HEC-ResSim is only necessary if more than one flood control reservoir exists within the watershed. If only one reservoir is within the watershed, the observed outflows can be directly mapped to HEC-RAS (HEC-ResSim can be removed from the modeling sequence,)as 100 percent of the reservoir AFDR will be allocated to the single reservoir. However, if there are two or more reservoirs within the same watershed, or if there are one or more reservoirs in upstream watersheds that provide benefits within the watershed being analyzed, then a HEC-ResSim alternative is needed for routing holdouts to properly allocate reservoir AFDR benefits to each reservoir.
There are two primary ways to allocate the damages reduced to each reservoir. The first way is to use HEC-ResSim to calculate a holdout time series at each computation point for each reservoir, which is then used by HEC-FIA to apportion the total damages reduced to each of the contributing upstream reservoirs. A reservoir holdout is a flow time series of what the reservoir "held out", or stored, during the event. A second approach is to assign, if it is known, a fixed apportioning percentage to each upstream reservoir. Refer to HEC-FIA Model Setup on how holdouts or fixed percentages are used to allocate damages reduced.