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Welcome to CivilGEO Knowledge Base
Compute Unsteady – Current Scenario Command
HEC-RAS analysis refers to a simulation that analyzes the flow of water in a river or channel over a specific time period. The simulation computes the evaluation of water levels, velocities, and other hydraulic parameters at different locations along the river during unsteady flow conditions, such as floods or varying discharges.
When all geometric and unsteady flow data are entered in the project, the user can perform the hydraulic calculations. In GeoHECRAS, the Compute Unsteady – Current Scenario command allows the user to perform an analysis of the current scenario (plan) for unsteady flow models.
Follow the steps below to use the Compute Unsteady – Current Scenario command:
- From the Analysis ribbon menu, click the Compute Unsteady dropdown menu and select the Current Scenario command. Alternatively, click the upper half part of the Compute Unsteady button.
- The Compute Unsteady – Current Scenario dialog box will be displayed.
The following sections describe how to use the Compute Unsteady – Current Scenario command and interact with the above dialog box.
Scenario Data
This section allows the user to define the scenario data to be used in the unsteady flow simulation.
The following options are provided in this section:
- Scenario (plan)
This dropdown combo box allows the user to select the scenario for which the analysis will be performed. The dropdown combo box lists all the unsteady flow scenarios that are already defined in the project. By default, the current scenario is selected. - Geometry
This read-only field contains the corresponding geometry for the selected scenario. - Unsteady flow
This read-only field contains the corresponding unsteady flow for the selected scenario.
Disregard following Elements during Analysis
This section allows the user to select elements to be disregarded or not considered in the simulation. The user can check/uncheck the checkbox(s) corresponding to each element to select/deselect them.
The following checkbox options are provided in this section:
- Lateral Structures
Selecting this checkbox causes the lateral structure elements, such as levees, flood walls, or lateral weirs to be disregarded from the simulation. - Storage Area Connection
Selecting this checkbox causes the storage areas such as lakes, reservoirs, or ponds connected with river or channel systems to be disregarded, assuming they have minimal impact on flow dynamics and are treated as isolated bodies of water. - Inline and Lateral Structure Breaches
Selecting this checkbox causes the inline and lateral structure breaches to be disregarded, assuming no structural failures or breaches along the river or channel system during the simulation. - Pumping
Selecting this checkbox causes the artificial extraction or addition of water to the river or channel system to be considered independently of the simulation to simplify the model.
Execute Processes
This section contains three components to be used in performing an unsteady flow analysis within HEC-RAS.
The following checkbox options are provided in this section:
- Geometric preprocessor
This checkbox option is used to process the geometric data into a series of hydraulic properties tables, rating curves, and a family of rating curves. This is done in order to speed up the unsteady flow calculations. During each iteration, the software interpolates the hydraulic variables from the tables instead of calculating hydraulic variables for each cross-section. - Unsteady flow simulation
This checkbox option is used to simulate the unsteady flow in the river system. - Output postprocessor
This checkbox option is used to post-process the output from the unsteady flow simulation. It computes detailed hydraulic information for a set of user-specified timelines during the unsteady flow simulation period. In general, unsteady flow computations only compute stage and flow at all of the computation nodes, as well as stage and flow hydrographs at user-specified locations. If this option is unchecked, then the user will only be able to view the stage and flow hydrographs and inundation mapping, no other output from HEC-RAS. By using this option, the user will have all of the available plots and tables for unsteady flow that HEC-RAS normally produces for steady flow. - Close dialog box while computing
This checkbox option allows the user to close the dialog box on clicking the [Compute] button.
Simulation Time Window
This section defines the starting date and time and ending date and time of the HEC-RAS hydrology simulation. The user can click on the [Date] button, which displays a calendar date selector from which the user can select a specific date. Similarly, the user can click on the [Time] button, at which point the software displays a 24-hour time selector from which the user can select a specific time.
Simulation Time Settings
This section contains the computational time step interval, hydrograph output interval, detailed output interval, and mapping output interval for defining the simulation time settings.
The following options are provided in this section:
- Computation time step interval
This option allows the user to define a fixed computational time step. The value for the fixed computational interval should be chosen with care and consideration as to how it will affect the simulation. Refer to this article in our knowledge base to learn how time step values can affect the stability of the model.Clicking on the [Advanced] button will display the Advanced Time Step Control panel of the Unsteady Flow Computational Options dialog box. This panel allows the user to use the Courant number method for the variable computational time step. Refer to this article in our knowledge base to learn how to use the Courant number method. - Hydrograph output interval
This option is used to define at which interval the computed stage and flow hydrographs will be written to HEC-DSS. It should be selected based on adequate number of points needed to define the shape of the computed hydrographs without losing information about the peak or volume of the hydrographs. This interval must be equal to or larger than the selected computation time step interval. - Detailed output interval
This option is used to define the intervals at which profiles of water surface elevation and flow are generated at user-specified intervals during the simulation. Profiles are not written for every computational time step because it would require too much space to store all of the information for most jobs. Also, when the postprocessor is run, the software will compute detailed hydraulic information for each one of the instantaneous profiles that are written. The selected interval must be equal to or greater than the computation time step interval. - Mapping output interval
This option is used to define the interval at which the user will be able to visualize mapping output within HEC-RAS Mapper.
Computational Options
This section allows the user to define the flow regime to be computed by the HEC-RAS unsteady flow analysis engine.
The Flow regime dropdown combo box lists the following options:
- Subcritical flow: This option requires only downstream boundary conditions. By default, this option is selected.
- Mixed flow: This option requires both upstream and downstream boundary conditions.
Computing Unsteady Flow Analysis
When all the options have been properly defined, click the [Compute] button. The dialog box will be closed, and HEC‑RAS immediately starts performing the analysis for the current scenario in a separate window showing the progress of the computations. The information that appears in the window is an indicator of software progress during the computations and a list of any computational messages that come up during the run. When the computations have been completed, the user can click the [Close] button to close the dialog box. If a problem is detected in the model data or analysis, the software will display a dialog box detailing the issue and will stop the process.
Note that a terrain surface must be assigned in the project before running the analysis. Otherwise, the following informational dialog box will be displayed when you click on the [Compute] button.
