1. Using a unit hydrograph to determine storm streamflow
Problem 2.2 as an example. Determine the storm hydrograph resulting from the rainfall pattern shown, given the triangular 1-hr unit hydrograph 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. 3
1.0
U (cfs) 2
iN (in/hr)
0.5 1 1 2 3 4 1 2 3 4 5 6
Time (hr)
Time (hr)

2. 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 ) SCS (p )

3. 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 mi2
Ct = 1.7
L = 27 mi
Lca = 15
Cp = 0.7

4. S-Curve Method – to construct a UH of any Duration

5. Figure 2-9

6. 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) 1 75 2
180 3
275 4
280 5
210 6
130 7 60 8 30 9 15 10 11