Kerby's Equation

Parameters

Flowpath Length - The length of the Sheet flow path.

Land Slope - The slope of the hydraulic grade line.

Catchment Retardance - Retardance roughness.

Calculation

where:

L  = Length of overland flow (ft)

S = Average catchment slope (ft/ft)

n  = Retardance Coefficient

Smooth pavement 0.02

Poor grass, bare soil 0.30

Average grass 0.40

Dense grass 0.80

t_c = time of concentration (mins)