The Effects of Water Surface Profiles on Manning's Roughness Coefficient

by P. Michael DePue, II, (M.ASCE),
Ta Wei Soong, (M.ASCE),

Document Type: Proceeding Paper

Part of: North American Water and Environment Congress & Destructive Water


The Manning Equation has wide application in engineering practice. Nonetheless, certain aspects of it are still not widely understood. Often, engineers deduce the value of Manning's Roughness coefficient from measurements of the water surface elevation and flow rate in a channel. This paper shows that engineers must consider the effects of the water surface profiles when making these calculations. Laboratory data is presented showing the influence of backwater effects on the measured roughness coefficient. It is demonstrated that an Ml water surface profile similar to that found near a dam can cause the back-figured roughness coefficient to be in error by as much as 23%. Clearly this level of error must be accounted for in calculations. The authors briefly discuss the ongoing development of a procedure to correct for this error.

Subject Headings: Hydraulic roughness | Water surface profiles | Flow profiles | Laboratory tests | Flow rates | Flow measurement | Channel flow

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