Gutter Flow Calculator (HEC-22)

Check whether a street gutter carries the design flow within the allowable spread: enter the cross and longitudinal slopes, the spread width, and Manning’s n.

Cross slope Sx (ft/ft)Decimal, e.g. 0.02 for 2%.

Longitudinal slope SL (ft/ft)Street grade as a decimal.

Spread T (ft)Width of flow from the curb face.

Manning’s n0.016 is the HEC-22 default for pavement.

Gutter Capacity (Triangular Section)

Item	Value

How the calculation works

HEC-22 modified Manning equation, uniform triangular gutter
Q (cfs) = (0.56 ÷ n) × Sx^1.67 × SL^0.5 × T^2.67
Depth at curb d (ft) = T × Sx
Flow area A (ft²) = T × d ÷ 2
Velocity V (ft/s) = Q ÷ A

Surface	Typical Manning’s n
Concrete pavement (HEC-22 default)	0.016
Asphalt pavement	0.016
Concrete gutter with asphalt street	0.017

Notes & limitations

Equation and n values per FHWA Hydraulic Engineering Circular No. 22 (HEC-22), Urban Drainage Design Manual, Chapter 4. The 0.56 constant is for US customary units.

Applies to a uniform cross-section triangular gutter. Composite gutters (parabolic or V-shaped with a depressed gutter section) use the HEC-22 composite-section equations instead.
Spread is usually limited by agency criteria (commonly half a travel lane for the design storm); verify the allowable spread before accepting the capacity.
This gives the full gutter capacity at the entered spread — compare it with the Rational Method runoff arriving at the inlet to check for bypass.

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