Linking a RAS model alternative takes some special treatment, as the RAS model alternative uses river mile identifiers for locations instead of location names. If the RAS model alternative has a location name in the Description field of the HEC-RAS Cross Section Data Editor (Figure 7.12), then HEC-RTS will try to match that with output from the previous model in the Program Order.

Figure 7.12 HEC-RAS Cross Section Data Editor
For steady flow analysis, the user will need to specify total (maximum) flow at HEC-RAS junction points. The user should have an HEC-HMS or HEC-ResSim junction at each handoff point, and use the total flow. For unsteady flow analysis, the user needs to specify incremental (local) flows at each junction, and HEC-RAS uses that to route the flow.
An important point to remember is that HEC-HMS and HEC-ResSim compute flows at junctions or subbasin outlets, so those flows must be added to the HEC-RAS model at that point (a lateral inflow hydrograph), not distributed along the stream (not a uniform lateral inflow hydrograph). Also, be sure to add the hydrographs from DSS at the correct location; some hydrographs need to be connected to a storage area, some a lateral inflow.
Hydrographs for HEC-RAS may come from HEC-ResSim or HEC-HMS. HEC-HMS generates "Local Flow", which is the flow that is generated between the upstream junction and downstream junction, not including stream flow coming into the downstream junction. (If there is a separate tributary entering a junction, the user must be sure to include that flow. Often that is a separate lateral inflow hydrograph to HEC-RAS, as HEC-HMS does not compute local and tributary as one dataset.) Generally, HEC-ResSim provides the flow out of reservoirs and HEC-HMS provides local flows. HEC-HMS computes locals throughout the basin, HEC-ResSim usually will only produce locals (copied from HEC-HMS) downstream of reservoirs modeled.
In the example used in this manual, the RAS model alternative will obtain most of the flows needed from the HMS model alternative, with two exceptions, where the flows will be obtained from the ResSim model alternative. Initially, the model linking for the RAS model alternative defaults to the extract list (similar to Figure 7.7).

        1. From the table, highlight all of the locations, with the parameter Flow, click Select Input Model Alternative (Figure 7.7), the Select Input Model Alternative dialog box will open (Figure 7.8).


        1. From the Input Model Alternative list (Figure 7.8), select the correct HMS model alternative. Click OK, the Select Input Model Alternative dialog box will close (Figure[ 7.8|Linking HEC-ResSim#_bookmark97]), and a Confirm Input Selection message window will appear (Figure 7.9). The message window is asking the user if this is the linking appropriate for the selected rows. Click Yes, the Confirm Input Selection message window will close (Figure 7.9).


        1. Now on the Model Linking Editor (Figure 7.13), all of the selected rows will now have a linkage to an HMS model alternative in the Input From (Figure 7.13) column.


Figure 7.13 Model Linking Editor – RAS Alternative Model Linking

        1. There are two locations that need to get flow from a ResSim model alternative. For example, in Figure 7.13, for the location DryCreek DryCreek RS 244007.8, from the Input From column (Figure 7.13), from the list, select a ResSim model alternative (i.e., ResSim-B:calibrate). This location will now get flow from the ResSim model alternative (the other location in the example is Russian CoyoteToDC RS 527387.7).


        1. Once the user has verified the linking for the RAS model alternative, click Apply (Figure[ 7.13|#_bookmark103]) to save the model linking for the RAS model alternative.