There are two general approaches for modeling bridges within 2D areas: (1) Simplified 1D/2D Bridge Modeling, and (2) Detailed Bridge Modeling.

The simplified 1D/2D approach is designed for cases where the details of the bridge hydraulics are not important and only overall energy/head losses are important. Those losses are applied along a single line of cell edges at the centerline of the bridge. The approach is faster and requires less resolution of bathymetry around the bridge, but can be more difficult to calibrate since there are more parameters in the 1D modeling approaches. The simplified 1D/2D approach can be applied for all types of bridge flows including pressurized flow and bridge overtopping, and can be used with all 2D solvers, including the Diffusion Wave Solver.

The detailed modeling approach utilizes multiple rows of 2D cells within the bridge footprint in order to resolve the bridge hydraulics. At low flow conditions, flow through the bridge is treated similarly to flow in any other part of a 2D area. As water levels rise and the bottom of the bridge deck is wetted, the 2D solver simulates the effects of pressurized flow. When water levels begin to overtop the bridge deck, a second set of 2D bridge cells becomes active to calculate water levels and velocities in the flow layer above the bridge deck. The detailed bridge approach is more computationally expensive and requires detailed geometry of the bridge, but does not require as much parameterization as the simplified approach. It should only be applied with the non-linear shallow water equations solvers SWE-ELM and SWE-EM. 

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.