Catch Basin Spacing Calculator

Storm drain inlets have to be spaced closely enough that ponding water never spreads past the allowable width into a travel lane, and this calculator works backward from that spread limit to find how far apart inlets can go. FHWA's HEC-22 Urban Drainage Design Manual is the governing reference for that allowable-spread limit: its Table 5.1 sets minimum design frequency and spread by road classification, from "Shoulder" on a high-speed interstate up to "1/2 Driving Lane" on a low-volume local street, and the Maximum Allowable Spread input here is meant to be pulled from that table (or a converted, roughly-equivalent value in feet) for the facility you're designing. It starts with the modified rational gutter-flow equation, Q = (Kc/n) × Sx^(5/3) × S^(1/2) × T^(8/3), which describes how much water a triangular gutter section can carry before it reaches your maximum allowable spread — steeper cross slopes and roadway grades let the gutter carry more flow at the same spread. From that flow, the calculator estimates a grate inlet's interception efficiency using a simplified velocity-based relationship (efficiency drops as flow velocity climbs, since fast-moving water skips over the grate rather than falling in), then converts the resulting intercept capacity into a contributing drainage area using the rational method, A = Q/(Ci), with a fixed pavement runoff coefficient of C = 0.9.

That area is translated into a linear spacing along the curb and reported both as a raw distance and as inlets needed per a standard 400-ft block. Treat this as a preliminary sizing tool, not a final HEC-22 design: it uses a fixed composite gutter roughness (n = 0.016), a simplified single-efficiency-curve grate model rather than manufacturer-specific interception curves, and assumes runoff only drains from one side of the road. Real drainage design should verify results against local IDF curves, actual grate hydraulic performance data, and full HEC-22 inlet-on-grade or inlet-in-sag procedures, especially near low points where ponding depth — not spread — governs. The Gutter Width input is collected but does not currently affect the calculation.