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Question

# A 4 h unit hydrograph of a catchment is triangular in shape with base of 80 h. The area of the catchment is 720 km2 . The base flow and ϕ index are 30 m3/s and 1 mm/h, respectively. A storm of 4 cm occurs uniformly in 4 h over the catchment. The peak flood discharge due to the storm is

A
210 m3/s
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B
230 m3/s
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C
260 m3/s
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D
720 m3/s
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Solution

## The correct option is A 210 m3/sCatchment area, A=720km2=720×106 m2 Excess rainfall = 1 cm Peak discharge=Qp Run off volume = Area under UH A×0.01=12(80×3600)×Qp ⇒720×106×0.01=12(80×3600)Qp ⇒Qp=50m3/s Depth of precipitation = 4 cm Infiltration in 4 hours = 4 x 1 = 4 mm = 0.4 cm Run off depth = 4 - 0.4 = 3.6 cm peak of DRH of 4 hours =3.6×peak of UH of 4 hrs =3.6×50=180 m3/s Peak of flood discharge = peak of DRH + base flow =180+30 =210 m3/s

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