There are two general approaches for modeling bridges in 2D: (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 the overall energy/head losses are important for modeling areas around the bridge. The simplified 1D/2D approach is faster, 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. The detailed modeling approach applies a fully 2D mesh to resolve the details of the bridge hydraulics. The detailed bridge approach is more expensive and requires detailed geometry of the bridge. Another limitation of detailed bridge modeling is that it is currently only applicable to low flow conditions. Fully 2D pressure bridge flow and bridge overtopping will be added in future version of HEC-RAS. Further details on the two approaches are described in detail in the sections below. The simplified 1D/2D bridge modeling approach is available for all of the 2D solvers including the Diffusive Wave Solver. This is considered acceptable since, the purposes of the simplified 1D/2D bridge modeling approach is to capture the overall bridge head losses and not the details of the flow hydraulics through the bridge. The detailed bridge modeling approach should only be applied with the non-linear shallow water equations solvers SWE-ELM and SWE-EM.
Simplified 1D/2D Bridge Modeling
In this approach, the bridge curves for modeling bridges in 1D are utilized to enforce flows through the bridge as a function of the computed head loss through the bridge. The bridge curves are generated using automatically generated 1D cross-sections upstream and downstream of the bridge. This approach is useful when only the head losses are of interest for the purposes of modeling areas other than the bridge itself. It is not appropriate for detailed modeling of bridges since the approach requires a relatively coarse mesh. The approach can be utilized for all hydraulic flow regimes including low flow with and without weir flow and pressure flow with and without overtopping. The bridge geometry including piers and deck are included in the 1D bridge geometry.
During each 2D solver iteration, face area-weighted water surfaces are computed at the upstream and downstream cross-sections. These water surface are then used to interpolate a bridge flow from the precomputed 1D bridge hydraulic curves. The solver then modifies the bed friction along the bridge centerline faces to match the bridge flow. The method enforces the total flow through the bridge and does not enforce a flow/velocity distribution through the bridge.
Detailed Bridge Modeling
In the detailed modeling approach, a high-resolution computational mesh is utilized to simulate the detailed bridge hydraulics. In version 6.4 and earlier, the method is limited to low flow conditions and cannot simulate pressured flow with or without overtopping. This approach is much more computationally expensive than the simplified 1D/2D approach. However, it provides the highest accuracy of bridge hydraulics. The bridge piers should be be included in the terrain.