An extreme rainstorm has occurred at the Dry Creek watershed

An extreme rainstorm has occurred at the Dry Creek watershed. The intensity of rainfall excess (k) is shown below. Also shown is the i-hour unit hydrograph for Dry Creek. Assume the baseflow in the creek at the time of the storm is a constant 15 cfs. The area under the direct runoff hydrograph is given as 2 x 10 cubic ft. Time (hr) 1-hr UH (cfs) 0 80 120 60 20 4 Time (hr) ie (in/hr) 0-2 2-4 1.0 Use this information to determine the following. (a) The watershed area in mi-. (b) The direct runoff hydrograph (in cfs) in tabular form. Please do not draw the hydrograph. (e) The peak discharge of the streamflow hydrograph.

Solution

Effectife rainfall in 0-2 hrs=2inch=1/6ft

Effectife rainfall in 2-4 hrs=3inch=1/4ft

Total Effectife rainfall =5/12ft

Area of watershed = A=(area under drh)/(total effective rainfall)

A=4.8*10⁷ ft² = 1.7218 mi²    Answer(a)

3

Hence, Peak streamflow discharge hydrograph = 46.667ft/s

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Time(hr)	UH of 1hr(cfs)	S Curve addition	S Curve	offset S curve	Y	DRH due to 1/6ft rain in 2hr(cfs)	DRH due to 1/4ft rain in 2hr(cfs)	Resulting DRH(cfs)	Base flow in(cfs)	streamflow hydrograph(cfs)
0	0		0	-	0	0	0	0	15	15
1	80	0	80	-	80	6,67	0	6.67	15	21.67
2	120	80	200	0	200	16.67	0	16.67	15	31.67
3	60	200	260	80	180	15	10	25	15	40
4	20	260	280	200	80	6.67	25	31.67	15	46.67
5	0	280	280	260	20	1.67	22.5	24.17	15	39.17
6	-	-	280	280	0	0	10	10	15	25
7	-	-	-	280	0	0	2.5	2.5	15	17.5
8	-	-	-	280	0	0	0	0		0