Calculate curb-and-gutter flow spread with FHWA HEC-22
For a uniform triangular section, gutter spread T is the horizontal distance from the curb to the water edge and curb depth is d = SxT. The calculator solves HEC-22 Equation 4-2 for flow or spread and also handles a depressed composite gutter with two cross slopes. Looking for a building gutter instead? Use the roof-gutter flow calculator.
For educational purposes only. Not a substitute for professional engineering judgment.
Interactive model
Interactive curb-and-gutter spread
Switch between uniform and depressed-composite HEC-22 sections, then explore how discharge, slopes, roughness, gutter width, and depression change spread, curb depth, and travel-lane encroachment.
Open full model (opens in a new tab)FHWA roadway gutter hydraulics
This calculator reproduces the conventional curb-and-gutter procedures in FHWA HEC-22 and the Curb & Gutter module of Hydraulic Toolbox. A uniform triangular gutter uses Equation 4-2. A composite gutter includes a steeper depressed width next to the curb and uses Equations 4-4 through 4-6.
For a composite section, the calculator iterates on spread until the flow in the depressed gutter and the flow on the adjacent pavement sum to the specified discharge. Depth and mean velocity are then computed from the actual two-slope cross-section.
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Last verified: August 2026