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Table 4.--Correlation coefficients for selected hydraulic characteristics [Coefficients were computed using untransformed data] Manning's Friction Water Bed mate- Hydraulic Hydraulic Dis- coefficient, slope, slope, rial size, radius, depth, charge, n Sf S d84 R D Q w n----- 1.00 0,71 0.68 0.64 -0.09 -0,04 -0.23 S ---- 1.00 ,99 .66 .02 .07 - ,12 f S __n 1.00 .62 - .02 ,04 -,14 c:fsc- 1.00 .33 .39 .12 Rn--- 1.00 .99 .91 Dn--- 1.00 ,88 Q----- 1.00 The method for predicting channel roughness uses multiple-regression analysis, which relates Manning's roughness coefficient to easily measured hydraulic characteristics of Colorado streams. The hydraulic characteristics used in the equation for predicting Manning's n for higher-gradient natural channels are defined below: Energy gradient, Sf--The energy gradient or friction slope is the slope of the energy line of a body of flowing water. The Manning equa- tion was developed for conditions of uniform flow in which the water and friction slopes are parallel, For these data, this condition was met because the two variables had approximately the same value, The correla- tion between the water and friction slopes was 0.99; therefore these two variables can be used interchangeably in the Manning equation provided the reach is reasonably uniform; Hydraulic radius, R--Hydraulic radius is a measure of the boundary area causing friction per unit of flow and is computed as the cross- sectional area of a stream of water perpendicular to flow divided by the wetted perimeter, Cross-section area and wetted perimeter are obtained from onsite surveys. By standard practice, the wetted perimeter does not include surface irregularities of submersed stream-bed particles. Hydraulic depth, D, the cross-sectional area divided by top width, gener- ally can be used in place of hydraulic radius in uniform reaches because the two variables had approximately the same value, and the correlation coefficient between them was 0.99. ". 34