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<br />-' <br /> <br />4.4 DESCRIPTION OF DAM <br />An arch dam was selected for the purposes of these studies because of the <br />suitable canyon topography and foundation conditions for an arch dam, the ability <br />of this design to incorporate the spillway and outlet works as part of the dam <br />structure, and the local availability of aggregate for concrete. A typical dam <br />layout for the Centennial damsite is shown in Figure 4.1. <br /> <br />The spillway is sized to pass the probable maximum flood (PMF), as required <br />by the regulatory agencies. The spillway would be incorporated into the central <br />portion of the dam. A large capacity outlet works would be provided to minimize <br />the frequency of spillway use and to permit large controlled releases from the <br />dam. Reservoir outlet gates located at several levels, would allow for selective <br />releases from different depths in the reservoir. Construction of the dam is <br />estimated to require about five years. <br /> <br />4,5 GEOLOGY <br />Preliminary geological investigations were conducted for a dam located at <br />the Centennial site. Criteria used to identify this site included canyon shape, <br />abutment weathering, bedrock jointing frequency, and the direction of bedrock <br />jointing in relation to the geometry of the dam. <br /> <br />Site i dent i fi cat i on was based upon revi ew of exi st i ng geo 1 ogi c maps and <br />~iterature, review of existing aerial photos, a helicopter aerial reconnaissance <br />of the canyon damsites, and brief field inspections. No subsurface geotechnical <br />investigations were conducted as part of this study. The damsite and reservoir <br />site are underlain predominately by crystalline rocks, typically banded gneisses, <br />with some granites and pegmatite dikes and veins. Preliminary geological <br />reconnaissance indicates that the Centennial site would be suitable for the <br />construction of a major storage facility. <br /> <br />""" <br /> <br />Virtually any reservoir of the sizes being contemplated in the canyon would <br />overlie a fault zone. In the case of the Centennial site, the reservoir overlies <br />the Black Hawk Fault. This is a vertical fault crossing the creek bed <br />approximately one-half mile upstream of the Centennial dam axis. According to <br />the Colorado Geological Survey, the Black Hawk Fault is not potentially active <br /> <br />'- <br /> <br />4-2 <br />