Critical Rainfall Intensity For Peak Rate Of Runoff Calculator

Critical Rainfall Intensity Formula:

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

Critical Rainfall Intensity (P_c):

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1. What is Critical Rainfall Intensity?

Critical Rainfall Intensity is the precipitation rate at which surface runoff initiates, leading to increased risk of flooding or other hydrological hazards. It represents the threshold rainfall intensity that triggers significant runoff in a catchment area.

2. How Does the Calculator Work?

The calculator uses the Critical Rainfall Intensity formula:

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

Where:

\( P_c \) — Critical Rainfall Intensity (cm/hr)
\( Q_R \) — Peak Drainage Discharge by Rational Formula (m³/s)
\( A_c \) — Area of Catchment (m²)
\( C_r \) — Runoff Coefficient (dimensionless, 0-1)

Explanation: The formula calculates the rainfall intensity that would produce the given peak discharge for a specific catchment area and runoff coefficient.

3. Importance of Critical Rainfall Intensity Calculation

Details: Calculating critical rainfall intensity is essential for flood prediction, urban drainage design, stormwater management, and infrastructure planning. It helps determine the rainfall threshold that could lead to flooding events.

4. Using the Calculator

Tips: Enter peak drainage discharge in m³/s, catchment area in m², and runoff coefficient (between 0 and 1). All values must be positive numbers with the runoff coefficient not exceeding 1.

5. Frequently Asked Questions (FAQ)

Q1: What is the significance of the constant 36 in the formula?
A: The constant 36 is a conversion factor that accounts for unit conversions between rainfall intensity (cm/hr), discharge (m³/s), and area (m²).

Q2: How is the runoff coefficient determined?
A: The runoff coefficient depends on surface characteristics - impervious surfaces have higher coefficients (0.7-0.95), while pervious surfaces have lower coefficients (0.05-0.35).

Q3: What are typical values for critical rainfall intensity?
A: Values vary widely based on catchment characteristics, but typically range from 1-10 cm/hr for urban areas and 0.5-5 cm/hr for rural areas.

Q4: Can this formula be used for all catchment sizes?
A: The rational formula is most appropriate for small to medium-sized catchments (typically less than 200-300 hectares).

Q5: How does land use affect critical rainfall intensity?
A: Urbanization with increased impervious surfaces raises runoff coefficients, which decreases the critical rainfall intensity needed to produce flooding.