Win TR20 Project Formulation Hydrology Using Win TR20

Win. TR-20 Project Formulation Hydrology Using Win. TR-20 for Small Watersheds What is Small ? ? Presented by: Win. TR-20 Development Team Win. TR-20 for small watersheds February 2015 1

What is small? l The smallest Sub-area less than 0. 02 square mile (12. 8 acres) l Tc equal to or less than 0. 1 hour Note: The minimum drainage area in Win. TR-20 is 0. 1 acre (0. 00015 sq mi). The minimum Tc is 0. 6 minute (0. 01 hour). Win. TR-20 for small watersheds February 2015 2

Win. TR-20 minimum discharge l The minimum practical discharge is 0. 001 cfs. Win. TR-20 for small watersheds February 2015 3

Small Watershed Example Win. TR-20 for small watersheds February 2015 4

Storm Analysis Data Win. TR-20 for small watersheds February 2015 5

Set Global Output Options Win. TR-20 for small watersheds February 2015 6

Win. TR-20 runoff volume l Win. TR-20 sums the hydrograph points to get runoff volume. l For this example, with CN = 77, rainfall of 5. 0 inches, runoff should be 2. 62 inches. Win. TR-20 for small watersheds February 2015 7

Win. TR-20 Output l Runoff Amount is 2. 623 inches. Win. TR-20 for small watersheds February 2015 8

Win. TR-20 Output Minimum discharge is 0. 5 cfs. l Runoff Amount is 2. 248 inches. l Win. TR-20 for small watersheds February 2015 9

Win. TR-20 Rainfall Tables The Type MSE 4 (Midwest/Southeast States Region 4) storm used in this example has a time interval of 0. 1 hour. l For Tc’s less than 0. 1 hour, the peak discharge does not increase much because rainfall intensity is already at a maximum. l The small increase of peak discharge is caused by the steepening of the unit hydrograph. l Win. TR-20 for small watersheds February 2015 10

The End Win. TR-20 for small watersheds February 2015 11

Win. TR-20 for small watersheds February 2015 12