There are two options that need to be enabled in the HEC-ResSim alternative that are not normally used in day-to-day CWMS forecasting. The modeler will have to enable both the Compute Unregulated Flows and the Compute Holdouts options in the Alternative Editor, as shown in Figure 1.


HEC-ResSim Alternative Editor - Unregulated Flows and Holdouts

Compute Unregulated Flows Option

When the Compute Unregulated Flows option is checked, HEC-ResSim computes an unregulated flow at downstream points of the reservoir. Once HEC-ResSim is run, you should compare the regulated flows to the unregulated flows. If rating curves are available and properly defined within the model, you should compare the resulting stages as well prior to running HEC-RAS.

There are several reasons to compare the unregulated flows and stages to the regulated flows and stages. One reason is to determine the magnitude of regulated versus unregulated conditions, to conceptualize the flood extent prior to running HEC-RAS. The second reason is to catch any calibration issues, so they can be corrected before moving to the next step in the process. Figure 2 is an example of a regulated versus an unregulated flow and stage hydrograph from a typical HEC-ResSim model.


HEC-ResSim Hydrograph Output - Regulated (blue) vs. Unregulated (red)

Compute Holdouts Option

The Compute Holdouts option in HEC-ResSim routes holdouts from each reservoir in the watershed to downstream computation points. Holdouts can be positive or negative. Positive holdouts signify that the reservoir is storing inflows and reducing flows downstream. Negative holdouts signify that the reservoir is evacuating storage, typically after the flows have receded downstream and there is capacity to release water without causing additional flooding. The timing and magnitude of holdouts in downstream reaches determines how much of the total FDR is attributed to each reservoir. Figure 3 is an example of multiple reservoir holdouts routed to an index gage location located downstream of all reservoirs in the system. HEC-FIA Model Setup explains how these holdouts are utilized in allocating FDR to each reservoir.


HEC-ResSim Reservoir Holdouts Routed to Index Gage Location


When upstream reservoirs exist outside of the CWMS watershed (and possibly outside of district boundaries), allocating FDR is more complicated. One way to handle this scenario is to create a new HEC-ResSim model containing all reservoirs needed to compute the holdouts. The resulting holdout time series data can be passed to the individual watershed HEC-FIA models to apportion FDR between all reservoirs. A second option is to retrieve the holdout time series data from the individual upstream watersheds. You will have to route, combine, and process the holdouts outside of HEC-ResSim and HEC-FIA. The results of this can be used in the HEC-FIA alternative by assigning fixed apportioning percentages in the holdout overrides table. HEC-FIA Model Setup discusses how to process holdouts outside of HEC-ResSim and apportion AFDR to reservoirs.