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Hydrology & Irrigation

Introduction and Basic Assumptions of Unit Hydrograph

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Question

An effective rainfall of 2 hour duration produced a flood hydrograph peak of $200 m^{3} / s .$ The flood hydrograph has a base flow of $20 m^{3} / s .$ If the spatial average rainfall in the watershed for the duration of storm is 2 cm and the average loss rate is 0.4 cm/hour, the peak of 2-hour unit hydrograph (in $m^{3} / s$ up to one decimal place) is _____
150

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Solution

The correct option is A 150
$Flood hydrograph peak = 200 m^{3} / s .$
$Base flow = 20 m^{3} / s .$
$Rainfall = 2 c m$
$ϕ = 0.4 c m / h r$

$Effective rainfall = 2 - 0.4 \times 2 = 1.2 c m$

$Peak of DRH = 200 - 20 = 180 m^{3} / s .$

$Peak of 2-h unit hydrograph$

$= \frac{Peak of DRH}{Effective rainfall}$

$= \frac{180}{1.2} = 150 m^{3} / s .$

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