The setup of the HEC-RAS alternatives is where a CWMS watershed being used to calculate AFDR can differ the most from a standard real-time CWMS watershed. Additional HEC-RAS alternatives are required to represent the hydraulics for a combination of with- and without-reservoirs and levees when trying to compute AFDR. For watersheds utilizing the more simplistic HEC-FIA Holdouts approach to compute AFDR, at least one new HEC-RAS alternative is required to represent the unregulated (without-reservoirs) scenario. Similarly, for the more detailed approach to compute AFDR results, at least three new alternatives are required to represent the unregulated, unregulated without-levee, and regulated without-levee scenarios. The results of each alternative will be fed to individual HEC-FIA alternatives.
The level of effort required to setup the HEC-RAS alternatives appropriately for either approach varies depending on the characteristics of the basin. For example, the most basic case where only headwater reservoirs exist in the basin might simply require copying the existing HEC-RAS alternative and changing the reservoir outflow boundary conditions to the unregulated flow. On the other hand, a basin with tandem reservoirs and levees with significant storage may require geometries to be reconfigured and boundary conditions be modified to best represent the unregulated and without-levee scenarios.
It is ultimately up to district personnel to determine what modifications to the HEC-RAS model are necessary to achieve the most realistic inundations for the without project scenarios. This section explains what to consider when setting up HEC-RAS alternatives for FDR computes and provides guidance on how to handle various types of complexities within the basin.