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n = Manning's roughness coefficient (see Pipe Roughness),

R = Hydraulic Radius,

S = Hydraulic gradient.

The equation is in metre-per-second units but the n values are the same as those specified in the foot-per-second format of the equation (e.g. smooth concrete pipe n = 0.012 approximately).  The formula is sometimes known as Strickler's formula and 1/n as the Strickler's coefficient.