Constant vs. Varying Roughness Coefficient
The Manning Equation can be applied using three methods, including the Design Method, the Lanfear-Coll Method, and the Stevens-Schutzbach Method. The roughness coefficient used in these methods is often treated as a constant value. However, historical research has shown that the roughness coefficient actually varies as a function of flow depth.
All three methods can be modified to use a varying roughness coefficient. The use of a constant or varying roughness coefficient can impact sewer capacity estimates by over 20%. Both versions are used in examples on this website and are identified using dashed or dotted lines as shown here.
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A technical paper describing the use of a varying roughness coefficient in more detail is available for download.
This technical paper was presented at the American Society of Civil Engineers Pipelines 2005 Conference. This conference was held in Houston, Texas on August 21-24, 2005.