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• Mr. Richard Mills <br />July 27, 1988 <br />Page 2 <br />flow resistance equation developed by Bathhurst et al. (1979) for flow <br />in high-gradient channels to evaluate flow conditions (velocity, depth, <br />etc.). Rock stability is then evaluated using the safety factor <br />approach (Stevens, et al., 1976). The overall procedure is similar to <br />the methodology utilized in the Office of Surface Mining (OSM) design <br />manual entitled Surface Mining Water Diversion Manual, (1982). A design <br />factor of safety of 1.5 was utilized to size channel riprap. The <br />resultant calculated hydraulic and geometric properties for the <br />reclaimed stream channel are: <br />Channel depth: 2.5 ft. <br />Bottom width: 12 ft. <br />Channel slope: 25% <br />Flow depth: 0.7 ft. <br />Velocity: 11.2 fps <br />Calculations are based upon a maximum gradient of 25% as shown on sheet <br />3. Any increase in actual gradient or discharge could destabilize <br />riprap protection. <br />• Please note that we have specified=a median riprap diameter of 24 <br />inches. This stone size safety factor of 1.5 for the 10-year discharge. <br />Riprap layer thicknesses shoul~be 3.0 feet. These preliminary <br />calculations were based on use of an angular rock Ffich has a relatively <br />high angle of repose. <br />Percent Finer Rock <br />By Weight Diameter <br /> (inches) <br />100 33 <br />35-50 24 <br />10-20 12 <br />2-10 8 <br />Due to the relatively steep slopes and fine-grained nature of the <br />underlying materials, a granular filter layer is required beneath the <br />riprap. Specific criteria for the channels will be dependent on actual <br />postmining channel materials. Generalized criteria for bedding <br />specified in the Urban Storm Drainage Criteria Manual specify either a <br />two layer filter consisting of Type I and Type II bedding meeting <br />. specifications contained in Table 1, or a one-foot thick layer of Type <br />II bedding. <br />