Parameters

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 Roughness

Horton’s Roughness value.

Calculation

Where:

L  = Length of overland flow (m)

S = Average catchment slope (m/m)

i  = Rainfall Intensity (mm/hr)

n  = Surface roughness (similar but not identical to Manning’s n)

Concrete or Asphalt 0.010 - 0.013

Bare Sand 0.010 - 0.016

Graveled Surface 0.012 - 0.030

Bare Clay-Loam Soil (eroded) 0.012 - 0.033

Sparse Vegetation 0.053 - 0.130

Short Grass Prairie 0.100 - 0.200

Lawns 0.170 - 0.480

 t_c = time of concentration (hrs)