Flowpath Length - The length of the Sheet flow path.
Land Slope - The slope of the hydraulic grade line.
Catchment Retardance - Retardance roughness.
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
tc = time of concentration (mins)