The internal surface of the PE pipe is smoother than that of concrete or clay pipe. For gravity applications, after some algebraic manipulation to the following Chezy-Manning equation, it can be demonstrated that the flow capacity of the PE pipe is 44% greater than that of concrete or clay pipe, considering the internal diameter of the old clay or concrete pipe equal to that of the replacement PE pipe.


where Q = the flow quantity, n = Manning roughness coefficient, A = the area of the pipe, rH = hydraulic radius, S = the slope of the energy line, which is parallel to the water surface and pipe invert if the flow is uniform. The n value ranges between 0.012 and 0.015 (on average about 0.013) for clay or concrete pipes, and is approximately 0.009 for PE (Lindeburg 1992).