In the case of 2D Bridges with the Pressure/Overtopping method, users have the same low flow (energy, momentum, and Yarnell) and high flow (energy and pressure/weir) bridge modeling approaches available for 1D bridge modeling (except the WSPRO low flow method is not available for 2D modeling). The HEC-RAS software takes the user input bridge data and modeling approaches, then develops a family of rating curves for the bridge, just as is does for 1D modeling. However, for 2D modeling, the bridge’s curves are used to obtain a water surface difference through the bridge for each set of cells being used to model the bridge.  This water surface difference is then equated to a force.  That force is distributed and put into a special version of the momentum equation for each set of cells spanning the bridge centerline.  So instead of calculating friction forces, pressure forces, and spatial acceleration forces, these forces are obtained from the bridge curves.  Then the 2D equations are solved as they are normally solved at any cell/face in the model.  This approach used for 2D modeling allows for equivalent forces to be computed for low flow, pressure flow, and combined pressure flow/weir flow, or even low flow/weir flow. 

The amount of force given to each cell is based on the percentage of the total flow passing through that particular set of cells.  This 2D modeling approach allows for varying flow, water surface, and velocity at each of the cells around the centerline of the bridge opening.  Therefore, the flow is still computed as two-dimensional flow through and over top of the bridge.  Flow can pass at any angle through the bridge opening based on the hydraulics of the flow and the number of cells being used to represent the bridge opening. 

To model a 2D bridge with the Pressure/Overtopping method inside of a 2D Flow Area, the SA/2D Area Conn geometry drawing tool is used.  The basic steps for adding this type of bridge are as follows:

  1. Draw a centerline for the bridge opening/embankment using the SA/2D Area Conn drawing tool in the Geometric Data editor, or by using the editing tools in RAS Mapper.  The bridge centerline must be drawn from left to right looking downstream.
  2. Develop an appropriate mesh (cell size and orientation) for the bridge, using the structure mesh controls.  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…)
  3. Enter the bridge data (deck and roadway; distance from upstream bridge deck to outside cross section’s piers; abutments; bridge modeling approach; Manning’s n values for the 1D bridge cross sections; and hydraulic tables controls (HTAB) into the SA/2D Area Conn editor.
  4. Pre-process the geometry to create the bridge curves.  Review the bridge family of rating curves for hydraulic accuracy.

Run the model and review the results.  Make any necessary changes to the data to improve the results.