Peak Rate of Runoff in Rational Formula Calculator

Rational Formula:

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

Runoff Coefficient (Cr):

(dimensionless)

Area of Catchment (Ac):

m²

Critical Rainfall Intensity (Pc):

cm/hr

Peak Drainage Discharge (QR):

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1. What is the Rational Formula?

The Rational Formula is a widely used method for estimating peak drainage discharge from small catchment areas. It calculates the maximum flow rate of stormwater runoff based on rainfall intensity, catchment area, and runoff characteristics.

2. How Does the Calculator Work?

The calculator uses the Rational Formula:

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

Where:

\( Q_R \) — Peak drainage discharge (m³/s)
\( C_r \) — Runoff coefficient (dimensionless)
\( A_c \) — Area of catchment (m²)
\( P_c \) — Critical rainfall intensity (cm/hr)

Explanation: The formula estimates the maximum flow rate by multiplying the runoff coefficient, catchment area, and rainfall intensity, then dividing by 36 for unit conversion.

3. Importance of Peak Runoff Calculation

Details: Accurate peak runoff estimation is crucial for designing drainage systems, stormwater management infrastructure, flood control measures, and urban planning to prevent flooding and water damage.

4. Using the Calculator

Tips: Enter runoff coefficient (0-1), catchment area in square meters, and critical rainfall intensity in cm/hr. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical runoff coefficient value?
A: Runoff coefficients range from 0.1-0.95 depending on surface type. Pavement has higher coefficients (0.8-0.95) while grassy areas have lower (0.1-0.3).

Q2: How is critical rainfall intensity determined?
A: Critical rainfall intensity is based on local rainfall data, storm duration, and return period, typically obtained from intensity-duration-frequency curves.

Q3: What are the limitations of the rational formula?
A: The formula is best suited for small catchment areas (typically < 80 hectares) and assumes uniform rainfall distribution and steady-state conditions.

Q4: Why divide by 36 in the formula?
A: The division by 36 converts units from (cm/hr × m²) to m³/s, accounting for time and area unit conversions.

Q5: When should this formula not be used?
A: For large catchment areas, complex terrain, or where storage effects are significant, more sophisticated hydrological models should be used.