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Civil Engineering QUESTION #2128
Question 1
In hydrology, the rational method for estimating peak runoff from a catchment is expressed as \(Q_p = \dfrac{C \cdot i \cdot A}{360}\) (SI units). The time of concentration \((t_c)\) is used to determine the rainfall intensity \(i\) because:
  • It represents the time for the entire catchment to contribute to runoff at the outlet simultaneously — using intensity for duration equal to \(t_c\) gives the maximum possible peak discharge for the design return period✔️
  • It is the time for water to infiltrate through the soil profile to the water table
  • It is used to calculate the recession limb of the unit hydrograph
  • It represents the lag time between the centroid of rainfall excess and the peak of the hydrograph
Correct Answer Explanation
In the rational method, \(t_c\) = time of concentration = time for water from the most remote point to reach the outlet. When storm duration = \(t_c\), the entire catchment contributes simultaneously, producing maximum peak flow. Using \(i\) corresponding to duration \(= t_c\) and the specified return period gives the design peak discharge. Storms shorter than \(t_c\) cannot fully contribute; longer storms have lower intensity.