The program calculates the starting headwater elevation by assuming an initial zero rate of change of storage in the basin. Eliminating storage as a variable, the continuity equation becomes
QI=QO+QWWith a known QI (stream discharge) and tailwater at the starting time, the program calculates a headwater elevation so that the flow through the bridge opening and over the roadway equals the QI.
If the above assumption (dS/dt=0) is reasonable, the user can leave this input data cell empty and use the default value calculated by the program as explained above. In some cases, however, this assumption may not be valid, (such as the case that the start time for the analysis is based on the results of a previous run). At the end of the previous run, the headwater elevation is known. By readjusting the starting time and ending time and inputting the initial headwater elevation as the end elevation of the previous run, the program can continue the run smoothly. In this case, the rate of storage change is not zero.
If the user inputs an unreasonable starting elevation, the program may be unable to arrive at a stable solution. If you are uncertain about the value of the starting elevation, leave this cell blank.