Determination of roughness coefficients for streams in Colorado

Most hydraulic calculations of flow in channels and overbank areas require an evaluation of flow resistance, generally expressed as Manning 's roughness coefficient. The degree of roughness depends on many factors. The report summarizes and relates several methods of estimating roughness and presents additional channel-roughness verification data on higher-gradient streams with slopes greater than 0.002. A procedure is outlined that enables the user to systematically evaluate the factors affecting natural, agricultural, and urban channel and overbank roughness. Two prediction equations are presented to aid in the calculation of coefficients for natural stable channels in which roughness changes dramatically with depth of flow. Roughness coefficients can be determined from low-to-high flow conditions as long as the channel remains fairly stable, sediment concentrations are not so great as to result in mudflows or debris flows, and stream slopes are less than 0.05. Because of extreme turbulence, large energy losses, and hence large roughness coefficients, flow in high-gradient, cobble- and boulder-bed mountain streams generally is subcritical. (USGS)