Kerby’s Formula

Generally, this method applies for lengths less than about 1200 ft.

Where:

• L=Length of flow path (ft)
• S=Surface slope (ft/ft)
• n=Coefficient (see accompanying table)
• tc=in minutes.

Typical values of n:
Smooth impervious surface	0.02
Smooth bare packed soil	0.10
Poor grass, cultivated row crops	0.20
Moderately rough bare ground	0.20
Pasture or average grass	0.40
Deciduous timberland	0.60
Conifer timberland or dense grass	0.80