The Free On-line Aquaculture Dictionary

Coefficient of  roughness (n) for pipes and channels - used in equations for flow calculation such as Mannings equation
Surface Value of "n"
Glazed pipes, plastic pipes, very smooth iron pipes, neat cement surface 0.01
Cast iron, rough brickwork, ordinary concrete 0.015
Channels with brick sides and concrete base 0.015
Earth canals and rivers free of stones and weeds 0.025
Earth canals and rivers with some stones and weeds 0.03
Canals and rivers with rough bases and much vegetation 0.04
Torrential rivers with beds of boulders and detritus 0.05
Channels  with very rough, heavy grasses and reeds 0.06