Using a unit hydrograph to determine storm streamflow

Using a unit hydrograph to determine storm streamflow 3 U (cfs) i. N (in/hr) Problem 2. 2 as an example. Determine the storm hydrograph resulting from the rainfall pattern shown, given the triangular 1 -hr unit hydrograph 2 1 0 0 1 2 3 Time (hr) 4 1. 0 0. 5 0 0 1 2 3 4 Time (hr) 5 6 Why? Unit Hydrographs are used in hydrologic design to determine the streamflow hydrograph that would result from a given rainfall input. When combined with design rainfall they are used to determine design flows so that control or containment structures (e. g. culverts, detention basins) can be properly sized.

Synthetic Unit Hydrographs The UH represents the integrated effect of size, slope, shape and basin storage characteristics in converting excess rainfall input into streamflow Empirical equations relate UH properties to basin characteristics Snyders (p 130 -133) SCS (p 133 -141)

Problem 2. 4 • A small watershed has the characteristics given below. Find the peak discharge Qp, the basin lag time tp, and the time base of the unit hydrograph TB using Snyder’s method A = 150 mi 2 Ct = 1. 7 L = 27 mi Lca = 15 Cp = 0. 7

S-Curve Method – to construct a UH of any Duration Hydrology and Floodplain Analysis, Fourth Edition By Philip B. Bedient, Wayne C. Huber, and Baxter E. Vieux Copyright © 2008 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Figure 2 -9 Hydrology and Floodplain Analysis, Fourth Edition By Philip B. Bedient, Wayne C. Huber, and Baxter E. Vieux Copyright © 2008 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Problem 2. 14 • Using Excel develop the S-curve for the given 3 -hr UH and then develop the 2 -hr UH from the 3 -hr UH Time (hr) U(cfs) 0 0 1 75 2 180 3 275 4 280 5 210 6 130 7 60 8 30 9 15 10 5 11 0 U(cfs) 300 200 100 0 0 5 10 15