Manning's Coefficient of Roughness (n) Discussion
The value of 'n' for smooth bore pipe is affected by depth of flow, velocity of flow and quality of construction. In controlled experiments, using clean water, values of 'n' under 0.009 have consistently been obtained for vitrified clay pipe and some other pipe materials. Many design engineers recommend that a more conservative value of 'n' be used in design because of (1) the buildup of slime and grease on all pipe surfaces, (2) the variations in 'n' due to variable flow conditions, (3) the deposition of debris, grit and other foreign materials which find their way into a sewer system, (4) the loss of hydraulic capacity of flexible pipe due to ring deflection and (5) misalignment due to construction or settlement. Based upon current data, it appears that 'n' values of 0.009 - 0.013 can be applied to all types of smooth bore pipe. After pipe lines have been in place for several years, measurements may indicate 'n' values which differ from the design value.
