The Manning Equation and Boundary Resistance

Uniform full pipe flow was selected to review application of the Manning equation and its use in predicting flow conditions. Manning's n was determined not to be a function of boundary roughness alone; it was found to increase with increasing conduit size and effective roughness and to decrease with the Reynolds number. The selection of correct effective roughness values for design is important to correct assessment of boundary resistance to flow and, for concrete-walled conduits, is one to two orders of magnitude less than values often attributed to that material.