Manning's Formula for Discharge in CPHEEO Manual on Sewerage and Sewage Treatment Second Edition Ministry of Urban Development, New Delhi, Dec 1993 Pg 46

Q = [3.118 x 10^(-6) x d^(2/3) x S^(1/2)] / n is incorrect may be typographical error

it should be

Q = [3.118 x 10^(-6) x d^(8/3) x S^(1/2)] / n

Where

Q = discharge in litres per second
S = Slope
d = diameter of pipe in mm n= Manning’s coefficient of roughness

Regards
Linesh Chungath
India