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<br />I <br /> <br />I <br />I <br />I <br /> <br />I <br /> <br />more resistent sandstones, or by old terrace deposits. The colluvium <br />and some alluvium are suitable for construction of earthfill or <br />rollercrete dams. The alluvial deposits along the Colorado River and <br />in the higher terrace deposits contain suitable material for concrete <br />aggregate and filter material. No major deposits of hard rock <br />sui table for rockfill dams are present in the area. Boulders and <br />cobbles for use as riprap could be processed from the older terrace <br />and some colluvial deposits. <br /> <br />I <br /> <br />3. Dam Types <br /> <br />I <br /> <br />Dam layouts were done at each of the proposed storage <br />locations. Three dam types were considered at each site: an embank- <br />ment (earthfill) dam, a rockfill dam and a roller'-compacted concrete <br />(RCC) dam. Concrete arch dams and concrete gravity dams were judged <br />as uneconomical from the outset of the study. <br /> <br />I <br /> <br />I <br /> <br />The engineering geology inves tiga tions concluded that rock- <br />fill dams were not economical at any of the proposed sites (except <br />possibly the Upper Beaver si te) because high quali ty rock was not <br />available nearby. For this reason no rockfill dam layouts were done. <br /> <br />I <br /> <br />The engineering geology investigations <br />sites as definitely not suitable for RCC dams. <br />alluvial foundations are unable to support <br />pressures exerted by RCC dams. For this reason <br />not done at some locations. <br /> <br />also established some <br />Some sites wi th deep <br />the higher bearing <br />RCC dam layouts were <br /> <br />I <br /> <br />I <br />I <br />I <br />I <br />I <br /> <br />4. Hydrologic and Hydraulic Criteria <br /> <br />The spillway and outlet works preliminary designs and layouts <br />were developed on the basis of simplified criteria, which produces <br />uniformity in the cost estimates. The hydrologic and hydraulic cri~ <br />teria used are summarized below: <br /> <br />a. <br /> <br />Spillway <br /> <br />o <br /> <br />Inflow design flood is the Probable Maximum Flood <br />(PMF) assuming no upstream reservoirs. <br /> <br />o <br /> <br />Flood surcharge limited to 8 ft whenever possible. <br /> <br />o <br /> <br />Freeboard maintained during PMF is 1 ft. <br /> <br />o <br /> <br />Spillway discharge limited to 100 cfs/ft of width. <br /> <br />I <br />I <br /> <br />IV-3 <br /> <br />I <br />