Flow Length - The length of the Sheet flow path.
Land Slope - The slope of the hydraulic grade line.
Catchment Roughness - Horton’s Roughness value.
B = Const (107 metric)
S = Average catchment slope (m/m)
L = Length of overland flow (m)
n = Horton's roughness (similar but not identical to Manning’s n)
Bare Soil .0275
Poorly Grassed .035
Average Grass .045
Dense grass 0.06
tc = time of concentration (mins)