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Understanding HEC-HMS Scenarios

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 box lists the existing terrain surfaces in the project and the corresponding map layers related to the selected terrain surface.

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:

    • None (default)
    • Flow
    • 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

Selecting the Delete Scenarios command will display the below dialog box.

The above dialog box displays all scenario plans, basin models, meteorological models, and control specifications associated with the HEC‑HMS project.

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.

The Delete Scenarios command 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.

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.

Scenario Summary

Selecting the Scenario Summary command will display the below dialog box.

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

The Scenario Summary dialog box 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.

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.

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

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

Multiple Plan Analysis

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.

To perform computations on multiple plans sequentially (one immediately after the other), expand the Compute Analysis dropdown combo box in the Analysis ribbon menu and select the Multiple Scenarios command.

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.

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, 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.

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 output results related to the selected element(s) will be displayed under the Plots, Data Tables, and Data Summary panels.

About the Author Chris Maeder

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