Flow Length - The length of the Sheet flow path.
Land Slope - The slope of the hydraulic grade line.
Event Intensity - The intensity of the rainfall.
Catchment Retardance - Retardance coefficient.
L = Length of overland flow (ft)
S = Average catchment slope (ft/ft)
i = Rainfall Intensity (in/hr)
Cr = Retardance Coefficient
Very smooth asphalt 0.007
Tar and sand pavement 0.0075
Tar and gravel pavement 0.017
Short grass 0.046
Dense grass 0.06
tc = time of concentration (mins)