C153629 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I I discharge was reduced from the inflow PMF maximum of 50,900 cfs to a spillway release maximum of 10,200 cfs. The reservoir level would rise 8.8 feet above the normal maximum (spillway crest) during the routing of the PMF. Figure 3-2 illustrates graphically the inflow and outflow relationship during the routing of the PMF. More detailed information regarding the parameters, assumptions, and results of the flood hydrology analysis is presented in Appendix A-1. 3.5 SEDIMENTATION There are many complex variables that determine the production of sedi- ments from a basin and what portion of that production gets retained in a manmade on-stream reservoir. The Rock Creek basin above the damsite has a good forested cover with the more open meadows and valleys having well established grass cover. The annual precipitation is sufficient to maintain this growth and as a result there is little natural ero- sion, and thus a low production of sediments. Average annual sediment yields were estimated by using empirical equa- tions developed for other basins of a similar nature, by examining recently measured stream transport volumes and by applying a unit sedi- ment yield factor developed by the Colorado Land Use Commission. Close comparisons were obtained between measured peak transport amounts dur- ing the runoff season and average annual projections by the unit yield methods. A 50-year period coinciding with normal project life used in planning studies was selected for computing the total volume. The 52-mi2 basin was estimated to yield about 262 acre-feet of sediment in the 50-year period. Depending on the reservoir size, its configuration and the annual operational characteristics, not all incoming sediment will be retained 3 - 15 6819W/502OW