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<br />.... <br />Field surveys and 75 current-meter measurements of discharge were <br />'" <br /> <br />made at 21 steep-gradient natural stream sites in the Rocky Mountain of <br /> <br />Colorado for the purpose of computing channel roughness by the Manning <br /> <br />formula and to evaluate other hydraul ic characteristics of steep-gradient <br /> <br />streams. These data indicate that Manning's roughness coefficient <br /> <br />decreases markedly with depth and increases with friction slope. <br /> <br />Multiple regression techniques were used to develop a method for pre- <br /> <br />dicting Manning's roughness coefficient from the easily measured friction <br /> <br />(or water) slope and hydraulic radius (or hydraul ic depth). The average <br /> <br />standard error of the prediction equation is 28 percent and ranges from <br /> <br />-24 to +32 percent. The equation appears to underestimate the higher <br /> <br />roughness values and ::0 overestimate the velocity and discharge of low <br /> <br />flows. The equation \~as verified using other available data on steep- <br /> <br />gradient s~reams. Substitution of the prediction equation into the <br /> <br />Manning equation yields regime equations that can be used to estimate <br /> <br />velocity and discharge without requiring a subjective estimate of <br /> <br />channel roughness. The limitations of the prediction equation and <br /> <br />regime equations are: <br /> <br />1. They are appl icable to natural main channels~aving relatively <br /> <br />stable bed and bank materials and average factors affecting roughness <br /> <br />coefficients. <br /> <br />2. The equations should be used within a range of slope from 0.002 <br /> <br />to 0.04 and a hydraulic radius from 0.5 to 7 ft (0.15 to 2.1 m). <br /> <br />5"S- <br />