Next, the required data for the ungaged lateral inflow should be entered. From the Unsteady Flow Analysis editor, choose Options and click on ungaged lateral inflow. (After ungaged inflow data has been entered, there will be a text message at the bottom of the unsteady run editor indicating that this plan has ungaged inflow. The ungaged lateral inflow editor can be accessed directly by clicking on this message.)

Figure 7 65. Selecting the Ungaged Lateral Inflow Option.

Now the data for the ungaged lateral inflow can be entered (Figure 7-66).

Figure 7 66. Ungaged Lateral Inflow Editor.

By default, the Optimization Mode and Optimization Target are set to Sequential and Stage, respectively (see hydraulic reference manual). The "Number of Iterations" is the maximum number of times that HEC-RAS will iterate (estimate ungaged inflow, lag and route the lateral inflow, run the unsteady model, and check for convergence) when computing the ungaged inflow. The "Flow Conv Criteria" is the flow tolerance (in cfs or m^3/s). RAS will iterate until the flow tolerance is met (based on average, least-squared difference—see hydraulic reference) or the maximum number of iterations has been met. If the user enters a value for the "Smoothing Window," then RAS will apply a running-average when computing the ungaged inflow (hydraulic reference…). The previous data (the information at the top of the editor) is only entered once and it applies to all of the known gage/routing reaches.
To enter information for a particular gage, the first step is to click on <New> and enter the name of the gage (or other descriptive name for the inflow into this routing reach). Next, clicking on <Set RS> allows the user to enter the location of this gage. (Important: this river station should match a river station that was entered on the unsteady flow editor for Internal Observed Stage and Flow!)
The unknown lateral inflows are added by clicking on <Add Lateral Inflow> (for a stream or point source) or by clicking on <Add Uniform Lateral Inflow> (for a disperse inflow). The "Contrib. Area" column is used to enter the contributing area for each of the unknown lateral inflows for the current reach. These areas are summed and then a percentage is assigned to each lateral inflow. This is only applicable if a particular known gage/routing reach has more than one unknown inflow. After RAS has determined the total unknown inflow for a given routing reach, the program will proportion the flow between the various lateral inflows based on the ratio of Contributing Areas. In the above example, the contributing area has been entered as a percentage. The uniform inflow stating at river station 70 will get 20% of the flow and the lateral inflow at RS 50 will get the remaining 80%. The contributing areas could also have been entered as a decimal fraction (0.2 and 0.8). Alternately, the user could enter the actual size of the contributing watersheds (e.g., 100 and 400 sq. mi. ). Since the division is proportionately based, the units of contributing area do not need to be specified. Regardless of how the area is entered, RAS will compute and display the percent flow for each area.
The "Lag Time" is used by the program when routing the unknown inflow. The amount of unknown inflow is computed at the gage location. However, this unknown inflow enters the river system upstream (that is, at an earlier point in time). The lag time is the amount that the program will shift the lateral hydrograph back in time when it routes the inflow and reruns the program. It should be the approximate travel time from the lateral inflow location to the gage station.
For each unknown inflow, the user can specify an optional maximum and minimum flow. The program will limit the lateral inflow to these values. This may be needed to prevent the program from computing hydraulically unreasonable flows.
Finally, an Optional DSS B part can be specified. After the inflows have been computed, RAS will write the lateral hydrograph out to DSS. The B part of the pathname will either be that specified by the user, or the default name. The default name is, "UNGAGED INFLOW #," where # is the river station for a particular lateral inflow (either 70 or 50 in the above example).
After all the lateral inflow information has been entered, additional routing reaches can be added (only applicable if the given RAS project has more than one known gage) by clicking the <New> button and repeating the process. Different routing reaches can be selected from the drop down menu next to the Ungaged Area Name. Clicking <Delete> will remove the actively displayed routing reach.