Stormwater Runoff Calculator

Two independent methods are computed here. The Rational Method treats peak discharge as Q = CiA, where C is a dimensionless ratio of peak runoff rate to rainfall rate, i is average intensity over the time of concentration, and A is drainage area. Typical C values are 0.70 to 0.95 for asphalt and roofs and 0.10 to 0.25 for lawns; a composite catchment uses an area-weighted C. The method suits catchments below roughly 80 hectares and assumes storm duration at least equals the time of concentration. The SCS Curve Number method instead computes runoff depth from rainfall depth: S equals 25400 divided by CN minus 254 in millimetres, initial abstraction is 0.2S, and runoff is the square of P minus 0.2S divided by P plus 0.8S. Impervious surfaces carry CN 98, while pervious ground ranges from about 60 on hydrologic soil group A to 89 on group D. The two methods answer different questions and will not agree: the Rational Method gives an instantaneous peak flow for pipe sizing, the Curve Number method an event volume for storage and treatment sizing. The 0.2 initial abstraction ratio has been challenged by subsequent analysis of gauged data suggesting roughly 0.05 fits observations better, raising predicted runoff for small storms. Curve numbers also assume average antecedent moisture, so a saturated catchment produces considerably more runoff than shown. The pollutant concentrations applied here, 150 mg/L suspended solids, 0.3 mg/L phosphorus and 2.5 mg/L nitrogen, are event mean concentrations typical of mixed urban land use and vary by an order of magnitude between sites.

The Rational Method originates with Thomas Mulvaney, an Irish engineer who presented the relationship between rainfall intensity, catchment area and peak flow to the Institution of Civil Engineers of Ireland in 1851. Emil Kuichling applied and popularised it in the United States in an 1889 paper in the Transactions of the American Society of Civil Engineers describing sewer design for Rochester, New York, and it has carried his framing since. Its simplicity kept it in use for over a century despite the crudeness of a single lumped coefficient. The Curve Number method was developed by the US Soil Conservation Service, with Victor Mockus a central figure, and published in Section 4 of the National Engineering Handbook in 1954. It was built from rainfall-runoff records on small agricultural watersheds and later extended to urban land uses in Technical Release 55, Urban Hydrology for Small Watersheds, issued in 1975 and substantially revised in 1986; the agency is now the Natural Resources Conservation Service. Water quality entered the picture through the Environmental Protection Agency Nationwide Urban Runoff Program, which monitored catchments between 1979 and 1983 and established the event mean concentration concept. The 1987 Clean Water Act amendments added section 402(p), creating the NPDES stormwater permit programme, with Phase I regulations in 1990 and Phase II in 1999. Low impact development, which shifted practice from conveyance toward on-site infiltration, was formalised by Prince George's County, Maryland in the early 1990s under Larry Coffman.