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<br />.. . <br /> <br />.' <br />. <br /> <br />17r.1"'l <br />- ,,(. <br /> <br />that the majority of this material. tends to accumulate along the base of- <br />hills lopes and never reaches the channel. Consequently, potential aggrada-. <br />tion under reduced - flow conditions downstream of a project depends pri- <br />marily on 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 proV'ides a ". . . self regulation <br />of sediment supply to the quanticy 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 F.ig. 1).. _ -Therefore, the self-regulating mechanism discussed above <br />would .control sggradation of coarse material in the reach between the <br />proj ect 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 concerng to be addressed from application of the <br />channel maintenance procedure are transport of fine ..gravels snd sand <br />particles and vegetation encroachment. <br /> <br />SEDIME~7 SUPPLY AND TRANSPORT CAPACITY IN ROCK CREEK <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 />3-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 />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 material systems <br />with little suspended load considering the data used in its development <br />(4) and subsequent evaluation 0,6). 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 (1.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 (I) 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 sedi!:1ent supply (1.e.. channel conditions pro- <br />vided excess transport capacity). Results for the coarse sand size <br />