Using the TR-55 tabular hydrograph method (see TR-55 reference manual for details), hydrographs for a selected basin or outlet may be computed. The Compute Hydrograph(s) button is chosen once all of the necessary input for basins and outlets have been entered and a new set of hydrographs will be computed.
If there is a basin selected when computing hydrographs, then a hydrograph for only the selected basin will be computed. However, if there is an outlet selected when computing hydrographs, then a new hydrograph for each upstream basin will be computed along with the routed hydrographs at outlet points.
There may be a slight difference between hydrographs computed by WMS-TR-55 implementation and those in the TR-55 reference manual. This difference occurs because the Ia/P value is rounded to the nearest hydrograph value for the standard version of TR-55. In WMS hydrographs are determined by linear interpolation between two hydrographs using the actual Ia/P value.
Discrepancies Between the TR-55 Hydrograph and the Unit Peak Discharge Method
One question that is frequently asked about TR-55 hydrographs is why the peak flow computed from the TR-55 unit peak discharge method is different from the peak flow on the TR-55 computed hydrograph. Though the TR-55 tabular hydrograph method, which is used to compute TR-55 hydrographs, may give similar results to the TR-55 unit peak discharge method, which is used to compute a peak flow, the peak flows from these two different methods will usually be different. WMS uses the method described in Exhibit 5 in the TR-55 manual to compute a hydrograph using the tabular hydrograph method. Hydrograph ordinates used in the the TR-55 tabular hydrograph method were originally obtained from a series of TR-20 runs with different times of concentration and other values, as described in the TR-55 manual. See the TR-55 manual for more information.
WMS uses exhibit 5 in the TR-55 manual for the tabular hydrograph method.
Values for the Tc, Tt, Ia/P, and Am*Q are used as input values for this table, and all hydrograph values are linearly interpolated or extrapolated from the values for Tc, Tt, and Ia/P. The difference between the WMS method and the method described in the manual is the WMS hydrograph is linearly interpolated. The method described in the manual just rounds the values for Tc, Tt, and Ia/P. Using the manual method gets the same basic results, but the WMS results will be more accurate and can go beyond the minimum and maximum values for Tc, Tt, and Ia/P (this is not recommended).
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