Field calculator
Rational method runoff calculator (Q = CiA)
The rational method is the most common way to estimate the peak rate of stormwater runoff from a small drainage area, and it is the starting point for sizing swales, inlets, pipes, and detention. The formula is Q = C x i x A: Q is the peak flow in cubic feet per second, C is the runoff coefficient (the fraction of rain that runs off instead of soaking in, roughly 0.10 to 0.30 for lawns and open ground and 0.70 to 0.95 for pavement and roofs), i is the rainfall intensity in inches per hour for the design storm at the site time of concentration, and A is the drainage area in acres. It works cleanly in these units because one acre-inch per hour is almost exactly one cfs. Enter the three values to get the peak flow. Treat the result as a planning estimate for small sites: the runoff coefficient, the design storm and its intensity, the time of concentration, and whether the rational method is even accepted are all set by the local stormwater code and the civil engineer, and a composite C is needed when the area mixes surfaces.
Result
The rational method estimates peak stormwater runoff: Q = C × i × A, where Q is the peak flow in cubic feet per second, C is the dimensionless runoff coefficient (how much rain runs off rather than soaking in), i is the rainfall intensity in inches per hour for the design storm at the site time of concentration, and A is the drainage area in acres. One acre-inch per hour is almost exactly one cfs, which is why the formula works in these units. Use it for small drainage areas as a planning figure; the runoff coefficient, the design storm, the time of concentration, and the acceptable method are set by the local stormwater code and the civil engineer.
anvilfield.com/calculators/rational-method-runoff-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Runoff calculator (Q=CiA) FAQ
What is a detention pond?
A detention pond is a basin that holds the runoff from a developed site during a storm and releases it slowly through an outlet structure. It cuts the peak flow leaving the site to no more than the pre-development rate, protecting downstream channels and property. A dry detention basin empties between storms.
What is the difference between detention and retention?
Detention holds the storm and then empties, controlling the peak flow. Retention keeps a permanent pool of water that never fully drains, which adds water-quality treatment as sediment settles and pollutants break down. A detention basin is dry between storms; a retention pond stays wet. The permit and the engineer decide which a site needs.
What is an outlet control structure?
An outlet control structure is the riser, orifice, and weir assembly that sets how fast a basin releases water. It is usually multi-stage, with a low orifice for small storms and weirs for larger ones, so the release stays at or below each storm's limit. Its dimensions come from the engineer's stage-discharge design.
Why do detention ponds need maintenance?
Detention ponds fail mostly from no maintenance, not bad design. Sediment fills the storage, the outlet clogs and floods the basin, and trees take root in the embankment. Removing sediment, clearing the outlet, mowing, and inspecting after storms keep it working. The O&M plan is usually required by the permit and runs with the property.
Why is my detention basin not draining or flooding?
Check the outlet first. A clogged low-flow orifice or a debris-matted trash rack stops the basin from releasing, so it fills and floods. Clear the rack and the orifice and confirm it draws down. Never enlarge the orifice to drain faster, because that fails the release limit. Inspect the outlet after every storm.