A peak of flood hydrograph due to 6 hr storm is 470 m3/s. The average depth of rainfall is 8 cms. Assume an infiltration loss of 0.25 cm/hour and a constant base flow of 15 m3/sec. Estimate the peak discharge of 6-hour unit for this catchment.

Duration of rainfall excess = 6 hr

Infiltration loss = 0.25 cm/hr

Total loss for 6 hr = 0.25 ∗ 6 = 1.5 cm

Total depth of rainfall = 8 cm

Rainfall excess = 8 – 1.5 = 6.5 cm

Peak of flood hydrograph = 470 m3/s

Constant base flow = 15 m3/sec

Peak of DRH = 470 – 15 = 455 m3/sec

Peak of 6 hr unit hydrograph = Peak of DRH/Rainfall excess

∴ Peak of 6 hr unit hydrograph = 455/6.5 = 70 m3/sec

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