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VELOCITY PROFILE, WATER-SURFACE SLOPE, AND BED-MATERIAL SIZE FOR SELECTED STREAMS IN COLORADO By Jean P. Marchand, Robert D. Jarrett, and Larry L. Jones ABSTRACT Existing methods for determining the-mean velocity in a vertical do not address the conditions present in high-gradient, shallow-depth streams common to mountainous regions such as Colorado. This report presents velocity- profile data that were collected for 11 streamflow-gaging stations in Colorado using both a standard Price type AA current meter and a prototype Price Model PAA current meterl. Computational results are compiled that will enable mean velocities calculated from measurements by the two current meters to be com- pared with each other and with existing methods for determining mean velocity. Water-surface slope, bed-material size, and flow-characteristic data for the 11 sites studied also are presented. INTRODUCTION Purpose Conventional current-meter measurement of discharge involves summing the discharge computed in each of 25 to 30 subsections spanning the stream cross section (Rantz and others 1982). The subsection discharges are computed by multiplying the subsection area by the corresponding subsection mean velocity. Accurate stream discharge measurement depends on several factors including reliable determination of subsection mean velocity and depth, and also the number of subsections. This report presents data associated with mean velo~ i1ptprmin~tion ----- -- - - - --- High-gradient and shallow-depth streams such as those common to the mountainous regions of Colorado are characterized by very turbulent flow and relatively large bed material. Limited velocity-profile data was collected in high-gradient streams (slopes greater than 0.01) during previous studies (Jarrett, 1984). Jarrett's study indicated that the velocity profiles, or vertical-velocity curves, associated with these streams were nonlogarithmic. Most methods for determining mean velocity assume a logarithmic velocity profile. These methods may underestimate the mean velocity of nonlogarithmic velocity-profile data on high-gradient streams. Previous velocity-profile investigations usually have been limited to smooth flumes or moderately sloped, deep streams (O'Brien, 1937; Tracy and Lester, 1961; Savini and Bodhaine, 1971; Sarma and others, 1983). However, Bridge and Jarvis (1977) lAny use of brand names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. 1