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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />.' <br />, I <br />I <br /> <br />by the reservoir. The trap efficiency or percent retention of the in- <br />coming sediments will be high if the reservoir is large and maintained <br />full. The high ratio of reservoir volume to average annual inflow for <br />the Rock Creek reservoir of approximately 2.2 provides a trap effi- <br />ciency of about 9S percent. <br /> <br />The sediment material will not consolidate fully when deposited in a <br />reservoir and this "bulking" causes it to occupy greater reservoir <br />storage space than the volumes computed. Using an 0.8 bulking factor <br />and the 9S percent trap efficiency, the maximum storage volume occupied <br />was estimated to be: <br /> <br />262 acre-feet x 0.9S/0.8 or 311 acre-feet. <br /> <br />In order to round to the nearest values of total storage, a sediment <br />reserve space of SOO acre-feet was selected. This very low volume <br />represents only one percent of the total storage. The predominantly <br />sandy nature of the sediment with low suspended materials will deposit <br />readily in the upper reaches of the reservoir and at the inlets of the <br />numerous feeder creeks that run directly in the reservoir. Very little <br />material will be passed on to downstream channel reaches. <br /> <br />More detailed information regarding sedimentation studies for Rock <br />Creek Reservoir is presented in Appendix B of this report. <br /> <br />3.6 DAM AND APPURTENANT DESIGNS <br /> <br />A. Dam Site <br /> <br />The damsite is located in a narrow section of Rock Creek Canyon about <br />3 miles downstream of the Highway 134 Rock Creek crossing. The stream- <br />bed at the damsite is about 90 feet wide and both abutments slope <br />upward from the streambed at about I.S horizontal to 1.0 vertical. <br /> <br />3 - 16 <br /> <br />681g,.j/S02~ <br />