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<br />. <br /> <br />2~8~ <br /> <br />Narrows Reservoir <br /> <br />Narrows Reservoir will back water up the South Platte River a <br />distance of approximately 18 river miles at the top of the flood <br />control pool. The reservoir will have a surface area of 25,245 acres <br />and a total capacity of 973,185 acre-feet of storage at the top of <br />flood control pool, elevation 4428.5 feet. Of this capacity, <br />475,000 acre-feet is allocated to flood control, 75,000 acre-feet <br />to joint-use by flood control and irrigation, 373,025 acre-feet <br />to irrigation and 50,160 acre-feet to inactive and dead storage. <br />The reservoir will also provide 575,373 acre-feet of surcharge <br />from the top of the flood control pool to maximum water surface <br />elevation 4447 feet. See exhibit 6. <br /> <br />Dam Embankment <br /> <br />. <br /> <br />The dam embankment will consist of two pervious outer zones of <br />sand and gravel fill material supporting an inner zone of relatively <br />impervious-selected material. The inner zone will contain approxi- <br />mately 4,400,000 cubic yards of compacted earthfill embankment. <br />Approximately 3,900,000 cubic yards of sand and gravel will be used <br />in the pervious upstream and downstream outer zones. The material <br />is available from borrow areas near the dam. Selected embankment <br />material will be compacted in 6-inch layers by tamping rollers. <br /> <br />The upstream face of the dam will be protected by a three- <br />foot layer of riprap on an 18-inch gravel blanket. The downstream <br />slope will be protected with 12 inches of topsoil seeded to grass. <br />Total volume of the embankment, including riprap, bedding and <br />topsoil is about 8,840,000 cu. yds.. The Feasibility Design Draw- <br />ing, Exhibit 5, shows the dam section in more detail. <br /> <br />Foundation Explorations <br /> <br />. <br /> <br />Eleven holes were drilled along the axis of the Narrows damsite <br />by the Bureau of Reclamation for the Corps of Engineers in 1943 and <br />1944. In October 1947, the Bureau began extensive subsurface invest~ <br />gations to determine the character of the foundation materials, the <br />seriousness of abutment leakage, and the coefficient of permeability <br />of the channel-fill material. A bedrock surface contour map and a <br />geologic cross-section along the axis were drawn from the geologic <br />data provided by the drill-holes. A series of water level measure- <br />ments in the drill-holes furnished sufficient additional data to <br />draw a water table surface contour map. The location of drill-holes <br />and a geologic section along the axis of the dam are shown on <br />exhibit 7. <br /> <br />17 <br />