A runoff coefficient is a factor used in some methods to estimate the portion of rainfall that becomes surface runoff from an area.
Definition
A civil engineering firm may use a runoff coefficient as an analysis input when estimating runoff from a site or drainage area. The selected value can reflect land cover and other conditions as represented by the chosen method, and the calculation may combine coefficients across areas. Project work can include selecting and documenting values, mapping contributing areas, and explaining the method and assumptions. Firms may use different coefficients because methods, local criteria, surface classifications, and design purposes differ. A coefficient simplifies real site behavior; it is not a measured property of a parcel, a universal value for a surface, or a guarantee of actual runoff. The calculated flow remains dependent on the rainfall input and other method assumptions. Owners can track whether the value and method are stated consistently across calculations and review documents. Stormwater criteria and permits may involve the EPA.
How owners use it
- Review whether the selected method and coefficient assumptions are documented in the drainage analysis.
- Compare proposed land-cover assumptions across design alternatives or site revisions.
- Example: 2.0 acres × 1.0 inch × 0.5 coefficient = 1.0 acre-inch of estimated runoff depth-volume equivalent under that simplified calculation.
- Include coefficient selection and supporting criteria in the drainage scope and deliverables.
