Users can model bridges directly inside of 2D flow areas.  Bridges inside of 2D flow areas can handle the full range of flow regimes, from low flow to pressure flow, and combined pressure flow and flow going over top of the bridge deck or roadway. Users enter bridge data inside of 2D Flow Areas similarly to the modeling of bridges in a 1D model.  There are two methods in 2D for simulating 2D bridges: (1) 1D Family of Rating Curves, and (2) 2D Pressure/Overtopping. 

To model a bridge inside of a 2D Flow Area, the SA/2D Area Conn geometry drawing tool is used.  The basic steps for adding a bridge to a 2D model are the following:

  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 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.
  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.
  5. Add the bridge piers and abutments to the terrain. This step is not required but highly recommended. If the these features are not added to the terrain, the model will still produce the same head loss through the bridge but the velocities through the bridge will be incorrect. More specifically, it will appear as if the flow is going through the abutments and bridge piers. 

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

Draw the Bridge Centerline

To add a bridge inside of a 2D flow area, open the Geometric Data editor (accessed from the HEC-RAS main window by clicking the Edit | Geometric Data menu command) and select the SA/2D Area Conn drawing tool, from the tool bar across the top of the editor.  Draw the centerline of the bridge from left to right looking downstream.  In the example shown in Figure below, the bridge centerline is only laid out for the bridge opening.

The user has the choice of just modeling the main bridge opening with this structure (including the bridge abutments), or also including the entire road embankment (left and right of the bridge opening).  If the approaches for the entire roadway are included in the structure, then the family of rating curves will include flow over the left and right roadway approaches, as well as flow through and over the bridge.  However, if the user chooses to model just the main bridge opening with the structure, then the family of bridge rating curves will only include flow going through and over the main bridge opening.

The left and right embankments can be modelled as either normal 2D flow cells and faces (solving the general 2D equations), or separate hydraulic structures can be laid out for the left and right roadway approaches.  Using separate hydraulic structures for the roadway approaches allows the user to change the elevation of the roadway, instead of using the raw terrain elevations; furthermore, using separate hydraulic structures allows the weir equation to compute the overflow (if desired) and breaching of the embankments.  Additionally, these separate SA/2D hydraulic connections can be used to breach the roadway embankments, if desired.

Figure 3-49. Example Bridge Centerline for Bridge Opening Only.

The Bridge Centerline is used to extract information from the faces under the bridge deck.  In order for the face to be selected as part of the bridge geometry, the center of the bridge face must be within the bounds of the centerline.

Cell faces are used to extract the ground profile under the terrain.  Cell faces that have the center of the cell face under the centerline are included in the bridge computations

Complete the Bridge Data

After locating the bridge (in RAS Mapper) you will need to complete the bridge data through the Geometric Data Editor through the SA/2D Connection Editor.  The data requirements will depend on the bridge method: 1D Family of Rating Curves or 2D Pressure/Overtopping.