Question

The 4-hr unit hydrograph for a watershed is given as below. Time (hr) 2 4 6 8 10 12 14 Unit hydrograph (m3/s/10 mm) A design storm has 1.5 cm/hr in first 2 hr and 1.0 cm/hr in the second 2 hr. The abstraction rate is 0.5 cm/hr. Base flow is 10 m/s. Calculate the total runofi hydrograph.

Answer 1

Answer:-

Given That:-

The 4 - hr unit hydrograph for a watershed is given.

A design storm has 1.5 cm/hr in first 2 hr and 1.0 cm/hr in the second 2 hr. The abstraction rate is 0.5 cm/hr. Base flow is 10 m³/s.

Time (hr)	4 hr $\mho$ H	4 hr	logged	Unit hydrograph	sum
0	0				0
2	15				15
4	45	0			45
6	65	15			80
8	50	45	0		95
10	25	65	15		105
12	10	50	45	0	105
14	0	25	65	15	105
		10	50	45	105
		0	25	65
			10	50
			0	25
				10
				0

Time (hr)	S-Curve ordinate	S-Curve lagged 2 hr	Difference	2HR UH
0	0	0	0	30
2	15	15	15	60
4	45	45	30	70
6	80	80	35	30
8	95	95	15	20
10	105	105	10	0
12	105	105	0	0
14	105	105	0

Calculate the total runoff hydrograph.

Given R₁ = 2 $\times$ 1 cm/hr = 2 cm = 20 mm

R₂ = 2 $\times$ 0.5 cm\hr = 1 cm = 10 mm

Time (hr)	UH	R1$\mho$ 2x$\mho$	R2$\mho$	R1$\mho$+ R2$\mho$	Base flow + R1$\mho$+ R2$\mho$
0	0	0	0	0	10
2	15	30	0	30	40
4	45	90	15	105	115
6	65	130	45	175	185
8	50	100	65	165	175
10	25	50	50	100	110
12	10	20	25	50	60
14	0	0	10	10	20
			0	0	10

Coordinates of total run off hydrograph = flood hydrograph.