Estimating ungaged inflow is only available for 1D Unsteady Flow modeling. In order to use this option, a given reach must have an upstream hydrograph boundary, a downstream hydrograph boundary, and one or more additional internal boundaries. The internal boundary (or boundaries) is typically a stream gage location. An estimate of the ungaged inflow can be made between the upstream boundary and the gage (or between two gages). The ungaged inflow is estimated by creating a Double Boundary Condition(s) (DBC) at the location of the gage(s) (the UNET program referred to this as a Null Internal Boundary Condition), and breaking the given reach up into one or more "routing reaches." A routing reach is a section of river between two gages, or between a gage and the upstream boundary.
The ungaged inflow is optimized to reproduce either a stage hydrograph or a flow hydrograph at the DBC station. When optimizing the stage hydrograph, the reproduction of flow is secondary, being dependent on the calibration of the model. Similarly, when optimizing the flow hydrograph, the reproduction of stage is secondary, also being dependent on the calibration of the model. Optimizing stage is generally used for flood forecast modeling, where stage accuracy is the primary goal. Optimizing flow is used whenever the observed flow record must be maintained, such as a period of record frequency analysis. In either case, the ungaged inflow compensates for all the errors in the measurement of stage and flow, for systematic changes in roughness and geometry that may not be included in the model, and any other errors in calibration, data, or the numerical solution. Hence, great care should be exercised when using this feature.
In order to compute the ungaged inflow, the user should start with a calibrated HEC-RAS river model. In addition, the user will have to specify: observed internal hydrographs (stage or stage and flow); the location and distribution of the ungaged flows; maximum number of ungaged flow iterations; tolerances; simultaneous or sequential optimization; ungaged hydrograph time interval; and optional maximum and minimum ungaged inflow. (This is covered in detail below.) After the data has been entered, HEC-RAS can compute the ungaged inflow in a single program execution (the program will automatically lag the inflows and rerun the model). The final ungaged lateral inflow hydrograph(s) will be output to DSS. The results can be viewed from inside HEC-RAS, or used with any other DSS compatible program.