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<br />The equation for n, using the intermediate diameter is <br /> <br />n = <br /> <br />(0.0926)R1/6 <br />R <br />1.16 + 2.0 log ---d <br />84 <br /> <br />(5 ) <br /> <br />where <br /> <br />d84 = the particle intermediate diameter, in feet, that equals or <br />exceeds that of 84 percent of the particles. <br /> <br />I, <br />i <br /> <br />Limerinos (1970) selected reaches having a minimum roughness other than that <br />caused by bed material. Therefore, his values correspond to the base values <br />given by Benson and Dalrymple (1967) and shown in table 2. <br /> <br />Equation for predicting roughness coefficients of higher-gradient natural <br />channels <br /> <br />Existing guidelines for selecting roughness coefficients were developed <br />primarily for lower-gradient streams having relatively large flow depths. <br />Many streams in Colorado have higher-gradient channels (slopes greater than <br />0.002) and shallow flow depths. In these streams, most of the flow is in the <br />ma in channe 1. <br /> <br />Available verified data are very limited for high-gradient streams <br />(slopes greater than 0,01); however, many streams channels in Colorado are <br />much steeper. Estimating n values using existing guidelines are difficult for <br />these steeper streams. Therefore, for this report, additional verification <br />data were collected on streams with slopes greater than 0.002 in Colorado to <br />complement existing data. This report summarizes hydraulic studies of hydrau- <br />lics on high-gradient streams (Jarrett, 1984). <br /> <br />Current-meter discharge measurements and onsite surveys were made at <br />21 higher-gradient natural stream sites in Colorado to compute channel rough- <br />ness by the Manning formula, These sites, shown in figure 6, were selected to <br />provide a wide range in channel types and flow depths and to represent average <br />main channel flow resistance. Photographs of typical higher-gradient stream <br />channels studied are shown on figures 7 to 9. The maximum discharges at these <br />sites were equivalent to floods having about a 1 to 25-year recurrence <br />interval, A detailed description of fieldwork and computational procedures <br />used in computing the discharges are discussed in Benson and Dalrymple (1967), <br />Wolman (1954), and Barnes (1967). <br /> <br />23 <br />