hydraulics
The Manning formula is an

engineering formula useful for determining the velocity of water flowing in an open channel. It is

empirical, and relies on the semiperimeter R, defined as (A/P) where A is the cross sectional area of flow and P is the wetted perimeter:

R^(2/3)
V= --------- S_{o}^(1/2) (S.I. Units)
n

where S

_{o} is the slope and n is a special

*roughness factor* (the empirical

fudge factor). Manning's n for various surfaces:

Concrete culvert, clean 0.011
Concrete culvert, rough 0.017
Earth, straight and uniform 0.022
Earth bottom, rubble sides 0.030