The Holdouts Compute built into HEC-FIA may not be appropriate for some watersheds. If the watershed has significant agricultural damages, large levees where the simplifying assumptions are inappropriate, or where damages reduced must be allocated among many watersheds in a specific way, the model results should be processed outside of HEC-FIA to compute FDR results.

Damage Reaches and State Boundaries

Whether running the HEC-FIA model using the Holdout Compute or as Individual Computes (described in this section), the damage reaches and state boundaries can remain the same for either process. However, damage reaches should be configured to compute with and without-levee estimates.

You should use the HEC-RAS geometry extent shapefile to derive the damage reach areas, as this method is more efficient for larger and more complex watersheds. If these damage reach areas span multiple states, the reach areas should be split at the state boundaries to appropriately report AFDR. Engineering judgment should be used to determine which gages to use represent damage reaches and the extent of the damage reach areas.

Leveed Areas

The leveed area shapefile can be a storage area shapefile exported from HEC-RAS or a shapefile that was generated using GIS software. The shapefile should only include federal levees, as those are the only levees for which AFDR numbers are reported to Congress. Once the leveed area shapefile has been developed, the damage reach areas should be reviewed. If any leveed areas span multiple damage reaches, adjust the damage reach boundaries so that the leveed area falls within one damage reach.

Inundation Configurations

Any of the standard grids or cross sections type of ICs can be used to compute economic damages for individual FDR computations. Selecting the appropriate ICs will depend on which type of inundation data is available from HEC-RAS and whether agriculture damages reduced are needed. Grid type ICs are highly recommended over cross section type ICs due to simplicity and additional robustness of the method of the grid type compute.

Alternatives

The results for each HEC-RAS scenario must be run through the HEC-FIA model independently. This approach requires one HEC-FIA alternative for each HEC-RAS alternative. For watersheds with levees, a total of four alternatives are required (see HEC-RAS Model Setup). Similarly, only two alternatives are required for watersheds with reservoirs only. Each of the HEC-FIA results from the alternatives must be analyzed outside of HEC-FIA to apportion the FDR to each reservoir and levee in the system. Results for each alternative can be viewed from the Economic tab of the Aggregated Consequences Report.

Further information on reviewing and processing the results can be found in Interpreting Consequence Results.