Question

A 1” of rainfall excess falls on to a determined watershed in 1 hour resulting in the following unit hydrograph. Estimate the hydrograph if you know that the precipitation event resulted in the rainfall excess as shown in the second table. Take the base flow value as 0.25 inch/hour.

Table 1

Time (hr)	Flow(cfs)
1	20
2	110
3	210
4	160
5	110
6	60

Table 2

Time (hr)	1	2	3	4
Precipitation	0.6	1.1	1.6	0.6

Answer 1

AT 1hr, rainfall is 0.6 "

For 1-2 hr. , rainfall is 1.1"

For 2-3 hr. , rainfall is 1.6"

For 3-4 hr. , rainfall is 0.6"

TIME(hr.)	FLOW (U.H.) IN CFS	U.H. * 0.6"	U.H.* 1.1"	U.H.* 1.6"	U.H.* 0.6"	DRH (COL.3+4+5+6)	FLOOD HYDROGRAPH (DRH+0.25)
1	20	12	-	-	-	12	12.25
2	110	66	22	-	-	88	88.25
3	210	126	121	32	-	279	279.25
4	160	96	231	176	12	515	515.25
5	110	66	176	336	66	644	644.25
6	60	36	121	256	126	539	539.25
7	-	-	66	176	96	338	338.25
8	-	-	-	96	66	162	162.25
9	-	-	-	-	36	36	36.25