The main difference between a CWMS watershed used for AFDR calculations and a standard real-time CWMS watershed lies in how HEC-RAS alternatives are set up. Additional HEC-RAS alternatives are needed to model hydraulics for various combinations of reservoirs and levees when computing AFDR.
For watersheds using the simpler HEC-FIA Holdouts method, at least one new HEC-RAS alternative is needed to represent the unregulated scenario (without reservoirs). For the more detailed AFDR approach, at least three new alternatives are required to represent the unregulated, unregulated without levees, and regulated without levees scenarios. The outputs from each HEC-RAS alternative serve as inputs for corresponding HEC-FIA alternatives. A diagram illustrating how HEC-RAS integrates into the CWMS AFDR process is provided below.
The effort to set up these HEC-RAS alternatives varies based on basin characteristics. For example, a basin with only headwater reservoirs might simply require copying an existing HEC-RAS alternative and adjusting reservoir outflow boundary conditions to simulate unregulated flow. In contrast, basins with multiple reservoirs and levees involving significant storage may need geometry adjustments and boundary condition changes to accurately represent unregulated and no-levee scenarios.
Ultimately, district staff must decide which modifications to the HEC-RAS model are necessary to produce realistic inundation results for without-project scenarios. This section outlines key considerations and offers guidance on handling different complexities when setting up HEC-RAS alternatives for AFDR calculations.