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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?

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? Correct Answer 70 m<sup>3</sup>/s

Concept:

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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 loss

Peak hour period = peak of given DRH/Excess rainfall

Calculation:

Infiltration loss = 0.25 × 6 = 1.5 cm (As rain fall occurs for 6 hours)

Excess rainfall = 8 - 1.5 = 6.5 cm

Peak of flood hydrograph when 6.5 cm excess rainfall occurs for 6 hour period is 470 m3/s

Peak 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.

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The correct answer is