Understanding HEC-RAS Scenarios

HEC‑RAS provides the ability to work with multiple scenarios (or plans) within a single project. Each scenario associates a specific geometry file and flow file with a specific set of conditions, such as pre-developed and post-developed phases of a project.

Elements of a Project

A HEC‑RAS project is comprised of one or more scenarios. Scenarios are formulated by selecting particular geometry data and flow data. HEC‑RAS stores the geometry data and flow data in separate files. Modifications can be made to the geometry data and/or flow data to represent specific conditions, and then saved as new files.

The following diagram shows the interaction between the various files contained within a HEC‑RAS project.

Elements of a HEC‑RAS Scenario

The dropdown combo box adjacent to the Scenario Manager dropdown menu displays the Short ID of the currently selected scenario. The read-only field below the Short ID dropdown combo box displays the Scenario (plan) or Long ID of the current 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, click the dropdown combo box arrow of Scenario Manager. It will display four commands: Scenario Manager, Duplicate Current Scenario, Delete Scenarios, and Scenario Summary.

Scenario Manager

The Scenario Manager dialog box is used to manage the geometry and flow data (i.e., geometry and flow files) associated with the current scenario. In addition, new scenarios, geometry data, and flow data can be created, as well as copied from existing data. Selecting the Scenario Manager command will display the below dialog box.

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

Each of these sections contains individual elements that are used to define the corresponding section data. For example, both the Scenario (plan) and Geometry data sections allow the user to provide a description detailing the data. Each section allows the user to name the data. However, scenario (plan) data has both a Long ID and a Short ID for naming its data.

In the Geometry Data section, the user can insert the associated terrain surface from the dropdown.

The 2D land cover layer checkbox entry allows the user to associate the project scenario with the land use coverage layer. The available 2D land cover layers of the project will be displayed in the 2D land cover layer dropdown combo box. Refer to this article in our knowledge base to learn more about creating 2D land cover layers.

The 2D infiltration layer checkbox entry allows the user to associate the project scenario with the 2D infiltration layer. The available 2D infiltration layers of the project will be displayed in the 2D infiltration layer dropdown combo box. Refer to this article in our knowledge base to learn more about computing infiltration layers.

To define the steady or unsteady flow data, the user can enable either the Steady Flow Data or Unsteady Flow Data radio button for the selected scenario.

Scenario Summary

In the Scenario Manager dialog box, clicking on the [Summary] button will display an informational dialog box that provides a detailed overview of the project: number of scenarios (plans), number of geometry definitions, number of flow definitions, and details of the current plan (such as Element Details, Boundary Conditions, Defined Flows, and Storage Area Elevations).

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.

Duplicate Current Scenario

The Duplicate Current Scenario command makes an identical copy of the current scenario (plan). The user can elect to make a copy of the current geometry and steady flow data. This is helpful when the user needs to create different alternative designs for comparison — when comparing different bridge opening designs, for example.

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 select which plans, geometries, and flow data to remove from the HEC‑RAS project. This is helpful in order to remove data that is no longer relevant to a project or planned for inclusion in a reviewing agency project submittal. In addition, this command compresses the database, removing data no longer in use and reducing the project file size.

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

Multiple Plan Analysis

GeoHECRAS can perform analysis on multiple scenarios at the same time. This is useful, for example, when an existing bridge’s flow regime is compared to a proposed replacement bridge’s analysis. A scenario would consist of selecting the flow data and one of the geometry data. Computations on the multiple plans can then be performed sequentially (one immediately after the other) using the Multiple Scenarios command.

Note that the software provides separate Multiple Scenarios command for steady and unsteady flow models.

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

Displaying Multiple Scenario Results

After the scenario computations have been performed, the software can display the associated results using the following output parameters:

To view the output results, select the Profile Plots command icon from the Results ribbon menu.

Alternatively, select the Profile Plot command from the Profile Plots dropdown menu.

The Profile Plot window will be displayed. From the Profile Plot window, select Plans from the Options menu. The Plan Selection dialog box will be displayed and the user can then select the scenarios (or plans) to display.