1. What is Storm Water Runoff?

Storm water runoff is precipitation that flows over the ground surface when it rains. It's an important consideration in urban planning, civil engineering, and environmental management as it can cause flooding, erosion, and water pollution.

2. How Does the Calculator Work?

The calculator uses the Rational Method formula:

Where:

• \( Rainfall \) — Depth of rainfall (inches)
• \( Area \) — Drainage area (acres)
• \( Coefficient \) — Dimensionless runoff coefficient (0-1)

Explanation: The runoff coefficient represents the fraction of rainfall that becomes surface runoff, accounting for factors like surface type, slope, and soil conditions.

3. Importance of Runoff Calculation

Details: Accurate runoff estimation is crucial for designing drainage systems, sizing detention ponds, flood risk assessment, and complying with stormwater management regulations.

4. Using the Calculator

Tips: Enter rainfall depth in inches, area in acres, and an appropriate runoff coefficient (typically 0.05-0.95). All values must be valid (positive numbers, coefficient between 0-1).

5. Frequently Asked Questions (FAQ)

Q1: What are typical runoff coefficients?
A: Pavement: 0.9-0.95, Lawns: 0.1-0.35, Forests: 0.05-0.25. The coefficient depends on surface type and slope.

Q2: How do I convert acre-inches to other units?
A: 1 acre-inch = 3630 cubic feet = 27,154 gallons = 102.8 cubic meters.

Q3: What rainfall value should I use?
A: Use design storm values (e.g., 10-year 24-hour storm) from local rainfall data or IDF curves.

Q4: Are there limitations to this method?
A: This simplified method works best for small areas (<200 acres). For larger areas, more complex models like SCS Curve Number may be needed.

Q5: How does soil type affect runoff?
A: Sandy soils have lower coefficients (more infiltration) while clay soils have higher coefficients (more runoff).