Coefficient Of Runoff Given Peak Rate Of Runoff Calculator

Rational Formula:

\[ C_r = \frac{36 \times Q_R}{A_c \times P_c} \]

Peak Drainage Discharge (Q_R):

m³/s

Area of Catchment (A_c):

m²

Critical Rainfall Intensity (P_c):

cm/hr

Runoff Coefficient (C_r):

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1. What is the Rational Formula?

The Rational Formula is a method used in hydrology to estimate peak discharge from a catchment area. It relates runoff coefficient, rainfall intensity, and catchment area to determine the maximum flow rate during a storm event.

2. How Does the Calculator Work?

The calculator uses the Rational Formula rearranged to solve for runoff coefficient:

\[ C_r = \frac{36 \times Q_R}{A_c \times P_c} \]

Where:

\( C_r \) — Runoff Coefficient (dimensionless)
\( Q_R \) — Peak Drainage Discharge (m³/s)
\( A_c \) — Area of Catchment (m²)
\( P_c \) — Critical Rainfall Intensity (cm/hr)

Explanation: The formula calculates the proportion of rainfall that becomes surface runoff rather than being absorbed by the ground or evaporating.

3. Importance of Runoff Coefficient Calculation

Details: Accurate runoff coefficient estimation is crucial for urban drainage design, flood prediction, stormwater management, and hydraulic structure design.

4. Using the Calculator

Tips: Enter peak drainage discharge in m³/s, catchment area in m², and critical rainfall intensity in cm/hr. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical range for runoff coefficients?
A: Runoff coefficients typically range from 0.05-0.95, with higher values for impervious surfaces like pavement (0.8-0.95) and lower values for pervious surfaces like lawns (0.05-0.35).

Q2: How does land use affect runoff coefficient?
A: Impervious surfaces increase runoff coefficients, while vegetated areas decrease them due to higher infiltration and evaporation rates.

Q3: What is critical rainfall intensity?
A: Critical rainfall intensity is the precipitation rate at which surface runoff begins, indicating the threshold where the ground can no longer absorb rainfall.

Q4: Are there limitations to the Rational Formula?
A: The formula assumes uniform rainfall distribution and is best suited for small catchment areas (typically less than 200 acres or 80 hectares).

Q5: Why is the constant 36 used in the formula?
A: The constant 36 converts units to maintain dimensional consistency between m³/s, m², and cm/hr in the calculation.