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",-. - 24 - I g t l I t ~ In computing the sediment storage capacity required in the proposed floodwater-retarding structures, consideration was given to the amount of reduction of sediment production which will result from proposed land treatment measures to be installed in the ,,,atershed, It is estimated that these measures will reduce the sediment storage at the proposed reservoirs by ~n average of about 15 per cent, Consideration was given to the fact that some land treaL~ent measures in this plan will become effective immediately after construction, while others will require several years from the start of the project to become effective. A sediment trap efficiency of 90 per cent was assumed for all floodwater-retarding structures, Capacity equivalents for sediment storage in the proposed reservoirs, as finally estimated, ranged from 0,12 to 0,30 inches (Table 3), I 1< t I Field data were obtained on rates and amounts of past floodplain damages along Cherry Creek and its tributaries from sanding, scouring, streambaruc erosion, and chill\nel entrenchment. Estimates of future rates of floodplain damage from erosion and sedimentation, both with and ,.!ithout a "atershed project, were based on the size and frequency of flood flows to be expected during the life of the project. , Geologic Investigations . , A brief geologic reconnaissance was made of all proposed dill" sites. No unusual geologic or soils conditions that might result in serious construction problems were observed. Conditions are generally favorable for vegetated earth emergency spillways, One or two sites may require some rock excavation in the emergency spillway area. Materials underlying the dam sites will usually consist of clayey sand or sandy clay bedrock overlain by alluvial deposits consisting mainly of sand and gravel, At several of the sites where a high water table is present, foundation drains will be necessary. Adequate amounts of suitable borrow material are expected to be available at all sites, J ~ t I (-: , t f ~ It is expected that geologic conditions at the structure sites in this watershed lrill be generally similar to those encountered at the sites in the West Cherry Creel, Vlatershed, Complete foundation and borrow area investigations have been made at most of the vlest Cherry Creek sites and soil samples have been analyzed by the Soil Nechanics Laboratory, Detailed foundation and borrow area investigations, including exploration with power auger or core drill, ,rill be made at all dam sites prior to design and construction, Laboratory tests will be made to determine the stability and suitability of foundation materials and the suitability of the earth materials to be used in the ~"bankment of the dams, Emergency spillways will be under-excavated and then back-filled and compacted lnth soil suitable for supporting a vegetative cover,