WinTR-20 Project Formulation Hydrology Using WinTR-20 for Small Watersheds What is Small ?? Some individuals are using WinTR-20 for subdivision planning. It can be done, but with caution. This presentation shows how to set minimum hydrograph values and time increments to provide valid results for small drainage areas. TR-20 (DOS version) had severe limitations with this type of application, whereas WinTR-20 does not have those limitations. Presented by: WinTR-20 Development Team WinTR-20 for small watersheds March 2009 WinTR-20 for small watersheds March 2009

What is small? The smallest Sub-area less than 0.02 square mile (12.8 acres) Tc equal to or less than 0.1 hour WinTR-20 for small watersheds March 2009

Minimum discharge and time step The minimum practical discharge is 0.001 cfs. A minimum discharge of less than 0.001 cfs will cause the output to be in exponential units (such as 0.245E-03). It is difficult to read, but nevertheless computationally valid. WinTR-20 for small watersheds March 2009 WinTR-20 for small watersheds March 2009

Small Watershed Example WinTR-20 for small watersheds March 2009

Storm Analysis Data WinTR-20 for small watersheds March 2009

Set Global Output Options To provide the maximum precision of WinTR-20 results, select Hydrograph Print Precision as 3. Enter a Minimum Hydrograph Display flow of 0.001 (3 decimal places). Note that this is same as the Minimum Hydrograph Value. Leave the Print Time Increment blank. This will cause the hydrograph to be printed at the internal time increment of 6 percent of the time of concentration. Peak and Hydrograph Output codes may be set for Sub-Areas and Reaches individually or as Global selections. WinTR-20 for small watersheds March 2009 WinTR-20 for small watersheds March 2009

WinTR-20 runoff volume WinTR-20 sums the hydrograph points to get runoff volume. For this example, with CN = 77, rainfall of 5.0 inches, runoff should be 2.62 inches. With a high minimum hydrograph cutoff value, the summing of hydrograph points does not include the beginning and ending hydrograph tails, thus underestimating the runoff volume. WinTR-20 for small watersheds March 2009 WinTR-20 for small watersheds March 2009

WinTR-20 Output Runoff Amount is 2.623 inches. With a higher minimum discharge, the beginning and ending tails would be cut off. Runoff volume would be less than the correct value. Also of interest is the internal time increment. In this example, it is 0.006 hour or 6 percent of the Tc. WinTR-20 for small watersheds March 2009 WinTR-20 for small watersheds March 2009

WinTR-20 Rainfall Tables The Type II storm used in this example has a time interval of 0.1 hour. For Tc’s less than 0.1 hour, the peak discharge does not increase much because rainfall intensity is already at a maximum. The small increase of peak discharge is caused by the steepening of the unit hydrograph. WinTR-20 for small watersheds March 2009

WinTR-20 for small watersheds Data for this example are in the WinTR20_training_MD\workshops\starting data_files\ directory. File name: small_drainage_area.inp WinTR-20 for small watersheds March 2009

The End WinTR-20 for small watersheds March 2009

WinTR-20 for small watersheds March 2009