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

Concept:For a hydrograph analysis, TIME of concentration is defined as the time duration from the END of excess rainfall to the point of inflection.Excess rainfall = Rainfall – infiltration lossPeak hour period = peak of given DRH/Excess rainfallCalculation:Infiltration LOSS = 0.25 × 6 = 1.5 cm (As rain fall occurs for 6 hours)Excess rainfall = 8 - 1.5 = 6.5 cmPeak of flood hydrograph when 6.5 cm excess rainfall occurs for 6 hour period is 470 m3/sPeak of Direct Run off Hydrograph (DRH) when 6.5 cm excess rainfall occurs for 6 hour period = peak of flood Hydrograph – BASE flow = 470 – 15 = 455 m3/s.Now, peak of 6 hour unit hydrograph means that peak of DRH when 1 cm excess rainfall occurs for 6 hour period = 455/6.5 = 70 m3/s.∴ Peak of 6 hour unit hydrograph = 70 m3/s.