What is Stormwater Runoff?

Stormwater runoff is the flow of water that occurs when excess meltwater or rainwater flows over the earth's surface. This typically happens when the ground is saturated or when surfaces are impervious, such as roofs, driveways, and parking lots. Understanding the volume of this runoff is critical for civil engineering, environmental protection, and residential landscaping projects.

How to Use the Stormwater Runoff Calculator

This tool uses the standard volumetric calculation (V = A × R × C) to estimate the total amount of water generated during a storm event. To use the calculator:

• Area: Enter the total square footage of the site or catchment area.
• Rainfall: Enter the amount of rainfall in inches (often derived from local storm event data).
• Runoff Coefficient: Select the surface material. This represents the percentage of water that stays on the surface rather than soaking into the ground. For example, asphalt captures nearly all water (0.95), while a forest absorbs most of it (0.10).

Importance of Calculating Runoff Volume

Calculating runoff is essential for sizing retention ponds, drainage pipes, and rain gardens. Excess runoff can lead to soil erosion, flash flooding, and water pollution. By accurately estimating volume, property owners and planners can design mitigation strategies that manage water flow effectively and protect local watersheds. Using high-quality runoff data helps in meeting local environmental regulations and preventing property damage.

Frequently Asked Questions

What is a Runoff Coefficient?

The runoff coefficient (C) is a dimensionless value that relates the amount of runoff to the amount of precipitation received. It varies based on permeability, vegetation, and ground slope.

Why are my results in gallons?

While cubic feet are the standard for engineering, gallons are often more relatable for homeowners sizing rain barrels or small drainage systems. Our tool provides both for your convenience.