign Reaches command allows the user to manually draw/assign polylines on the Map View as reaches one after another until completed.

Follow the steps below to use the Draw/Assign Reaches command:

1. From the Input ribbon menu, select the Routing Reaches dropdown menu and then choose the Draw/Assign Reaches command.
   
2. The following dialog box(s) will be displayed.
   • Draw Reaches:
     
   • Assign Reaches:
     

Note that pressing the [Ctrl+E] keys will run the Draw Reaches command directly on the Map View.

The following sections describe how to use the Draw/Assign Reaches command and interact with the above dialog box(s).

Drawing Reaches

Drawing Reach Polylines

The Draw Reach Polylines section is used to draw the reaches on the Map View using polylines. To draw reach polylines, follow the steps below:

1. From the Draw Reach Polylines section, click the [Draw] button, and the dialog box will temporarily disappear. Use the Draw curvilinear polyline checkbox option to draw the polyline using curvilinear segments.
   
2. The status bar (shown under the Map View) will prompt you to draw a reach on the Map View. Draw the reach polyline on the Map View. While drawing elements, the user can use the [Ctrl] key to switch between the curvilinear and the linear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.
   Notes:
   • To draw the reaches more accurately, use the mouse roller wheel to zoom into a specific location.
   • The Snap Mode option can be enabled to cause the drawn reach to snap to the nearest HEC-HMS element.
3. After completing the reach drawing, the Draw Reaches dialog box will be redisplayed, and the Reach polylines read-only field will be changed from Not Drawn to Drawn.
   
4. The user can also reverse the direction of the drawn/assigned reach by clicking the [Reverse Direction] button.

Assigning Reaches

Selecting Reach Polylines

The Select Reach Polylines section can be used to manually assign multiple polylines on the Map View as reaches. To assign reach polylines, follow the steps below:

1. From the Select Reach Polylines section, click the [Pick] button and the dialog box will temporarily disappear.
   
2. The status bar (shown under the Map View) will prompt you to select a reach polyline from the Map View. Click on the previously drawn reach polyline on the Map View to select it.
   
3. Following the selection of a reach polyline, the Assign Reaches dialog box will be redisplayed, and the Reach polylines read-only field will be changed from Not Selected to 1 Selected.
   

Reach Specifications

This section is common to both the Draw Reaches and Assign Reaches dialog box and is used to specify the reach ID for each drawn/assigned reach. The user can assign these IDs either manually or automatically using some predefined formats.

Follow the steps below to assign reach IDs to the reaches:

1. If a reach was drawn/assigned while the Reach ID radio button option was selected, the user can manually enter the reach ID in the corresponding field as shown below.
   
2. Alternately, the user can enable the Auto-name reach ID radio button option to automatically name every newly drawn/assigned reach as per the user’s predefined naming formats as shown below.
   
   The different reach naming formats present in the Auto-name reach ID option are as follows:
   • Reach ID prefix: This option allows a prefix to be added to the start of the reach ID.
   • Reach ID digits: This option permits specification of a set number of digits to use for the reach ID. For example, using 3 digits causes the reach ID to be of the format 001, 002, etc.
   • Next available reach ID: This entry defines the next element ID number to be used.
   • Reach ID increment: This entry defines the increment to use when numbering elements. The default value is 1.
   • Reach ID suffix: This option allows a suffix to be added to the end of the reach ID.
   • Reach ID preview: This entry provides a preview of the reach naming specifications defined above.
3. After providing the Reach ID, press the [Enter] key or click the [Apply] button.
   
   Note that if the auto-name option is enabled and the user returns to the dialog box, the [Apply] button is disabled since the just drawn/assigned reaches have already been named and created.

Assign Reach Slope, Downstream Invert Elevation, Flow Direction & Missing Junctions

This section is common to both the Draw Reaches and Assign Reaches dialog box and allows the user to assign an invert elevation to the reach using a terrain model. Define this section before drawing/assigning the reaches so that the invert elevations can be assigned.

To assign an invert elevation, the Terrain elevation source dropdown combo box supports the following surface types:

• Elevation grids
• LandXML data
• TIN surfaces

The user can also apply an elevation offset to raise or lower the reach by the specified amount by checking the Apply elevation offset checkbox. On selecting this checkbox, the entry field next to it becomes available for entering an elevation offset value. A negative offset value will lower the reach by the specified amount.

In the Assign Reaches dialog box, an additional Create junctions (if missing) at confluences checkbox option is provided. If the user assigns the reach polylines where the junction(s) are not available, then checking this checkbox option will automatically insert the junction(s) at the confluences.

The Reach Table Edit command allows the user to edit the reach parameters in one dialog box for common hydrology routing methods. When the user selects this command, a modeless dialog box will be displayed. This dialog box contains an editable data grid listing all the reaches in the current scenario. When the user selects a row in the data grid, the corresponding reach will be zoomed to the Map View (if not visible) and highlighted. In addition, the user can select a reach from the Map View, and the corresponding row in the data grid will be shown selected.

Follow the steps below to use the Reach Table Edit command:

1. From the Input ribbon menu, click the Routing Reaches dropdown menu and then select the Reach Table Edit command.
   
2. The Reach Table Edit dialog box will be displayed.
   

The following sections describe how to use the Reach Table Edit command and interact with the above dialog box.

Reach Parameters

The Reach Parameters section will display the editable data grid columns listing the reach parameters contained in the current scenario. Based upon the selected routing method in the Project Options backstage page, as shown below, the data grid columns listing will be changed.

The user can select the following routing methods from the Project Options backstage page and the associated parameters will be listed in the data grid of the Reach Table Edit command.

Refer to this article in our knowledge base to learn more about reach routing methods.

Exporting Data to Excel or PDF

The user can export the data grid column values to Microsoft Excel or PDF documents. To export data into Excel or PDF, right-click on the data grid column and select Export Table to Excel or Export Table to PDF command from the displayed context menu.

Sending Data to the Windows Clipboard

The user can send the data grid column values to the Windows clipboard. Moving a data grid column value to the clipboard allows those values to be pasted into another software. To transfer the data to the Windows clipboard, right-click on the data grid column and select the Copy Table to Clipboard command from the displayed context menu. The data will automatically be sent to the Windows clipboard.

After editing the desired reach parameters for common hydrology routing methods, the user can click the [Close] button to close the dialog box.

Reaches define the elements (i.e., pipes, ditches, streams, etc.) that route flows through the stormwater network model. In GeoHECHMS, the Reach Data command allows the user to add new reach and edit reach data in a project.

Follow the steps below to view or modify the reach data:

1. From the Input ribbon menu, select the Reach Data command.
   Alternatively, the user can either double-click on the reach polyline from the Map View or choose the Reach Data command from the Routing Reaches dropdown menu of the Input ribbon menu.
   
2. The Reach Data dialog box will be displayed.
   

The following sections describe the Reach Data command and how to interact with the above dialog box.

Selecting Reach

This section allows the user to select the reach in order to define the reach data. The user can create a new reach, copy existing reach data to a new reach, and delete a reach. In addition, the user can navigate between reaches and enter a description detailing the defined reach.

The following entries are provided in this section:

• Reach ID
  This dropdown combo box lists all the reaches that are defined in the model. The user can select the desired reach from the dropdown combo box. Click the pencil icon to edit the reach ID. The user can navigate between the previous and next reaches using the up and down arrow buttons. Alternatively, the user can click the […] button to select the reach from the Map View. Note that the up and down arrow buttons will be disabled (i.e., grayed out) when the model contains only a single reach.
• Description
  This text field allows the user to enter additional information that describes the selected reach.
• New
  The [New] button allows the user to create a new reach. On clicking this button, the dialog box will temporarily disappear. A prompt will be displayed on the status bar informing the user to draw the reach in an upstream to downstream direction on the Map View. Once finished, press the [Enter] key, or right-click and select Done from the displayed context menu. While drawing a reach, the user can press the [Esc] key to abort the creation of a new reach and return the dialog box to its previous state.

The dialog box will be redisplayed. Next, enter the reach name in the Reach ID entry and click the [Accept changes] button. The software checks that the defined ID is unique. If not, a warning dialog box is displayed, and the user is then returned to the Reach ID field to change the ID.

• Copy
  The [Copy] button allows the user to copy existing reach data to a new reach. When this command is executed, the software automatically provides a unique default name for the duplicated reach. The cursor is then placed into the Reach ID. The user can go with the default name or enter a different valid and unique ID before moving on to add any other data.
• Delete
  The [Delete] button allows the user to delete the current reach data from the model. Clicking the [Delete] button causes the following confirmational dialog box to be displayed.
  

Click the [Yes] button to delete the selected reach data.

Click the [No] button to abort the deletion process.

Reach Specifications

The following sections describe the reach. Click on the dropdown selector at the Reach Specifications entry to display the various data panels that define the reach data.

General Specifications

This panel allows the user to define the routing and loss/gain methods to be used for the current reach.

Note that the routing and loss/gain methods are defined for the project in the Scenario Manager and will be displayed automatically in the corresponding dropdown entries. However, the user can change the desired method for the selected reach from this dialog box. Refer to this article in our knowledge base to learn how to use the Scenario Manager dialog box.

The following options are provided in this data panel:

• Routing method
  This dropdown combo box allows the user to select the routing method for the selected reach. While a reach element conceptually represents a segment of stream or river, the actual calculations are performed by a routing method contained within the reach. The dropdown combo box provides the following routing methods.
  1. None
  2. Kinematic Wave
  3. Lag Time
  4. Lag Time & Attenuation
  5. Modified Puls
  6. Muskingum
  7. Muskingum Cunge
  8. Normal Depth
  9. Straddle Stagger

Refer to this article in our knowledge base to learn about various HEC-HMS routing methods.

• Loss/gain method
  This dropdown combo box allows the user to select the loss/gain method for the selected reach. While a reach element conceptually represents a segment of stream or river, optional modeling of interactions with the subsurface is performed by a loss/gain method contained within the reach. The dropdown combo box provides the following loss/gain methods.
  1. None
  2. Constant Loss/Gain
  3. Percolation Loss

Refer to this article in our knowledge base to learn about various HEC-HMS loss/gain methods.

• Initial conditions
  This dropdown combo box entry sets the amount of stored water in the storage area at the start of the simulation. The dropdown combo box lists the following conditions:
  1. Discharge
  2. Inflow = Outflow

If you use the first option, you will also have to enter a discharge value. If you use the second option, it will be assumed that the initial outflow is the same as the initial inflow to the reach from upstream elements. This is essentially the same as assuming a steady-state initial condition.

• Initial discharge
  This entry field defines the initial discharge being released from the storage area. This entry is only available when the Initial conditions entry has been set to the Discharge option. Otherwise, this entry is disabled (i.e., grayed out).
• Length
  This entry field defines the length of the selected reach. The user can either enter the value manually or click the […] button to measure the length from the Map View. Clicking on the […] button allows the user to draw a measurement polyline representing the flow length of the selected reach. However, initially the software will assign the length from the defined reach polyline.

If the user graphically edits the reach polyline, then the length value automatically updates based upon the Recompute dimensions on element edits checkbox setting in the Options backstage page. Refer to this article in our knowledge base to learn about the Options backstage page.

Clicking on the [Recalc] button causes the software to automatically update this field value with the corresponding digitized reach polyline length. Similarly, the [Recalc All] button causes the software to update the lengths for all digitized reach polylines.

Note that this entry is disabled (i.e., grayed out) for the following routing methods: Lag Time, Lag Time & Attenuation, Modified Puls, Muskingum, and Straddle Stagger.

• Slope
  This entry field defines the slope of the selected reach. The software will automatically compute the slope based upon the connected element elevations (where possible). However, the user can override the assigned slope by manually defining the value. If the connected elements’ invert elevations change, then the software automatically updates this value based upon the Recompute dimensions on element edits checkbox setting in the Options backstage page. Refer to this article in our knowledge base to learn about the Options backstage page.

Clicking on the [Recalc] button causes the software to automatically update this field value with the corresponding slope based upon changed elevations. Similarly, the [Recalc All] button causes the software to update the slopes for all reaches.

Note that this entry is disabled (i.e., grayed out) for the following routing methods: Lag Time, Lag Time & Attenuation, Modified Puls, Muskingum, and Straddle Stagger.<

• Downstream connection
  This entry field defines the downstream element that the selected reach drains to. Clicking on the […] button allows the user to select the downstream element from the Map View. The following elements can be selected.
  1. Diversion
  2. Junction
  3. Reach
  4. Reservoir
  5. Sink

Note that a reach can connect downstream to another reach. Multiple reaches can be chained together to represent a long river reach with changing geometry. However, a reach cannot connect downstream to a subbasin.

• Downstream invert elevation
  This entry field defines the invert elevation at the downstream end of the selected reach.

If the selected reach is not connected to another reach (i.e., connected to a junction, etc.), then this field is read-only, showing the invert elevation of the connected element.

If the selected reach is connected to another reach, then this field is editable. The software will attempt to compute this value by using the upstream invert elevation (if available) and the defined reach length and slope values. The user can also override the computed invert elevation by manually defining the value.

Note that this entry is disabled (i.e., grayed out) for the following routing methods: Lag Time, Lag Time & Attenuation, Muskingum, and Straddle Stagger.

Computational Results

This section allows the user to see the analysis results for the current reach that was computed by HEC-HMS.

Note that the Detailed Results section provides additional analysis results for the current reach that was computed by HEC-HMS.

DSS Data

This section includes an external DSS file for referencing HEC-HMS computational results. It allows the user to easily copy the references and paste them into GeoHECRAS.

• DSS file name: This entry defines the external DSS file to be used for reading the data.
• Data path: This entry denotes the data path within the DSS data file, which contains the paired data.

Routing Data

The Routing Data panel is used to enter the data for the selected routing method for the defined reaches. This panel is displayed when the Routing Data option is selected in the Reach Specifications dropdown combo box. Note that the Routing Data panel content changes based upon the routing method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

Loss/Gain Data

The Loss/Gain Data panel is used to enter data for the selected loss/gain method for the defined reaches. This panel is displayed when the Loss/Gain Data option is selected in the Reach Specifications dropdown combo box. Note that the Loss/Gain Data panel content changes based upon the loss/gain method selected in the General Specifications section.

Refer to this article in our knowledge base to learn more about this panel.

The Renumber Interconnected Reaches command allows the user to automatically renumber routing reaches and connected routing junctions along a defined flow path.

Follow the steps below to use the Renumber Interconnected Reaches command:

1. From the Input ribbon menu, click the Routing Reaches dropdown menu and then select the Renumber Interconnected Reaches command.
   
2. The Renumber Interconnected Reaches dialog box will be displayed.

The following sections describe the Renumber Interconnected Reaches command and how to interact with the above dialog box.

Selecting Reaches

The Select Reaches panel allows the user to select reaches that define the river path. The user can use the checkbox contained within the column header to select and deselect all reaches.

Selecting Multiple (Interconnected) Reaches

The Select Multiple (Interconnected) Reaches section is used to sequentially number reaches and (by option) junctions along the selected path that has reaches connected end-to-end with each other. This section allows the user to select multiple reaches defined in the project that make up the river path.

If a reach is already selected on the Map View before running this command, the same reach will be shown selected within the table. The user can also select/deselect the desired reach by checking/unchecking the checkboxes corresponding to each reach that defines the river path.

Alternatively, the user can click the [Pick] button to select the most downstream and upstream reach on the Map View and the software will automatically determine all connected reaches between them that make up the river path. On clicking the [Pick] button, the Renumber Interconnected Reaches dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the most downstream and upstream reach on the Map View. Once finished, press the [Enter] key or right-click and choose Done from the displayed context menu. The dialog box will be redisplayed, and the total number of selected reaches will be displayed in the Total selected read-only field.

Notes:

• The user can select multiple reaches from the Map View before running the Renumber Interconnected Reaches command by holding the [Ctrl] key while selecting the desired reaches.
• Only those reaches that contain irregular cross sections are listed.
• Multiple reaches that reference the same irregular cross section geometry are not listed.

Manual Editing

This section contains a data grid listing all IDs of the selected reaches and junctions. The data grid also lists the reach distance and total reach distance in miles and feet. The values in the last two columns of the data grid provide a preview of the reaches and junctions naming specifications defined in the Reach Renumbering and Junction Renumbering panels.

In addition, the data contained in a data grid can be copied to the clipboard or exported as a Microsoft Excel or PDF document by using the Copy Table to Clipboard, Export Table to Excel, or Export Table to PDF commands from the right-click context menu.

Reach Renumbering

This panel allows the user to number the reaches based upon river chainage or incrementally.

The following options are provided in the data panel:

• Reach ID prefix: This option allows a prefix to be added to the start of the reach ID.
• Reach ID suffix: This option allows a suffix to be added to the end of the reach ID.
• Reach ID preview: This entry provides a preview of the reach naming specifications defined above.
• Numbering direction: This dropdown combo box defines the direction in which the reach IDs are increasing. The following options are available:
  1. Increasing Downstream
  2. Increasing Upstream (HEC-RAS style) (default)

Based on the option selected, the content of the Use reach distance subsection may change.

Use Reach Distance

The Use reach distance subsection is used to number the reach by using the river chainage along the defined river path. By default, this radio button option is shown selected on selecting the Reach Renumbering panel.

The following entries are provided in this subsection:

• Downstream reach ID: This entry provides a reach ID that will be used as the downstream most reach station.

Note that if the Increasing Downstream option is selected in the Numbering direction dropdown combo box, then the Downstream reach ID entry will be changed in the Upstream reach ID as shown below. In addition, the numbering methodology is altered to account for the selected direction.

• Upstream reach ID: This entry provides a reach ID that will be used as the upstream most reach station.
• Distance units: This dropdown combo box defines the unit for the reach length. Two available options are:
  1. Feet
  2. Miles
• Decimal precision: This optional checkbox defines the decimal precision that will be used in determining the reach stations. By default, this checkbox is disabled (i.e., grayed out). If enabled, then the user can enter a value ranging from 0 to 15.

Clicking on the [Preview] button will cause the software to fill the reach IDs in the New Reach ID column of the Manual Editing data grid.

Use Auto Increment

The Use auto increment subsection is used to number the reach using a fixed increment. Select the Use auto increment radio button option to enable the content of this subsection.

The following entries are provided in this subsection:

• Reach ID digits: This option permits specification of a set number of digits to use for reach ID. For example, using 3 digits causes the reach ID to be of the format 001, 002, etc.
• Next available reach ID: This entry defines the next element ID number to be used.
• Reach ID increment: This entry defines the increment to use when numbering elements. The default value is 1.

Junction Renumbering

This panel allows the user to number the junctions based upon river chainage or incrementally. By default, the Junction Renumbering checkbox panel is checked. If the panel checkbox is unchecked, then the content of this panel will be disabled (i.e., grayed out) and the junction renumbering cannot be performed.

The following options are provided in the data panel:

• Junction ID prefix: This option allows a prefix to be added to the start of the junction ID.
• Junction ID suffix: This option allows a suffix to be added to the end of the junction ID.
• Junction ID preview: This entry provides a preview of the junction naming specifications defined above.
• Numbering direction: This dropdown combo box defines the direction in which the junction IDs are increasing. The following options are available:
  1. Increasing Downstream
  2. Increasing Upstream (HEC-RAS style) (default)

Based on the option selected, the content of the Use reach distance subsection may change.

Use Reach Distance

This subsection is similar to that of the Use reach distance subsection of the Reach Renumbering panel. Hence, refer to the first subsection of the Reach Renumbering panel to learn more about it.

Use Auto Increment

The Use auto increment subsection is used to number the junction using a fixed increment. Select the Use auto increment radio button option to enable the content of this subsection.

The following entries are provided in this section:

• Junction ID digits: This option permits specification of a set number of digits to use for junction ID. For example, using 3 digits causes the junction ID to be of the format 001, 002, etc.
• Next available junction ID: This entry defines the next element ID number to be used.
• Junction ID increment: This entry defines the increment to use when numbering elements. The default value is 1.

Clicking on the [Preview] button causes the software to fill the junction IDs in the New Junction ID column of the Manual Editing data grid.

When all the required data have been properly defined in the Renumber Interconnected Reaches dialog box, click the [Apply] button. The software will then assign the user-defined changes to the reach stations along a defined flow path. Click the [Close] button to close the dialog box.

Note that when the user clicks the [Preview] button and then navigates to select a different reach or clicks the [Close] button without applying the changes, the Renumber Entities confirmational dialog box will be displayed.

Clicking on the [Apply] button will apply the user-defined changes to the reach station.

Clicking on the [Discard] button will discard the user-defined changes and allows the user to redo the process.

In developing HEC-HMS models, there are essentially two requirements - a runoff-generation component and a routing component. Routing is an essential component of any hydrology modeling project for the derivation of time series of flows into the oceans and studies of climate/land use change on water resources.

While a reach element conceptually represents a segment of stream or river, the actual calculations are performed by a routing method contained within the reach. Several routing methods are defined in GeoHECHMS. Each method implements a hydrologic routing methodology comparable to a hydraulic approach that implements the full unsteady flow equations.

The routing method for a reach can be selected from the Reach Specifications section of the Reach Data dialog box.

Follow the steps given below to select a routing method:

1. From the Input ribbon menu, select the Reach Data command.
   
   Alternatively, the user can either double click on the reach polyline from the Map View or choose the Reach Data command from the Routing Reaches dropdown menu of the Input ribbon menu.
   
2. The Reach Data dialog box will be displayed.
   
3. From the Routing method dropdown combo box, select a routing method from the list of available choices.
   
4. To enter the data for the selected routing method, select the Routing Data option from the Reach Specifications dropdown combo box.
   
5. The Routing Data panel of the Reach Data dialog box will be displayed. Note that the Routing Data panel content changes based upon the routing method selected in the General Specifications section of the Reach Data dialog box.

The following sections describe different routing methods and how to define the data for each method in the Routing Data panel.

Routing Method: None

When None is selected for the Routing method, the Routing Data dropdown combo box entry is disabled (i.e., grayed out).

If the user chooses the None method, the reach will translate flow instantaneously and without attenuation.

Routing Method: Kinematic Wave

The Kinematic Wave routing method approximates the full unsteady flow equations by ignoring inertial and pressure forces. It is assumed that the energy slope is equal to the bed slope. Consequently, this method is best suited to fairly steep streams. It is often used in urban areas where natural channels have been modified to have regular shapes and slopes.

When Kinematic Wave is selected for the Routing method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Manning’s roughness
  This entry defines the Manning’s roughness for the channel. Clicking on the […] lookup button will display an Open Channel Manning’s Roughness lookup table dialog box.
  
• Number subreaches
  This spin control button is used to define the number of subreaches in the reach element. The number of subreaches is used as a hint to the software when it determines the correct distance step to use during routing calculations. Criteria based on the steepness of the inflow hydrograph and other factors are used to automatically determine the correct distance and time steps for solving the kinematic wave equation. This entry's default value is 2 but may be increased by the user.
• Index method
  This dropdown combo box lists the following conditions:
  1. Celerity (Wave Velocity)
  2. Discharge (default)
• Maximum (index) discharge
  This entry represents the maximum expected flow in the reach. This value is used to develop a storage-discharge relationship for the reach using 1.5 times this value and the defined reach geometry.
  
  This entry is enabled when the Index method entry is set to Discharge. Otherwise, this entry is disabled (i.e., grayed out).
• Maximum (index) celerity
  This entry is used to compute the travel time. This entry is enabled when the Index method entry is set to Celerity. Otherwise, this entry is disabled (i.e., grayed out).
• Element shape
  This dropdown combo box lists the following shapes:
  1. Circular Pipe
  2. Deep Rectangle
  3. Rectangle
  4. Trapezoid (default)
  5. Triangle

  The above listed shapes are provided for specifying the cross section shape. The circle shape cannot be used for pressure flow or pipe networks but is suitable for representing a free water surface inside a pipe. The deep shape should only be used for flow conditions where the flow depth is approximately equal to the flow width. Depending on the shape you choose, additional information will have to be entered to describe the size of the cross section shape. This information may include a diameter (Circular Pipe), channel width (Deep Rectangle, Rectangle), trapezoid bottom width (Trapezoid), trapezoid side slope (V:H) (Trapezoid) or side slope (Triangle).

Note that the Trapezoid bottom width and Trapezoid side slope (V:H) options will be replaced by other parameters according to the shape selected from the Element shape dropdown combo box.

Routing Method: Lag Time

The Lag Time routing method only represents the translation of flood waves. It does not include any representation of attenuation or diffusion processes. Consequently, it is best suited to short stream segments with a predictable travel time that does not vary with flow depth.

When Lag Time is selected for the Routing method, the following data panel is shown.

Reach routing lag time is computed based upon the flow velocity in the reach routing element.

Lag time is the amount of time (i.e., travel time) that the inflow hydrograph will be translated as it moves through the reach.

Routing Method: Lag Time and Attenuation

The Lag Time and Attenuation routing method is a hydrologic storage routing method based on a graphical routing technique that is extensively used by the National Weather Service. The method is a special case of the Muskingum method where channel storage is represented by the prism component alone with no wedge storage (i.e., Muskingum X = 0). The lack of wedge storage means that the method should only be used for slowly varying flood waves. Like all hydrologic routing methods, it does not account for complex flow conditions such as backwater effects and/or hydraulic structures.

When Lag Time & Attenuation is selected for the Routing method, the following data panel is shown.

The following input parameters are provided in the data panel.

• Initial conditions
  This dropdown combo box entry sets the amount of stored water in the storage area at the start of the simulation. The dropdown combo box lists the following conditions:
  1. Discharge
  2. Inflow = Outflow (default)

  If you use the first option, you will also have to enter a discharge value. If you use the second option, it will be assumed that the initial outflow is the same as the initial inflow to the reach from upstream elements. This is essentially the same as assuming a steady-state initial condition.
  
  

• Initial discharge
  This entry defines the initial discharge being released from the storage area. This entry is only available when the Initial conditions entry has been set to the Discharge option. Otherwise, this entry is disabled (i.e., grayed out).
• Inflow lag time method
  This dropdown combo box lists the following methods:
  1. Constant Lag (default)
  2. Variable Lag
• Inflow lag Time
  This entry allows the user to define the travel time of the flood wave as it moves downstream. It is only available when the Constant Lag option is selected for the Lag method entry. Otherwise, this entry is disabled (i.e., grayed out).
• Inflow lag time function
  This entry is only available when the Variable Lag option is selected for the Lag method entry. Otherwise, this entry is disabled (i.e., grayed out). Clicking on the […] lookup button will display the Inflow Lag Data dialog box. This allows the user to define an inflow lag curve.
• Outflow attenuation method
  This dropdown combo box lists the following methods:
  1. Constant Attenuation (default)
  2. Variable Attenuation
• Outlflow attenuation duration
  This entry allows the user to define the attenuation of the flood wave. It is only available when the Constant Attenuation option is selected for the Attenuation method entry. Otherwise, this entry is disabled (i.e., grayed out).
• Outlfow attenuation function
  This entry is only available when the Variable Attenuation option is selected for the Attenuation method entry. Otherwise, this entry is disabled (i.e., grayed out). Clicking on the […] lookup button will display the Outflow Attentuation Curve Data dialog box. This allows the user to define an outflow attenuation curve.

Routing Method: Modified Puls

The Modified Puls routing method is also known as storage routing or level pool routing. It uses conservation of mass and a relationship between storage and discharge to route flow through the stream reach. Attenuation is achieved through the storage and delayed release of water in the reach instead of through a rigorous conservation of momentum approach. It can be useful for representing backwater due to flow constrictions in a channel so long as the backwater effects are fully contained within reach.

When Modified Puls is selected for the Routing method, the following data panel is shown.

The following input parameters are provided in the data panel.

• Storage discharge rating curve
  This dropdown combo box allows the user to select an already defined storage discharge rating curve. Clicking on the […] define button will display a Storage Outflow Curve Data dialog box. This allows the user to define a new storage discharge rating curve.
• Number subreaches
  The number of subreaches affect attenuation where one subreach gives the maximum attenuation and increasing the number of subreaches approaches zero attenuation. This parameter is necessary because the travel time through a subreach should be approximately equal to the simulation time step for an idealized channel. An initial estimate of this parameter can be obtained by dividing the actual reach length by the product of the wave celerity and the simulation time step. For natural channels that vary in cross section dimension, slope, and storage, the number of subreaches can be treated as a calibration parameter. The number of subreaches may be used to introduce numerical attenuation, which can better represent the movement of floodwaves through the natural system. Here, the default value is 1 but may be optionally increased.
• Initial conditions
  This dropdown combo box lists the following conditions:
  1. Discharge
  2. Inflow = Outflow (Default)

  If the first option is used, a discharge value is to be entered. The initial storage in the reach will be calculated from the specified discharge and the storage-discharge function. If the second option is used, it will be assumed that the initial outflow is the same as the initial inflow to the reach from upstream elements. This is essentially the same as assuming a steady-state initial condition. The initial storage will be computed from the first inflow to the reach and storage-discharge function.
  
  

• Initial discharge
  This entry defines the initial discharge value. This entry is only available when the Discharge option is selected for the Initial conditions entry. Otherwise, this entry is disabled (i.e., grayed out).
• Depth discharge rating curve (optional)
  This dropdown combo box allows the user to select an already defined depth discharge rating curve. Clicking on the […] define button will display the Depth Discharge Rating Curve Data dialog box. This allows the user to define a new depth discharge rating curve.
  
  

  Note that when this option is used, it is necessary that the user defines a downstream invert elevation in the General Specifications panel. The downstream invert elevation is added to the flow depth to compute the water surface elevation.

Routing Method: Muskingum

The Muskingum routing method uses a simple conservation of mass approach to route flow through the stream reach. However, it does not assume that the water surface is level. By assuming a linear, but non-level, water surface it is possible to account for increased storage during the rising side of a flood wave and decreased storage during the falling side. By adding a travel time for the reach and a weighting between the influence of inflow and outflow, it is possible to approximate attenuation.

When Muskingum is selected for the Routing method, the following data panel is shown.

The following input parameters are provided in the data panel.

• Reach travel time
  This entry defines the travel time of a reach in minutes.
• Attenuation coefficient
  This entry defines the weighting between inflow and outflow influence; it ranges from 0.0 up to 0.5. In practical application, a value of 0.0 results in maximum attenuation and 0.5 results in no attenuation. Most stream reaches require an intermediate value found through calibration.
• Number subreaches
  This spin control button defines the numbers of subreaches in the reach element. The number of subreaches affect attenuation where one subreach gives more attenuation and increasing the number of subreaches decreases the attenuation. The number of subreaches may be used to introduce numerical attenuation, which can be used to better represent the movement of floodwaves through the natural system. Here, the default value is 1 but may be optionally increased.

Routing Method: Muskingum-Cunge Routing

The Muskingum-Cunge routing method is based on the combination of the conservation of mass and the diffusion representation of the conservation of momentum. It is sometimes referred to as a variable coefficient method because the routing parameters are recalculated every time step based on channel properties and the flow depth. It represents the attenuation of flood waves and can be used in reaches with a small slope.

When Muskingum Cunge is selected for the Routing method, the following data panel is shown.

The following input parameters are provided in the data panel.

• Time step method
  This dropdown combo box lists the following conditions:
  1. Auto DX Auto DT (default)
  2. Specified DX Auto DT
  3. Specified DX Specified DT

  When the Auto DX Auto DT method is selected, the program will automatically select space and time intervals that maintain numeric stability. Alternatively, when the Specified DX Auto DT method is selected, the program will use the specified number of subreaches (i.e., DX) while automatically varying the time interval to take as long a time interval as possible while also maintaining numeric stability. When the Specified DX Specified DT method is selected, the program will use the specified number of subreaches and subintervals throughout the entire simulation.
  
  

• Number subreaches
  This spin control button defines the numbers of subreaches in the reach element. This entry is enabled when the Time step method entry is set to Specified DX Auto DT or Specified DX Specified DT. Otherwise, this entry is disabled (i.e., grayed out). Here, the default value is 1 but may be optionally increased.
• Number subintervals
  This entry is enabled when the Time step method entry is set to Specified DX Specified DT. Otherwise, this entry is disabled (i.e., grayed out). The default value is 1 Hour but may be changed from the options listed under this dropdown combo box.
• Index method
  This dropdown combo box lists the following conditions:
  1. Celerity (Wave Velocity)
  2. Discharge (default)

  The index method is used in conjunction with the physical properties of the channel and the previously mentioned time step method selection. The program's selected index method and specified parameters will be used to discretize the routing reach in both space and time.
  
  

• Maximum (index) discharge
  This entry represents the maximum expected flow in the reach. This value is used to develop a storage-discharge relationship for the reach using 1.5 times this value and the defined reach geometry.
  
  This entry is enabled when the Index method entry is set to Discharge. Otherwise, this entry is disabled (i.e., grayed out).
• Maximum (index) celerity
  This entry is enabled when the Index Method entry is set to Celerity. Otherwise, this entry is disabled (i.e., grayed out).
• Manning's roughness
  This entry defines the Manning’s roughness for the channel. Clicking on the […] lookup button will display an Open Channel Manning’s Roughness lookup table dialog box.
• Element shape
  This dropdown combo box lists the following shapes:
  1. 8-Point Cross Section
  2. Circular Pipe
  3. Rectangle
  4. Tabular Cross Section
  5. Trapezoid (default)
  6. Triangle

Note that the Trapezoid bottom width and Trapezoid side slope (V:H) options will be replaced according to the shape selected from the Element shape dropdown combo box.

Based on the selected shape, the interface of the Reach Data dialog box will change. When a user selects 8-Point Cross Section or Tabular Cross Section as an element shape, the Cross Section Geometry section will get enabled. This section is located below the Muskingum Cunge Specifications section.

The 8-point shape requires a cross section simplified with only eight station-elevation values. The cross section is usually configured to represent the main channel plus left and right overbank areas. A separate Manning’s n value is entered for each overbank. The cross section should extend from the channel invert up to the maximum water surface elevation that will be encountered during a simulation.

If the tabular shape is used, you will also have to select multiple curves that describe how discharge, area, and top width changes with elevation. These curves must be defined as elevation-discharge, elevation-area, and elevation-width functions, respectively, in the paired data manager before they can be used in the reach element. These curves must be monotonically increasing. Within each of the curves mentioned above, the x-axis defines the elevation, while the y-axis defines the variable of interest. Elevations must be monotonically increasing.

Refer to this article in our knowledge base to know more about 8-Point Cross Section and Tabular Cross Section.

Routing Method: Normal Depth Routing

The Normal Depth routing method uses a Modified Puls routing approach where storage-discharge relationships are developed using a normal depth assumption for the reach. The user enters geometric data for the channel. HEC-HMS computes the storage-discharge relationship for the given channel using Manning's equation for normal depth. HEC-HMS computes the number of Modified Puls subreaches by dividing the travel time by the simulation time interval.

When Normal Depth is selected for the Routing method, the following data panel is shown.

The following input parameters are provided in the data panel.

• Manning’s roughness
  This entry defines the Manning’s roughness for the channel. Clicking on the […] lookup button will display an Open Channel Manning’s Roughness lookup table dialog box.
  
  

  Note that previously defined Manning’s n values from the previously defined cross section should be carried over to the new cross section - reach as they are created.

• Maximum (index) discharge
  This entry represents the maximum expected flow in the reach. This value is used to develop a storage-discharge relationship for the reach using 1.5 times this value and the defined reach geometry.
• Element shape
  This dropdown combo box lists the following shapes:
  1. 8-Point Cross Section
  2. Circular Pipe
  3. Rectangle
  4. Trapezoid (default)
  5. Triangle
• Trapezoid bottom width
  This entry is used to define the bottom width of the trapezoidal channel.
• Trapezoid side slope (V:H)
  This entry is used to define the side slope of the trapezoidal channel. The side slope is dimensionless and entered as the units of horizontal distance per one unit of vertical distance.

Note that the Trapezoid bottom width and Trapezoid side slope (V:H) options will change according to the shape selected from the Element shape dropdown combo box.

Routing Method: Straddle Stagger Routing

The Straddle Stagger method uses empirical representations of translation and attenuation processes to route water through a reach. Inflow is delayed a specified amount of time. The delayed flows are averaged over a specified amount of time to produce the final outflow.

When Straddle Stagger is selected for the Routing method, the following data panel is shown.

The following input parameters are provided in the data panel.

• Lag time
  This entry specifies travel time through the reach. Inflow to the reach is delayed in time by an amount equal to the specified lag.
• Attenuation duration
  This entry specifies the amount of spreading in a flood peak as it travels through the reach. The delayed inflows are averaged over this specified time duration. The duration parameter loses physical meaning when it is greater than twice the lag time.

Pros and Cons of HEC-HMS Routing Methods

Kinematic Wave Routing

Lag Time Routing

Lag Time & Attenuation Routing

Modified Puls Routing

Muskingum Routing

Muskingum-Cunge Routing

Normal Depth Routing

Straddle Stagger Routing

While a reach element conceptually represents a segment of stream or river, optional modeling of interactions with the subsurface is performed by a loss/gain method contained within the reach. A loss/gain method represents losses from the channel, additions to the channel from groundwater, or bi-directional water movements, depending on the specific implementation of a method. The following loss/gain methods are provided in GeoHECHMS:

• Constant Loss/Gain
• Percolation Loss

Each method included in the software provides a different level of detail and not all methods are equally adept at representing a particular stream. Further, some loss/gain methods are only compatible with certain routing methods because of differing data requirements.

The user can select the loss/gain method for a reach using the Reach Data dialog box.

Follow the steps given below to select a loss/gain method:

1. From the Input ribbon menu, select the Reach Data command.
   
   Alternatively, the user can either double click on the reach polyline from the Map View or choose the Reach Data command from the Routing Reaches dropdown menu of the Input ribbon menu.
   
2. The Reach Data dialog box will be displayed.
   
3. Click on the Loss/gain method dropdown combo box to select a loss/gain method from the list of available choices.
   
4. To enter the data for the selected loss/gain method, select the Loss/Gain Data option from the Reach Specifications dropdown combo box.
   
5. The Loss/Gain Data panel will be displayed. Note that the Loss/Gain Data panel content changes based upon the loss/gain method selected in the General Specifications section.
   

The following sections describe different loss/gain methods and how to enter the data for each method in the Loss/Gain Data panel.

Loss/Gain Method: None

When None is selected for the Loss/gain method, the Loss/Gain Data dropdown combo box entry is disabled (i.e., grayed out).

If the user chooses the None method, the reach will perform routing calculations without including any losses or gains to the channel.

Loss/Gain Method: Constant Loss/Gain

The constant loss/gain method uses an empirical relationship to calculate channel loss using a fixed flow rate reduction and a ratio of the flow. It does not include any capability to represent gaining streams. A fixed flow rate is subtracted from the routed flow, and then the remainder is multiplied by a ratio. The reduced flow becomes the outflow for the reach.

Note that this method is compatible with all routing methods.

When Constant Loss/Gain is selected for the Loss/gain method, the following data panel will be displayed.

The following input parameters are provided in the data panel:

• Infiltration rate
  This entry field specifies the amount of flow to be subtracted from the inflow. It may be zero or greater than zero. If the specified value is zero, then no flow rate reduction will occur. If the specified value is greater than the inflow for a time step, then no outflow will result for that interval.
• Infiltration reduction fraction
  This spin control is used to reduce the inflow in linear proportion to the flow rate. After the constant value is subtracted from the routed inflow, the remainder is multiplied by the value one minus the fraction. The fraction must be between zero and one.

Loss/Gain Method: Percolation Loss

The percolation method uses a constant infiltration rate in combination with the inundated area in the reach to compute channel loss. It does not include any capability to represent gaining streams.

Note that this method is only compatible with the Modified Puls and Muskingum Cunge routing methods.

When used with the Modified Puls method, the optional elevation-discharge function and invert elevation must be specified in the routing parameters. The inundated area is calculated differently depending on the routing method. In combination with the Modified Puls method, the current storage and outflow are combined with the elevation-discharge curve to compute the inundated area. With the Muskingum-Cunge method, the wetted perimeter is multiplied by the reach length to compute area.

To learn more about different routing methods, refer to this article in our knowledge base.

When Percolation Loss is selected for the Loss/gain method, the following data panel will be displayed.

The following input parameter is provided in the data panel:

• Infiltration rate
  This entry field specifies percolation in terms of a flow rate per area. The inundated area is multiplied by the rate to determine the channel loss for each time interval.

The Connect Downstream command allows the user to automatically assign subbasin connections to the next downstream junction. It also allows the user to manually assign subbasin connections to downstream elements such as diversions, junctions, storage areas, reaches, manholes, pipes, etc.

Follow the steps given below to use the Connect Downstream command:

1. From the Input ribbon menu, click the Drainage Subbasins dropdown menu, and then select the Connect Downstream command.
   
2. The Connect Downstream dialog box will be displayed.
   

The following sections describe the Connect Downstream command and how to interact with the above dialog box.

Selecting Subbasins

The Select Subbasins section includes a table that lists all the subbasins contained within the current scenario and their associated downstream elements.

If a subbasin is already selected on the Map View prior to running this command, the same subbasin will be shown selected within the table.

Alternatively, click the [Pick] button adjacent to Manual selection to interactively select subbasins from the Map View. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select subbasins from the Map View. Then the user will be returned to the dialog box and the total number of selected subbasins will be displayed in the Total selected entry.

The Connected To column lists the downstream connection for each subbasin. Alternatively, the user can manually select the downstream connection element using the [Pick] button adjacent to the subbasin entry. Clicking on the [Pick] button will cause the dialog box to temporarily disappear, allowing the user to select the downstream element from the Map View.

The following elements can be selected:

• Diversions
• Junctions
• Sinks
• Storage Areas
• Reaches
• Manholes
• Pipes
• and more...

Once the element is selected, the connection line will be drawn and the user will then be returned to the dialog box. The corresponding row contained in the data grid will be updated with the corresponding downstream element.

Selecting Terrain Surface

The Terrain surface dropdown combo box lists all the elevation grids (i.e., DEMs) loaded in the project. Select the elevation grid to be used in computing the downstream junction for each subbasin.

Computing Downstream Connection

On clicking the [Compute] button, the software will create a hidden buffer for each selected subbasin boundary and determine all the junctions contained within the buffered area.
Based on the number of available junctions, the software will perform one of the following actions:

• Case 1: One Junction Available
  If the software finds one junction contained within the selected subbasin, it will assign that junction as the downstream connection for the subbasin.
• Case 2: More than One Junction Available
  If there is more than one junction, the software will analyze each junction location and compare it to the selected terrain surface. The software will then determine which junction has a lower elevation. The junction with the lowest elevation will then be assigned to the subbasin as the downstream connection point.
• Case 3: No Junctions Available
  If there is no junction within the buffer, then no downstream connection will be made.

Note that if the user has manually connected a downstream element for a subbasin, then the software will remember the connection at the project level.

Junctions define the elements (i.e., nodes, manholes, confluences, etc.) where rivers, streams, and stormwater pipes combine flow as it is being routed through the stormwater network model. Junction elements can have one or more inflows, but only one outflow. In addition, subbasins can be connected to junctions.

In GeoHECHMS, the Junction Data command allows the user to add new junctions and edit junction data in a project.

1. From the Input ribbon menu, select the Junction Data command.
   Alternatively, the user can either double-click on the junction node from the Map View or choose the Junction Data command from the Routing Junctions dropdown menu of the Input ribbon menu.
   
2. The Junction Data dialog box will be displayed.
   

The following sections describe how to use the Junction Data command and interact with the above dialog box.

Selecting Junction

The Select Junction section allows the user to select the junction for defining the junction data. The user can create a new junction, copy existing junction data to a new junction, and delete a junction. In addition, the user can navigate between junctions and enter a description detailing the defined junction.

The following entries are provided in this section:

• Junction ID
  This dropdown combo box lists all the junctions that are defined in the model. The user can select the desired junction from the dropdown combo box. Click the pencil icon to edit the junction ID. The user can navigate between the previous and next junctions using the up and down arrow buttons. Alternatively, the user can click the […] button to select the junction from the Map View. Note that the up and down arrow buttons will be disabled (i.e., grayed out) when the model contains only a single junction.
  
• Description
  This text field allows the user to enter additional information that describes the selected junction.
  
• New
  The [New] button allows the user to create a new junction. On clicking this button, the dialog box will temporarily disappear. A prompt will be displayed on the status bar informing the user to place a junction on the Map View. Click on the Map View to place the junction. Following the placement of a junction, the dialog box will be redisplayed. Next, enter the junction name in the Junction ID entry field and click the [Accept changes] button.
  
  The software checks that the defined ID is unique. If not, a warning dialog box is displayed as shown below. Click the [OK] button and the user is then returned to the Junction ID field to change the ID.
  
• Copy
  This button causes the software to copy existing junction data to a new junction. On clicking the [Copy] button, the software automatically provides a unique default name for the duplicated junction. The user can use the default name or enter a different valid and unique ID before moving on to add any other data.
  
• Delete
  This button allows the user to delete the current junction data from the model. On clicking the [Delete] button, the following confirmational dialog box is displayed.

Click the [Yes] button to delete the selected junction.

Click the [No] button to abort the deletion process.

Junction Specifications

This section allows the user to define the data to be used for the current junction.

The following options are provided in this section:

• Invert elevation
  This entry field defines the invert elevation of the selected junction. The other basin elements such as reach that can connect to the junction do require an invert elevation based upon the specific parameters.
  
• Downstream connection
  This entry field defines the downstream connection that the selected junction drains to. Clicking on the […] button allows the user to select the downstream elements from the Map View. The following elements can be selected:
  1. Diversion
  2. Junction
  3. Reach
  4. Sink
  5. Storage Area
  The user can click the [Clear] button to cancel the previous selection and redo the entire process.

Computational Results

This section allows the user to see the analysis results for the current junction that were computed by HEC-HMS.

Note that the Detailed Results section provides additional analysis results for the current junction that were computed by HEC-HMS.

DSS Data

This section displays information related to the output DSS (Digital Storage System) file generated for the HEC-HMS computational results.

• DSS file name: This entry defines the external DSS file to be used for reading the data.
• Data path: This entry denotes the data path within the DSS data file which contains the paired data.

The Renumber Interconnected Junctions command allows the user to automatically renumber routing junctions and connected routing reaches along a defined flow path.

Follow the steps below to use the Renumber Interconnected Junctions command:

1. From the Input ribbon menu, click the Routing Junctions dropdown menu and then select the Renumber Interconnected Junctions.
   
2. The Renumber Interconnected Junctions dialog box will be displayed.
   

The following sections describe the Renumber Interconnected Junctions command and how to interact with the above dialog box.

Selecting Reaches

The Select Reaches panel allows the user to select reaches that define the flow path.

Selecting Multiple (Interconnected) Reaches

The Select Multiple (Interconnected) Reaches section is used to sequentially number reaches and junctions along the selected flow path that has reaches connected end-to-end with each other.

If a reach(s) is already selected on the Map View before running the command, the same reach will be shown selected within the data table. The user can also select/unselect the desired reach by checking/unchecking the checkboxes corresponding to each reach that defines the flow path.

Alternatively, the user can click the [Pick] button to manually select the downstream most and upstream most reach on the Map View, and the software will then automatically determine all the junctions and reaches that connect with each other and form a river path.

Once the downstream and upstream reaches are selected, the dialog box will be redisplayed and the total number of selected reaches will be displayed in the Total selected read-only field.

Notes:

• The user can select multiple reaches from the Map View before running the Renumber Interconnected Junctions command by holding the [Ctrl] key while selecting the desired reaches.
• Only those reaches that contain irregular cross sections are listed.
• Multiple reaches that reference the same irregular cross section geometry are not listed.

Manual Editing

This section contains a data table that lists the IDs of the selected reaches and junctions. The data table also lists the reach distance and total reach distance in miles and feet. The user can enter new reach and junction IDs in the table columns. Alternatively, the user can use the Reach Renumbering and Junction Renumbering panels to generate new reach and junction IDs as per the defined naming specifications.

Junction Renumbering

This panel allows the user to number the junctions based upon river chainage or incrementally.

Junction Stationing

This section contains the following junction stationing options:

• Junction ID prefix: This option allows a prefix to be added to the start of the junction ID.
• Junction ID suffix: This option allows a suffix to be added to the end of the junction ID.
• Junction ID preview: This entry provides a preview of the junction naming specifications defined above.
• Numbering direction: This dropdown combo box defines the direction in which the junction IDs are increasing. The following options are available:
  1. Increasing Downstream
  2. Increasing Upstream (HEC-RAS style)

Use Reach Distance

The Use reach distance subsection is used to number the junctions by using the river chainage along the defined flow path. By default, this radio button option is selected.

The following options are provided in this subsection:

• Downstream junction ID: This entry provides a junction ID that will be used as the downstream most reach station.
  
  Note that if the Increasing Downstream option is selected from the Numbering direction dropdown combo box, then the Downstream junction ID entry will change into Upstream junction ID. In addition, the numbering methodology will be altered to account for the selected direction.
• Distance units: This dropdown combo box defines the unit of reach length. The available options are:
  1. Feet
  2. Miles
• Decimal precision: This optional checkbox entry defines the decimal precision that will be used in determining the reach stations. By default, this checkbox is unchecked. If checked, then the user can enter a value ranging from 0 to 15.

Clicking the [Preview] button will cause the software to automatically fill in the junction IDs in the New Junction ID column of the Manual Editing data table.

Use Auto Increment

The Use auto increment subsection is used to number the junction using a fixed increment. The user can select the Use auto increment radio button option to enable the content of the subsection.

The following options are provided in this subsection:

• Junction ID digits: This option allows the user to set the number of digits to be used for junction ID. For example, using 3 digits causes the junction ID to be of the format 001, 002, etc.
• Next available Junction ID: This entry defines the next element ID number to be used.
• Junction ID increment: This entry defines the increment to use when numbering elements. The default value is 1.

Reach Renumbering

This optional panel allows the user to number reaches based upon river chainage or incrementally. By default, the checkbox on the Reach Renumbering panel is checked. If this panel is unchecked, then the content under this panel will be disabled (i.e., grayed out) and the reach renumbering cannot be performed.

Reach Stationing

This section contains the following reach stationing options:

• Reach ID prefix: This option allows a prefix to be added to the start of the reach ID.
• Reach ID suffix: This option allows a suffix to be added to the end of the reach ID.
• Reach ID preview: This entry provides a preview of the reach naming specifications defined above.
• Numbering direction: This dropdown combo box defines the direction in which the reach IDs are increasing. The following options are available:
  1. Increasing Downstream
  2. Increasing Upstream (HEC-RAS style)

Use Reach Distance

The Use reach distance subsection is used to number the reach by using the river chainage along the defined flow path. By default, this radio button option is selected when the Reach Numbering panel is checked.

The following options are provided in this subsection:

• Downstream reach ID: This entry provides a reach ID that will be used as the downstream most reach station.
  
  Note that if the Increasing Downstream option is selected from the Numbering direction dropdown combo box, then the Downstream reach ID entry will change into Upstream reach ID. In addition, the numbering methodology will be altered to account for the selected direction.
  
  
• Distance units: This dropdown combo box defines the unit of reach length. The available options are:
  • Feet
  • Miles
• Decimal precision: This optional checkbox entry defines the decimal precision that will be used in determining the reach stations. By default, this checkbox is unchecked. If checked, then the user can enter a value ranging from 0 to 15.

Clicking the [Preview] button will cause the software to automatically fill in the reach IDs in the New Reach ID column of the Manual Editing data table.

Use Auto Increment

The Use auto increment subsection is used to number the reach using a fixed increment. The user can select the Use auto increment radio button option to enable the content of this subsection.

The following options are provided in this subsection:

• Reach ID digits: This option permits specification of a set number of digits to use for reach ID. For example, using 3 digits causes the reach ID to be of the format 001, 002, etc.
• Next available Reach ID: This entry defines the next element ID number to be used.
• Reach ID increment: This entry defines the increment to use when numbering elements. The default value is 1.

Once all the required data inputs have been defined, click the [Apply] button and the software will assign the user-defined changes along a defined flow path.

When defining a reach, the user might use multiple cross sections or reach geometries to define each reach. In this situation, several reaches are connected with junctions to represent the extent of the reach in which the defined cross section or reach geometry should be applied. If a junction is not located at the correct location on the river reach, the user can use the Georeference Junctions command to automatically map that junction to a desired location.
The Georeference Junctions command provides three options for mapping junctions to the desired locations, as listed below:

• Snap to Alignment Point
• Slide Along Alignment Polyline
• Draw on Map View

Follow the steps below to use the Georeference Junctions command:

1. From the Input ribbon menu, click on Routing Junctions dropdown menu and select the Georeference Junctions command.
2. The Georeference Junctions dialog box will be displayed.

The following sections describe how to georeference a junction to a desired location.

Selecting Junction to Georeference

The Select Junction to Georeference section allows the user to select the junction that is to be georeferenced. The user can select the junction from the Junction ID dropdown combo box listing all the junctions contained within the model. Alternatively, the user can click the [Pick] button to select the junction from the Map View.

After clicking the [Pick] button, the Georeference Junctions dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can then interactively select a junction from the Map View.

Note that the user can select only one junction at a time to georeference.

Upon selecting a junction, the user is immediately returned to the dialog box, and the selected junction is highlighted on the Map View.

Note that the Junction ID dropdown combo box updates automatically to match the selected junction.

The user can click the [Clear] button to cancel all the previous selections and redo the entire process.

Snapping Junction to an Alignment Point

If an existing alignment point for the junction exists on the Map View, the Snap to Alignment Point option can be used to snap the HEC-HMS junction to the alignment point so that the HEC-HMS junction matches that of the alignment point.

The user can click the [Pick] button to select the alignment point from the Map View. On clicking the [Pick] button, the Georeference Junctions dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can then interactively select the alignment point from the Map View.

Upon selecting the point, the user is immediately returned to the dialog box, and the status of Select alignment point read-only field will be changed from Not Selected to Selected. The user can select only one alignment point to use for georeferencing.

Once the junction alignment point is selected, the user can click the [Snap] button. On clicking the [Snap] button, the software will snap the selected junction to the selected alignment point.

Sliding Junction Along an Alignment Polyline

If a HEC-HMS junction has been snapped to an alignment point (on the defined reach) on the Map View but is not precisely located where it should be, the Slide Along Alignment Polyline option can be used to slide the junction along the defined river reach. Select the Slide Along Alignment Polyline radio button option to enable this section.

The user can click the [Pick] button to select the alignment polyline from the Map View. On clicking the [Pick] button, the Georeference Junctions dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the alignment polyline from the Map View.

Once finished, press the [Enter] key or right-click and select Done from the displayed context menu. The dialog box will be redisplayed, and the status of the Select alignment polyline element read-only field will be changed from Not Selected to Selected. The user can select only one alignment polyline to use for georeferencing.

Next, click on the [Slide] button. The Georeference Junctions dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next.

The user can then “slide” the junction along the defined river reach on the Map View. However, the command doesn’t allow sliding when an adjacent cross section is encountered.

Once finished, the user can press the [Enter] key or right-click and select Done from the displayed context menu. The dialog box will be redisplayed, and the junction will be placed at the new location.

The user can also check the Recompute adjacent node element elevations checkbox option to recompute the elevation of the adjacent node type elements while georeferencing the junction. By default, this checkbox is checked.

Note that the user cannot use the “slide” functionality to change the location of the termination (upstream most or downstream most) junctions on multiple reaches.

Drawing Junction Node on Map View

The Draw on Map View radio button option allows the user to draw a junction node on the Map View to which the selected junction is to be georeferenced.

On selecting the Draw on Map View radio button option, the [Draw] button becomes enabled (no longer grayed out). The user can click the [Draw] button to interactively draw the junction node on the Map View.

On clicking the [Draw] button, the Georeference Junctions dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user on what to do next. The user can then draw the junction node on the Map View. Once finished, the user can press the [Enter] key or right-click and choose Done from the displayed context menu. The dialog box will be redisplayed, and the selected junction will be georeferenced to the drawn node.

Assigning Junction Elevation

The Assign Junction Elevation section uses elevation data to assign an invert elevation to the georeferenced junction. The user should define the data for this section before georeferencing junctions. Alternatively, the user can use the Assign Junction Elevations command to assign invert elevations to the already drawn junctions. Refer to this article in our knowledge base to learn more about this command.
For assigning elevation data, this section only supports the following surface types:

• Elevation grids
• LandXML data
• TIN surfaces

It does not support the following surface types:

• CAD drawings
• GIS contours

The user can also apply an elevation offset to raise or lower the selected junction by the specified amount by checking the Apply elevation offset checkbox. On selecting this checkbox, the entry field next to it becomes available for entering an elevation offset value. For example, the user can define a negative offset value to lower the junction for specific circumstances, such as in the case of representing a manhole bottom elevation.