Use the FHWA HEC-21 horizontal-grade equations to calculate maximum inlet spacing, minimum inlet perimeter, design discharge, contributing area, and curb depth for flat or near-flat roadway decks.
HEC-21 derives this method for zero longitudinal slope. For a grade above 0 but no greater than 0.003, use the result as a flat-deck check alongside the applicable on-grade or vertical-curve procedure. Grades above 0.003 are outside this calculator's scope.
Enter the five-minute storm intensity and flat-deck geometry to calculate both independent FHWA design limits.
For educational purposes only. Not a substitute for professional engineering judgment.