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 Retardance

Retardance coefficient.


Calculation



Where:

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

Concrete 0.012

Tar and gravel pavement 0.017

Short grass 0.046

Dense grass 0.06

 t= time of concentration (mins)