Choose peak runoff flow, drainage area, rainfall intensity, or runoff coefficient using the approximate US-unit rational method Q ≈ C × I × A.

Rational-method peak flow estimate: Q ≈ C × I × A in cfs, in/hr and acres. Select rainfall intensity for the catchment time of concentration.

Related Calculators

Storm Water Runoff Formula

The following approximate US-unit equation estimates peak storm water runoff flow in cfs using intensity in in/hr and area in acres. It uses the conventional rounded unit factor of 1 instead of the dimensional factor of approximately 1.00833.

Q = C × I × A
  • Where Q is the estimated peak storm water runoff flow (cfs)
  • C is the Runoff Coefficient (dimensionless)
  • I is the Rainfall Intensity (in/hr)
  • A is the Drainage Area (acres)

To estimate peak runoff flow, multiply the runoff coefficient, rainfall intensity, and drainage area. Use a rainfall duration matching the catchment time of concentration and check local applicability; this method does not calculate a storm volume or hydrograph.

What is Storm Water Runoff?

Definition:

Storm water runoff is water that flows over the ground surface. This calculator estimates its peak flow rate, rather than total storm volume. Site-specific hydrology and applicable design requirements are still needed when assessing flood risks or sizing drainage.

How to Calculate Storm Water Runoff?

Example Problem:

The following example outlines the steps and information needed to calculate the Storm Water Runoff.

First, determine the drainage area. In this example, the drainage area is 50 acres.

Next, determine the rainfall intensity. Assume the rainfall intensity is 2 in/hr.

Then, select the runoff coefficient. For this example, we use a runoff coefficient of 0.45.

Finally, calculate the storm water runoff using the formula above:

Q = C * I * A

Q = 0.45 * 2 * 50

Q ≈ 45 cfs

FAQ

What factors influence storm water runoff?

Several factors such as rainfall intensity, land slope, soil composition, and vegetation cover affect storm water runoff; urbanization and impervious surfaces also play a significant role.

How can storm water runoff be reduced?

Storm water runoff can be reduced by implementing green infrastructure solutions like permeable pavements, rain gardens, and retention basins, which help increase water infiltration and decrease peak flows.

Why is calculating storm water runoff important?

Calculating storm water runoff is crucial for designing efficient drainage systems, managing flood risks, and ensuring sustainable development in both urban and rural areas.