Stormwater Runoff Calculator
Compute peak stormwater runoff flow rate in cubic feet per second using the Rational Method (Q = CiA). Used by engineers to size drainage infrastructure like storm sewers, channels, and detention basins.
About this calculator
The Rational Method is the most widely used approach for estimating peak runoff from small drainage basins (typically under 200 acres). The formula is Q = C × i × A, where Q is peak flow rate in cubic feet per second (cfs), C is the dimensionless runoff coefficient representing the fraction of rainfall that becomes runoff, i is rainfall intensity in inches per hour for the design storm duration equal to the time of concentration, and A is the drainage area in acres. The runoff coefficient C depends on land surface type: impervious surfaces like rooftops or pavement use C ≈ 0.85–0.95, while wooded or grassy areas use C ≈ 0.20–0.35. Rainfall intensity is obtained from intensity-duration-frequency (IDF) curves for the project location and chosen return period (e.g., 10-year or 100-year storm). The time of concentration is the travel time for runoff to move from the most remote point in the watershed to the outlet.
How to use
Suppose a 5-acre commercial parking lot (C = 0.85) experiences a design storm with rainfall intensity i = 3.5 in/hr, with a time of concentration of 15 minutes. Step 1: Identify values — C = 0.85, i = 3.5 in/hr, A = 5 acres. Step 2: Apply the formula: Q = C × i × A = 0.85 × 3.5 × 5. Step 3: Q = 14.875 cfs. This peak flow of approximately 14.9 cfs must be accommodated by the storm sewer or detention system serving the lot. A larger, more pervious area would yield a lower C and therefore a lower peak flow.
Frequently asked questions
How do I select the correct runoff coefficient for my drainage area?
The runoff coefficient C reflects how much of the rainfall becomes surface runoff rather than infiltrating the soil or being absorbed by vegetation. Common values: business districts and dense urban areas C = 0.70–0.95, residential suburban areas C = 0.25–0.50, lawns and parks C = 0.10–0.35, and forests C = 0.05–0.25. For a mixed-use watershed, calculate a weighted average C based on the proportion of each land use type. Always consult local municipal drainage standards, as many jurisdictions specify required C values for different zoning types.
What return period storm should I design for when using the Rational Method?
The appropriate return period (recurrence interval) depends on the consequence of failure and the type of infrastructure. Minor storm sewers in residential areas are commonly designed for the 10-year storm, while major trunk sewers, culverts under arterial roads, or drainage near critical facilities may require the 25-, 50-, or 100-year storm. Detention basins protecting downstream properties from flooding are often designed for the 100-year event. Local floodplain regulations and municipal drainage design standards typically specify minimum return periods for each facility type.
When is the Rational Method not appropriate for stormwater calculations?
The Rational Method is best suited for small, relatively homogeneous watersheds of 200 acres or less with short times of concentration. It assumes a steady, uniform rainfall intensity equal to the design storm and does not account for storage effects, non-uniform rainfall distribution, or the timing of runoff from different sub-areas. For large or complex watersheds, use hydrograph-based methods such as the NRCS (SCS) Curve Number method, TR-20, or continuous simulation models like SWMM. Any design involving significant detention storage or multi-subbasin routing should move beyond the simple Rational Method.