Question:medium

A 100-hectare watershed with uniform land use has a time of concentration of 2 hours. What will be the peak runoff rate at its outlet due to a storm with uniform rainfall intensity of 6 cm/h for a fixed return period? The runoff coefficient is 0.60.

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The Rational Method formula \(Q = CIA\) is simple, but unit conversion is the most common source of errors. Using the formula \( Q = \frac{CIA}{360} \) with I in mm/hr and A in hectares directly gives Q in m\(^3\)/s, which is a useful shortcut.
Updated On: Feb 19, 2026
  • 5 m\(^3\)/sec
  • 8 m\(^3\)/sec
  • 10 m\(^3\)/sec
  • 12 m\(^3\)/sec
Show Solution

The Correct Option is C

Solution and Explanation

Step 1: Understand the Rational Method formula. The Rational Method calculates the peak runoff rate for a small watershed using the formula: \[ Q = C \cdot I \cdot A \] where \(Q\) is the peak discharge, \(C\) is the runoff coefficient, \(I\) is the rainfall intensity (matching the time of concentration duration), and \(A\) is the watershed area.
Step 2: Ensure unit consistency. To obtain discharge \(Q\) in m\(^3\)/s, the input parameters must be in compatible units. A practical version of the equation is: \[ Q (\text{m}^3/\text{s}) = \frac{C \cdot I (\text{mm/h}) \cdot A (\text{ha})}{360} \]. The rainfall intensity is converted to mm/h: \(I = 6\) cm/h \( = 60\) mm/h.
Step 3: Apply the given values to the formula. The provided values are: Runoff coefficient, \(C = 0.60\); Rainfall intensity, \(I = 60\) mm/h; Area, \(A = 100\) ha. The calculation is: \[ Q = \frac{0.60 \times 60 \times 100}{360} = \frac{3600}{360} = 10 \text{ m}^3/\text{s} \]
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