Understanding 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 to represent a specific condition, 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 a 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.

Project File Structure

Elements of a HEC‑RAS Scenario

The Input ribbon menu shows the currently selected scenario (Short ID) and a description of the scenario (Long ID).

Scenario Manager

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

HEC-RAS Scenario Manager

From the Input ribbon menu, click the dropdown combo box arrow of Scenario Manager. It will display three commands: Scenario Manager, Duplicate Current Scenario, and Delete Scenarios.

HEC-RAS Scenario Manager

Scenario Manager

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

Scenario Manager dialog box

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

  • Scenario (or plan) data
  • Geometry data
  • Flow data (either steady flow or unsteady flow)

Each of these sections contain individual elements that are used to define the corresponding section data. For example, both scenario (plan) and geometry data 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 Scenario Manager, 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

Scenario Summary dialog box

For each scenario, a detailed summary of the elements that make up the scenario is provided:

  • Number of river reaches
  • Number of cross sections
  • Number of roadway crossings
  • Number of 2D elements
  • etc.

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.

Duplicate Current Scenario

Selecting the Duplicate Current Scenario command will display the below dialog box.

Duplicate Scenario

The Duplicate Current Scenario command makes an identical copy of the current scenario (plan). The user can optionally 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—like when comparing different bridge opening designs.

Delete Scenarios

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

Delete Scenarios dialog box

This dialog box will show all plans, geometry, and flow data associated with the HEC‑RAS project. The user can select which plans, geometries, and flow data to remove from the HEC‑RAS project. This is helpful in removing data that is no longer relevant to a project and is not to be included in a reviewing agency project submittal.

In addition, this command compresses the database, removing data no longer being used, causing the profile file size to shrink.

Note that after the scenario data has been deleted from the HEC‑RAS 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.

Multiple Plan Analysis

GeoHECRAS can perform an analysis on multiple scenarios at the same time. This is useful, for example, when a comparison of an existing bridge and a proposed replacement bridge are to be analyzed. 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 Analysis | Compute Steady | Multiple Scenarios ribbon menu command. (A similar command exists for unsteady flow scenarios.)

Computing Multiple Scenarios

Selecting the Multiple Scenarios ribbon menu command causes the Multiple Scenarios dialog box to be displayed.

Compute Steady - Multiple Scenarios

This dialog box allows the user to select which scenarios to be computed, the flow regime to be used in the computation, and which scenario result file should be loaded by default when displaying the output results. Clicking the [Compute] button will cause HEC‑RAS 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 overlaid on each other for the following output results:

  • Cross Section Plot
  • Profile Plot
  • General Profile Plot
  • Rating Curve
  • Stage & Flow Hydrographs
  • Summary Output

To view the output results, select the Results ribbon menu and choose the output menu command. For example, select Results | Profile Plot ribbon menu command.

Results Menu

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

Profile Plot - Plan Selection

About the Author Chris Maeder

Chris Maeder

Chris is an experienced civil engineering and software technology leader, with over 30 years industry experience. With proven expertise in global software development, he has built engineering teams that adapt quickly, focus on what’s important and, most importantly, deliver. He is a licensed professional civil engineer with extensive experience in water resource engineering. He has performed and supervised engineering projects in urban stormwater drainage, transportation and roadway drainage, storm sewer design, detention pond design, stormwater quality, green infrastructure, watershed management planning, wastewater sewers, water distribution networks, pump stations, FEMA flood studies, bridge and culvert design, bridge scour and armoring, dam failure analysis, seepage and groundwater modeling, and environmental permits.

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