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<br />'. . <br /> <br />. <br />.. <br /> <br />.. <br /> <br />001555' <br /> <br />. <br /> <br />. <br /> <br />.. ~hat the,:.majority of. this material t.ends to accumulate along :the base: .of: <br />hillslopes and never reaches, the channel. Consequently, potential:aggrada- <br />'tion under reduced' flow conditions downstream of a proj ect depends pri- <br />marilyon tributary sources of. coarse material. In the absence of signifi- <br />cant tributary supply, reduced discharges in the main channel reduce the <br />bed-load transport rate and the corresponding supply of coarse material <br />from the channel. As a result, no aggradation of coarse material occurs. <br />As concluded by Andrews," this mechanism provides. a" . . self regulation <br />of sediment supply to the quantity transported. . . ." <br /> <br />,.In the case of 'Rock Creek, the only major tributary entering below the <br />project is, Egeria Creek, which is well below the Forest Service boundary <br />(see Fig., 1),. . .Therefore, the self-regulating mechanism discussed above <br />would 'control aggradation of coarse material in the reach between the <br />project and Egeria' Creek. Consequently, aggradation "of coarse material <br />within the Forest boundary is not expected to occur. These conclusions <br />'.:.:.,...,suggest that the primary concerns'. to be addressed from application of the <br />channel maintenance procedure 'are transport of. fine..gravels:, and. sand <br />particles and vegetation encroachment. <br /> <br />SEDIMENT SUPPLY AND TRANSPORT CAPACITY IN ROCK CREEK <br /> <br />- <br />-- <br /> <br />,'During May 1985 the bedload and suspended load of Rock Creek were <br />: .measured seven times by the USFS. Bedload measurements were made with a <br />J-inch' Helly~Smith bedload sampler and suspended load samples with depth <br />integrating suspended sediment samplers. Bed load samples were analyzed <br />for particle size gradation, 'allowing determination of bed load transport <br />by size fraction. Data collection and analysis followed accepted USGS <br />guidelines. Although these results span only one runoff season, they are <br />indicative of the sediment supply provided to Rock Creek. <br />,. , <br /> <br />To estimate the transport capacity of Rock Creek, calculations were <br />made based on a combination of the. Einstein .suspended load calculation <br />,(2) and the Meyer-Peter, Muller (MPM) bedload equation (4). While a <br />certain amount of variability exists in any sediment transport calculation, <br />. this methodology. has. been. extensively applied and' .verified. The'. MPM <br />'~:,bedload . equation is suitable for' application to coarse. mate,rial systems' <br />with little suspended load considering the data used in its development <br />(4) and subsequent evaluation (3,6). The MPM equation was empirically <br />derived from experiments with both uniform and non-uniform sediments of <br />various density and channel slopes ranging from 0.04 to. 2 percent.] () <br />Research by Smart (5) indicated that the plane bed form of the MPM equation ~ <br />(used in this analysis) is applicable to steep slope channels (i.e., up to _ <br />.20.percent). <br /> <br />For application to Rock Creek, transport capacity calculations were <br />calibrated against the measured data by assuming that the supply of the two <br />coarsest size fractions (greater than 16mm and 8-16mm) were in equilibrium <br />with the transport capacity of that size fraction. This assumption is <br />supported by the conclusions of Andrew (1) as discussed above. Application <br />of the calibrated sediment transport relationships indicated that for all <br />size fractions (other than the two assumed in equilibrium) the transport <br />capacity greatly exceeded sediment supply (1.e., channel conditions pro- <br />vided excess transport capacity). Results for the coarse sand size <br />