Storm Drain Size Calculator

Storm drain sizing here proceeds in two steps. The Rational Method gives peak discharge Q = C x i x A, where C is a dimensionless runoff coefficient, i is rainfall intensity in inches per hour for a storm whose duration equals the time of concentration, and A is tributary area in acres. The unit conversion is very nearly unity, so Q emerges in cubic feet per second. Runoff coefficients run from about 0.10 to 0.20 for woodland and 0.25 to 0.35 for parks and turf, up to 0.80 for commercial districts and 0.85 to 0.95 for roofs and asphalt. Intensity is read from an intensity-duration-frequency curve, in the United States usually NOAA Atlas 14, for the chosen return period: commonly 10 years for local storm sewers and 25 to 50 years for trunk lines and cross-drains. The pipe is then sized with Manning's equation for a circular section flowing full, rearranged to solve for diameter, and the result is rounded up to the next commercially available size. Manning's n is about 0.012 to 0.013 for concrete and smooth-wall HDPE, and 0.022 to 0.024 for annular corrugated metal, so roughness choice alone can shift the required diameter by a full size increment. Velocity should stay above roughly 2 to 3 ft/s at design flow to keep sediment moving and below about 10 to 15 ft/s to limit abrasion. The Rational Method assumes uniform rainfall over the catchment and a constant coefficient, so it is generally restricted to areas under about 200 acres.

The rational formula is usually traced to Thomas James Mulvaney, an Irish engineer who in 1851 presented a paper to the Institution of Civil Engineers of Ireland relating peak flow from a catchment to rainfall intensity, area, and the time required for the whole catchment to contribute. Emil Kuichling, city engineer of Rochester, New York, brought the concept into American practice with an 1889 paper in the Transactions of the American Society of Civil Engineers on the relation between rainfall and sewer discharge in urban districts. A parallel British formulation by D. E. Lloyd-Davies followed in 1906, and in the United Kingdom the approach is still frequently called the Lloyd-Davies method. The hydraulic half of the calculation comes from Robert Manning, an Irish engineer who presented his uniform flow formula to the Institution of Civil Engineers of Ireland in 1889, with publication following in 1891. Manning was himself uneasy about the equation because it is dimensionally inconsistent, which is why the leading coefficient differs between metric and U.S. customary units, appearing as 1.486 in the foot-second system. Together these two nineteenth century results still underpin most municipal storm sewer design, with modern practice layering on continuous simulation models such as EPA SWMM for larger catchments or systems controlled by detention storage.