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<br />DETERMINATION OF ROUGHNESS COEFFICIENTS FOR STREAMS IN COLORADO <br /> <br />By Robert D. Jarrett <br /> <br />ABSTRACT <br /> <br />Most hydraulic calculations of flow in channels and overbank areas <br />require an evaluation of flow resistance, generally expressed as Manning's <br />roughness coefficient, n. The degree of roughness depends on many factors. <br />The report summarizes and relates several methods of estimating roughness and <br />presents additional channel-roughness verification data on higher-gradient <br />streams with slopes greater than 0.002. A procedure is outlined that enables <br />the user to systematically evaluate the factors affecting natural, <br />agricultural, and urban channel and overbank roughness. Two prediction <br />equations are presented to aid in the calculation of roughness coefficients <br />for natural stable channels in which roughness changes dramatically with depth <br />of flow. Roughness coefficients can be determined from low-to-high flow <br />conditions as long as the channel remains fairly stable, sediment concen- <br />trations are not so great as to result in mudflows or debris flows, and stream <br />slopes are less than 0.05. Because of extreme turbulence, large energy <br />losses, and hence large roughness coefficients, flow in high-gradient, cobble- <br />and boulder-bed mountain streams generally is subcritical. <br /> <br />INTRODUCTION <br /> <br />Purpose and Scope <br /> <br />The purpose of this report is to provide guidelines in the evaluation and <br />limitations of flow resistance in hydraulic analysis on streams in Colorado. <br />First, this report describes the requirements for locating cross sections and <br />reaches for a hydraulic study. Secondly, the report describes the factors <br />that affect the roughness characteristics of channels and overbanks. Thirdly, <br />flow resistance is described for the channel and overbank of a stream in the <br />natural, agricultural, and urban condition. Fourth, a systematic procedure <br />for calculating and selecting appropriate roughness coefficients is presented <br />in addition to examples. The report incorporates additional channel-roughness <br />verification data from a study of higher-gradient streams typical in Colorado. <br /> <br />Acknowledgments <br /> <br />The author gratefully acknowledges assistance of Eugene I. Jencsok, <br />Colorado Water Conservation Board, for help with project planning, data <br />collection, and report preparation. Many helpful review comments were made by <br />William P. Stanton, Colorado Water Conservation Board. <br /> <br />I <br />