CEMP-E
EI 11C201
1 March 1997
(1) Roughness coefficient. Values of n to be used in the formula range from 0.013
to 0.015. The lowest n values apply to new or relatively new pipe (in sections greater than 1.5 m
(5 feet)) with smooth interior surfaces, smooth bore, even joints, in excellent to good condition
and well constructed. Higher n values are required for older pipe with rough interior surfaces,
open or protruding joints, in fair to bad condition and poorly constructed. Values up to 0.017 are
often justified for very old pipe (such as brick or block sewers) in extreme deterioration, or pipe
very poorly constructed with improper alignment, sags and bellies, cracked or offset joints,
broken wall sections or internal corrosion. Some manufacturers of plastic and asbestos cement
pipe report n values of 0.009 to 0.011. However, due to uncertainties in design and construction,
plus a desire to provide a margin of safety, n values smaller than 0.013 will not normally be
permitted. Variation of n with depth of flow has been shown experimentally, and may be
considered in designing sewers to flow partially full. A solution to the Manning formula for full
pipe flow is shown in figure 3-1.
Figure 3-1. Chart for Manning formula
3-5
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