Section 4: Soil and Water Conservation Engineering
Runoff Estimation: Rational & SCS-CN Methods
Subtopic: Rational formula peak discharge
Question Statement:
A small agricultural watershed of area $A = 120\text{ ha}$ has a runoff coefficient $C = 0.35$. The design rainfall intensity for a duration equal to the time of concentration is $I = 80\text{ mm/h}$. Using the Rational method, the peak runoff rate is ________ $\text{m}^3/\text{s}$ (round off to two decimal places).
Answer Key:
9.33
Step-by-Step Solution & Explanation
Using the Rational formula in metric units:
$Q_p = \frac{C I A}{360}$
where $I$ is in $\text{mm/h}$, $A$ is in $\text{ha}$, and $Q_p$ is in $\text{m}^3/\text{s}$:
$Q_p = \frac{0.35 \times 80 \times 120}{360} = 9.33\text{ m}^3/\text{s}$