HEC-RAS 2D Bridge Modeling

When working with bridges with complex geometry, modeling the hydraulics in 2D provides a clearer understanding and a more accurate representation of the flow behavior at the bridge. Water may flow through multiple paths and directions at the bridge opening. A HEC-RAS 1D model will assume that the water surface elevation is constant across the bridge opening, whereas a HEC-RAS 2D model will show that the water surface elevation varies across the bridge opening as the flow makes its way through the opening. In addition, a 1D model assumes an average velocity at the bridge opening, while a 2D model will accurately represent the velocity field at the bridge. This is particularly important when computing potential bridge pier scour. And, there are other drawbacks to using a HEC-RAS 1D model for bridge analysis:

  • Ineffective flow areas to mark areas of non-conveyance
  • Energy losses from channel bends
  • Energy losses due to eddies at skewed bridges

The above concerns are automatically handled when using a 2D model. There are many bridge modeling situations that are effectively managed with a 2D model, but that would be problematic with a 1D model. For example, modeling of skewed bridges can cause additional energy losses and increased water surface elevations, conditions that would not be part of the analysis using a 1D model.

Skewed bridge opening

Skewed bridge opening

HEC-RAS 2D Bridge Piers

GeoHECRAS provides additional functionality to speed up 2D modeling of bridges, including stamping of bridge piers directly into the 2D flow area mesh. The following bridge pier shapes are provided:

  • Circular piers
  • Rectangular round nosed piers
  • Rectangular sharp nosed piers
  • Rectangular square nosed piers
  • Square piers

Once the bridge pier has been defined, the user can interactively move and rotate the piers to place them at the correct locations in the bridge opening. Then, the software will update the 2D mesh to incorporate the bridge piers into the mesh geometry.

Placing Bridge Piers into a HEC-RAS 2D Model

Follow these steps to stamp bridge piers to a 2D mesh:

  1. From the Input ribbon menu, select the 2D Flow Area Data menu item.
    2D Flow Area Data command
  2. The 2D Flow Area Data dialog box will be displayed.
    2D Flow Area Data dialog box
  3. From the dropdown combo box panel selector, select Bridge Piers.
    Select Bridge Piers from 2D Flow Area Specifications dropdown combo box
  4. The Bridge Piers panel will be displayed.
    Bridge Piers pannel
  5. From the Pier type dropdown combo box entry, select the bridge pier type to be placed into the 2D mesh.
    Select the bridge pier type
  6. Define the pier dimensions (diameter or width and length) that are to be used—based upon the bridge pier type selected.
  7. Click the [Place] button.
    Place a bridge pier
  8. The software will place the defined pier in the center of the 2D Flow Area Preview window.
    Bridge Pier on the 2D Flow Area Preview window
  9. Click on the bridge pier structure and then drag the pier to the correct location. The rotation handle can be used to rotate the bridge pier around its center point.
    Placing bridge piers at right locations

Stamping Bridge Piers into 2D Flow Area Mesh

After the bridge piers have been placed into the 2D flow area mesh, click the [Update] button. The software will then incorporate the bridge piers into the 2D mesh.

Update 2D mesh including piers

The Defined Piers data table lists the bridge piers contained in the 2D flow area. It includes the pier ID (name), pier type, and a checkbox to enable or disable a pier. To temporarily remove a pier from the HEC-RAS 2D computations, uncheck the Enabled checkbox corresponding to the pier.

The Defined Piers data table

About the Author Chris Maeder

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