1. What is Runoff Rate Calculation?

Runoff rate calculation estimates the rate of surface water runoff from a given area using the rational method formula. It's commonly used in civil engineering and hydrology for drainage system design.

2. How Does the Calculator Work?

The calculator uses the rational method equation:

Where:

• \( C \) — Runoff coefficient (dimensionless, 0-1)
• \( i \) — Rainfall intensity (cubic feet per second per acre)
• \( A \) — Drainage area (acres)

Explanation: The equation multiplies the runoff coefficient (representing surface imperviousness), rainfall intensity, and area to estimate peak discharge rate.

3. Importance of Runoff Rate Calculation

Details: Accurate runoff calculation is essential for designing effective drainage systems, stormwater management, flood control, and infrastructure planning.

4. Using the Calculator

Tips: Enter runoff coefficient (0-1), rainfall intensity in cfs/acre, and area in acres. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical runoff coefficient value?
A: Runoff coefficients range from 0.05-0.95 depending on surface type. Pavement might be 0.95, while grassy areas might be 0.05-0.35.

Q2: How is rainfall intensity determined?
A: Rainfall intensity is typically obtained from local rainfall data, intensity-duration-frequency curves, or meteorological records.

Q3: What are the limitations of the rational method?
A: The method assumes uniform rainfall distribution, constant intensity, and is best suited for small drainage areas (typically less than 200 acres).

Q4: Can this be used for commercial site development?
A: Yes, the rational method is commonly used for site development drainage design, though local regulations may specify additional requirements.

Q5: How does vegetation affect runoff coefficient?
A: Dense vegetation reduces runoff by increasing infiltration and interception, resulting in lower coefficient values compared to impervious surfaces.