Catch Basin Size Calculator

Sizing a catch basin starts with the peak flow it must intercept, and for small urban catchments that comes from the rational method, Q equals C times I times A. The formula owes its convenience to a numerical accident in customary units: one acre-inch per hour happens to equal 1.008 cubic feet per second, so runoff coefficient times intensity in inches per hour times area in acres returns cfs directly with no conversion factor. The method is generally restricted to drainage areas below about 200 acres, and many agencies set a lower ceiling of 20 to 100. Rainfall intensity is read from a local intensity-duration-frequency curve at a duration equal to the time of concentration, usually 5 to 15 minutes for a paved catchment, with a 5 minute floor applied by most manuals. Runoff coefficients run from 0.70 to 0.95 for asphalt and concrete, 0.75 to 0.95 for roofs, 0.35 to 0.70 for gravel, and 0.05 to 0.35 for lawns depending on soil and slope; mixed catchments use an area-weighted composite. Once peak flow is known, the structure follows from standard precast sizes of 18, 24, 30, 36 and 48 inches, an outlet pipe sized to carry the flow at 2.5 to 10 feet per second, and a sump below the outlet invert to settle grit before it reaches the sewer. Grates and frames in roadways must carry AASHTO HS-20 or HL-93 wheel loading. Two caveats matter. The runoff coefficient is not a physical constant and drifts upward for rarer storms, so frequency adjustment factors of 1.1 to 1.25 are applied for 25 to 100 year events, and grates clog with debris, making a 50 percent clogging allowance common. On streets it is usually allowable gutter spread, not basin dimensions, that dictates inlet spacing.

The rational method is generally credited to Thomas James Mulvany, an Irish engineer who set out the concept of time of concentration and the proportionality between rainfall rate and peak discharge to the Institution of Civil Engineers of Ireland in 1851. It entered American practice through Emil Kuichling, whose 1889 paper in the Transactions of the ASCE applied it to sewer design in Rochester, New York, and it reached British practice through Lloyd-Davies in 1906, which is why the same formula is known there as the Lloyd-Davies method. Its longevity is remarkable for a technique that predates modern hydrology by a century. Applying it required rainfall statistics that did not yet exist. Systematic rainfall frequency analysis in the United States matured under the Weather Bureau and culminated in Technical Paper 40, published by David Hershfield in 1961, which gave engineers national depth-duration-frequency maps; NOAA Atlas 14 now serves that role with far denser data. The catch basin as a structure is older than the calculation. Sumped inlets were built into the great nineteenth century combined sewer systems of London and the industrial cities of the northern United States specifically to keep street grit out of brick sewers that could not be easily cleaned. Modern American inlet design procedures, including grate capacity, gutter spread, and bypass flow calculations, were standardized by the Federal Highway Administration in HEC-12 and its successor HEC-22, the Urban Drainage Design Manual, which remains the reference for highway drainage design.