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Modeling 2D Bridge Hydraulics for Scour Analysis in HEC-RAS
Selecting Flows
Bridge scour analyses analyze multiple flows in a risk-informed framework.
See Page 2.1 of the FHWA HEC18 guidance to select the appropriate flows for your analysis. Please read all of section 2.1 of this guidance, the excerpt below is only a summary:
Bridge foundations for new bridges should be designed to withstand the effects of scour caused by hydraulic conditions from floods larger than the design flood. In 2010, the U.S. Congress recommended that FHWA apply risk-based and data-driven approaches to infrastructure initiatives and other FHWA bridge program goals... Prior to the use of these risk-based approaches, all bridges would have been designed for scour using the Q100 flood magnitude and then checked with the Q500 flood magnitude. Table 2.1 presents recommended minimum scour design flood frequencies and scour design check flood frequencies based on hydraulic design flood frequencies.
The Hydraulic Design Flood Frequencies outlined in Table 2.1 assume an inherent level of risk. There is a direct association between the level of risk that is assumed to be acceptable at a structure as defined by an agency's standards and the frequency of the floods they are designed to accommodate. The Scour Design Flood Frequencies presented in Table 2.1 are larger than the Hydraulic Design Flood Frequencies because there is a reasonably high likelihood that the hydraulic design flood will be exceeded during the service life of the bridge.
2. Detailed Bridge Modeling in HEC-RAS
i. Quasi-Steady 2D Flow Modeling
The FHWA bridge scour equations are steady flow equations.
They assume a single value for each hydraulic input.
The HEC-RAS 2D model is unsteady and starts with a dry terrain/mesh.
Set up each simulation as a quasi-steady simulation with the following steps:
- Define the entire flow boundary condition with the same flow
- Use either the warmup features or the Initial Condition Time (pictured below) to fill the model and achieve steady state
- Make sure the model is stable in the simulation time window.
- Use hydraulic results from a late, stable time step.
- Be careful of using Max results as they may include velocity and depth perturbations from instabilities.
ii. Selecting Hydraulic Equations
The objective of 2D bridge modeling is to develop the XS averaged hydraulics the FHWA toolbox requires with more detailed hydraulic modeling.
The SWE-EM equation with the Conservative Turbulence method will produce the most accurate hydraulic results under most conditions.
Increasing the accuracy of your hydrodynamics has a computational cost, but quasi-steady bridge models do not tend to be run time limited.
Therefore, users should generally choose accuracy in the accuracy-run time tradeoff associated with the hydrodynamic equations.
(Note: In recent versions of HEC-RAS 7.0 the run time for SWE-EM has improved substantially).
iii. Mesh Resolution, Time Step, and Convergence Analysis
Your bridge deck information may come from a larger model (e.g. a regional flood model with multiple bridges) that is optimized for run time not for resolving detailed velocities through the bridge. While you may be able to use the bridge geometry information and the terrain from an existing model, you will probably need to make a new mesh. Pier scour - in particular - uses velocity and depth from the maximum unit flow cell, so results will be sensitive to your mesh size (e.g. finer meshes are likely to compute higher max cell velocities and, therefore, more pier scour). Because bridge scour requires a steady flow in a relatively small domain, run times can be manageable even for relatively fine mesh resolutions.
More information on generating meshes around piers at multiple levels of detail is available here.
3. Selecting a 2D Bridge Modeling Approach
HEC has actively developed 2D bridge modeling since version 6.0.
Almost every release over the last few years has included new bridge modeling features and approaches.
The guidance and best practices have evolved quickly.
Before you try to compute bridge scour in HEC-RAS you must make sure you have selected an appropriate bridge modeling approach and have followed the best practice guidelines (e.g. always define a bridge deck and include piers in both the terrain and deck)
We summarize the bridge modeling options and some of the best practices for bridge scour here, but also see more, current, 2D bridge guidance here.
Bridge models in HEC-RAS 2D are defined as SA/2D Connections 
.
After you define and digitize a SA/2D Connection in RASMapper you can define the deck, pier, and abutment data in the Geometry editor.
The cross section associated with the bridge editor and the bridge deck information that the Hydraulic Toolbox requires will come from these data defined in the geometry editor.
There are two approaches to modeling bridges in HEC-RAS:
- 1D Family of Rating Curves (RCs)
- 2D Pressure Overtopping
(Note: HEC used to recommend low-flow "detailed modeling without a bridge deck and the piers stamped into the terrain. This is no longer recommended with the current, available, features.)
1D Family of Rating Curves (RCs)
| 2D Pressure and Overtopping
| |
|---|---|---|
| Approach | Computes 2D losses based on terrain and computes losses with the 1D bridge modeling algorithm. If the 1D bridge model computes more losses through the bridge this method adds enough distributed 2D losses to match the 1D solution. | Uses 2D hydraulics through the terrain (with optional minor losses) for flows below the bridge deck and a 2-layer, 2D, bridge deck model for pressure-weir flow. |
| Bridge Deck | HEC recommends defining a bridge deck in the geometry for all bridges that stay subcritical. | |
| Piers | Piers must be included in the bridge deck (geometry file) and should also be included in the terrain. Model will use deck piers for 1D analysis and the terrain piers for 2D analysis and then compare results. | Piers must be included in the terrain and may be included in the bridge deck (new in 6.7) |
| How the Model Uses Piers Defined in the Bridge Deck (i.e. Geometry) |
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| Note: Defining the piers in the bridge deck and the terrain (as terrain mods) is recommended for both approaches. | ||
| Recommended Application | Large models with multiple bridges that do not require detailed resolution 2D velocities through the bridge opening. | Detailed bridge hydraulics that need to resolve detailed 2D velocities through the bridge (e.g., scour studies) during pressure flow and/or overtopping regimes. |
| Cautions | Does not resolve supercritical flow. | Could be computationally expensive for large models. |
| More Documentation | ||
| Videos | ||
Use a Bridge Deck for All 2D Bridge Models and Define Piers in Bridge Editor AND Terrain
- HEC-RAS stores bridge information in the 2D/SA Connection element of the geometry file.
- Both bridge modeling approaches require this information (and if you use the bridge scour export tool it will read bridge geometry from this file).
- The 2D Pressure Overtopping method also benefits from stamping the piers in the terrain to compute detailed flow.
- The 1D Family of Rating Curves approach is not sensitive to the terrain under the bridge, so it will not "see" piers stamped into the terrain.
- Therefore, it is usually best practice to include piers in the geometry file (i.e. bridge deck) AND the terrain.
The subgrid approach in HEC-RAS allows users to choose coarse modeling grids to optimize run times for large-domain models with long flow records.
But bridge scour models are quasi-steady models with relatively small domains.
(Note: Modeling a bridge for scour analysis with a large model developed for other purposes is not recommended. These models should only be large enough to appropriately represent the upstream and downstream boundary conditions and avoid backwater effects on the reference lines.)