1. What Is Storm Runoff?

Storm runoff is the portion of rainfall that flows over the ground surface when the soil is saturated or when rainfall intensity exceeds the infiltration capacity of the soil. It's a critical factor in hydrology, urban planning, and stormwater management.

2. How Does The Calculator Work?

The calculator uses the rational method equation:

Where:

• \( P \) — Precipitation (inches)
• \( A \) — Area (square feet)
• \( C \) — Runoff coefficient (dimensionless, 0-1)

Explanation: The equation calculates the volume of water that will run off a surface during a rain event, accounting for how much water is absorbed versus how much flows over the surface.

3. Importance Of Runoff Calculation

Details: Accurate runoff estimation is crucial for designing drainage systems, preventing flooding, managing stormwater, and complying with environmental regulations.

4. Using The Calculator

Tips: Enter precipitation in inches, area in square feet, and select an appropriate runoff coefficient (typical values: 0.9 for paved areas, 0.3 for lawns, 0.2 for forests).

5. Frequently Asked Questions (FAQ)

Q1: What is a runoff coefficient?
A: The runoff coefficient (C) represents the fraction of rainfall that becomes surface runoff. It ranges from 0 (all water infiltrates) to 1 (all water runs off).

Q2: How accurate is this method?
A: The rational method provides reasonable estimates for small drainage areas (<200 acres). For larger areas, more complex models are needed.

Q3: What are typical coefficient values?
A: Common values: 0.95 for roofs, 0.85 for asphalt, 0.35 for grassland, 0.25 for woodland. The coefficient varies with slope and soil type.

Q4: Does this account for storm duration?
A: No, this simple calculator uses total precipitation. For time-varying analysis, hydrologic models like SCS or HEC-HMS are needed.

Q5: How can I reduce runoff?
A: Use permeable surfaces, rain gardens, green roofs, and other low-impact development techniques to increase infiltration.