HEC-RAS 6.7 has added the ability to simulate 2D bridges using a 2-layer 2D horizontal approach referred to simply as the Pressure/Overtopping method. In this method the computational mesh beneath the bridge is clones or copied and placed on top of the bridge deck. This allows for separate 2D flow calculations below and above the bridge. The flow through the bridge opening may be pressurized and the flow over the bridge deck may be plunging off the downstream edge of the deck. For more details on the computational methods utilized see the HEC-RAS 2D Technical Reference Manual - 2D Bridge with Pressure/Overtopping. The figure below shows a simple schematic with a side view of 2D bridge where the bridge is pressurized and overtopping. 


The steps below outline how to add a 2D bridge with pressure/overtopping. The example starts with the following initial mesh and terrain that does not contain the bridge:

  1. In RAS Mapper, add the bridge piers and abutments to the terrain. Unlike the 2D bridge with the 1D bridge curves, the pressure/overtopping method requires adding the piers and abutments to the terrain. 
  2. Open the Geometric Data editor, and draw a centerline for the bridge deck/opening using the SA/2D Area Conn drawing tool, or by using the editing tools in RAS Mapper. The bridge centerline should be drawn from left to right looking downstream. If it is not, the model will still run but the sign of flows and head-water and tail-water will be incorrect for output. The centerline should only cover the bridge opening and not the embankment or bridge roadway.
  3. Adjust the computational mesh near the bridge using breaklines and refinement regions.  Some hand editing may be required depending on the bridge and what else is near the bridge (i.e., levee, another bridge, railroad tracks, road, etc…).  The user may need to modify existing breakline(s) and/or refinement regions near the bridge. The figure below shows a mesh near a bridge before and after adding the bridge. Utilize a mesh resolution such that the edges of the bridge align with the computational faces. HEC-RAS does not enforce faces along the edges of the bridge but this is highly recommended and up to the user. Remember to only include the bridge desk in the SA/2D connection. The bridge abutment and roadway should be captured with the terrain. 
  4. From the Geometric Data editor, select the bridge SA/2D Connection and click on Edit Connection...
  5. From the SA/2D Connection Data editor, click on the Deck button to add the bridge deck low and high chord data. 


    The bridge distance is only used for outputting the bridge average head-water and tail-water elevations. The distance determines the distance upstream and downstream at which the head-water and tail-water elevations are computed. 
    Notice that the weir parameters are not utilized for the 2D bridge pressure/overtopping method. Instead, the method utilizes a plunging flow calculation at downstream edge of the bridge. See the HEC-RAS 2D Technical Reference Manual - 2D Bridge with Pressure/Overtopping for more information. 
  6. Click on the 2D Bridge Param button in the SA/2D Connection editor to open the Connection - 2D Bridge Parameters editor (see figure below). Enter the bridge deck minor loss parameters and deck Manning's roughness coefficient by . For more information on how the minor losses are computed at 2D bridges with pressure overtopping see the HEC-RAS 2D Technical Reference Manual - 2D Bridge with Pressure/Overtopping. 
  7. If modeling bridge scour, then bridge piers and abutment slopes should be entered. Bridge piers and abutment slopes are entered in the SA/2D Connection editor are only utilized for reference in the bridge scour calculations. They are not used by the hydraulic calculations. Bridge piers should be entered as terrain modifications (see for Using 2D HEC-RAS Results for Bridge Scour Analysis more details). 


Viewing 2D Bridge Results

Result Maps

When simulating 2D bridges with the pressure/overtopping method, there is a separate cloned mesh above the bridge deck. However, this mesh is created at runtime in the computational engine, and so the user-interface does not know about this mesh. This means that variables on the cloned mesh need to be output on the lower mesh (i.e. regular mesh in RAS Mapper) and the prefix Above Bridge is added to them (see figure below).

It's noted that the mapping output for Above Bridge variables only corresponds to the bridge deck on the cloned mesh above the bridge and everywhere else, the mapping output corresponds to the lower mesh. This is done in order to have a smooth and continuous map of output variables. For example, the figure below shows the  Above Bridge Velocity. Away from the cloned mesh above the bridge, the velocities are the same as the those in the regular Velocity layer. 


Time Series

To view time series of 2D bridge variables open the HEC-RAS Stage and Flow Hydrograph plotter by either clicking on the  button in the main HEC-RAS window or by right-clicking on a main result node in RAS Mapper and selecting Plot Results Time Series. To view results for 2D bridges click on the menu Node Type | SA/2D Connections. The left-side of the editor will show a list of the 2D bridges. The figure below shows an example of time series output for a 2D bridge with Pressure/Overtopping. 

Various output variables can be selected by clicking on the menu Options | Variables ... The output time series variables include the head-water and tail-water water surface elevations shown as Stage HW and Stage TW, respectively. The variable below and above the deck are averaged along the bridge centerline.  

Limitations

There are several limitations of the 2D Bridge Pressure/Overtopping method. Some of the limitations are due to the fact that it is a new in HEC-RAS for 6.7 while other limitations are due to the physics and numerical methods. Even thought the method is more physics-based than using the 1D bridge curves, the method is still simplified in that it represents the flow as a 2-layer depth-averaged flow. The low and high chords of the bridge deck are assumed to be constant in elevation. The approach also does not support 2D sediment, secondary flow, and debris flow. Another major limitation is that the user cannot interact or view the cloned mesh on top of the bridge deck. The cloned mesh is created at runtime and mapping output is separated into above and below bridge variables but is till all output on the lower mesh.