When to add an HEC-RAS 2D Culvert to your Hydraulic Model

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Lately, I have been working on several dam inundation studies. As part of this work, I have been developing HEC-RAS models to route breach flows through the downstream area. HEC-RAS Version 5.0.5 allows users to culverts to 2D Flow Areas. However, modeling every bridge or culvert in HEC-RAS can be very time-consuming. The good news is that you don’t need to model every road crossing in the inundation area as an HEC-RAS 2D culvert.

Bridges in Hydro-Flattened DEMs

It is common for dam-break analyses to assume that debris would block stormwater structures such as culverts and drains. However, debris probably wouldn’t block a large bridge opening during a dam-break scenario. Therefore, your model should allow flow through large bridge openings. Typically, you should make sure that bridges with design flows of the same order of magnitude as breach outflows (or larger) are removed from the terrain.

Hydro-flattened Digital Elevation Models (DEMs) do not have man-made hydraulic structures such as bridges. Geospatial scientists remove them during data post-processing.

However, some bridges slip through the cracks. In this case, you must add an HEC-RAS 2D culvert to model the hydrology properly.

Adding an HEC-RAS 2D Culvert

The process for adding an HEC-RAS 2D culvert to your model is as follows:

  1. Acquire dimensions from as-built drawings, GIS data, or Google Earth
  1. Outline the hydraulic structure in HEC-RAS using the SA/2D Connection tool
  2. Populate hydraulic structure data as shown below. Click the Culvert button on the left side of the Connection Data Editor to access the Culvert Data Editor.

Note that the US Station and DS Station refer to the center station along the structure’s upstream and downstream cross-section as shown below.

Interpreting the Results

Draw a Profile Line in RAS Mapper to view the modeled peak water surface elevation below the bridge deck.

This demonstrates the importance of ensuring that modeled flows can pass through large bridge openings. This ensures that your model captures the full inundation area. To analyze flow through an HEC-RAS Hydraulic Structure, navigate to the Geometry editor, right-click the SA/2D Connection and click Plot Stage and Flow Hydrograph.

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