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10 Feasibility Report for Dam Safety Modifications to DeWeese Dam April 22, 1997 ,-.----- '.... \.\.: Load, COmbination Slldiii \Re~uire~Factoi'sof~afety: ......Ten..ilet" . Stress Usual Normal Reservoir Pool Normal Tailwater Flood Reservoir Pool Flood Tailwater Normal Reservoir Pool Normal Tailwater Design Earthquake 1.0 N/A(2) 3.0 3.0 Unusual 2,0 2,0 2,0 Extreme 1,0 1.0 (1) Computed stresses must not be greater than the material allowable stresses divided by the required factor of safety. (2) Tensile stresses are not allowed for the usual load combination. Stability analyses for the design alternatives were performed using classical two-dimensional gravity analysis methods [5]. This approach results in a reasonably conservative design of the rehabilitation alternatives, and neglects any benefits from potential arch-action in the dam and crest. Hydrologic performance criteria for dams in Colorado are established by the SEa, These criteria dictate the flood event which a dam must be capable of withstanding with acceptable stability factors of safety, according to the dam's hazard classification [6]. The SEa has classified DeWeese Dam as an Intermediate, Class II dam. As such, the dam must be capable of safely passing the runoff from 50 percent of the Pl\1P, The term safely passing in the SEa criteria establishes the direct relationship between hydrologic performance and structural performance of the structure, Hydrologic studies performed by GEl [4], have estimated that the inflow design flood (IDF) from the 50 percent Pl\1P produces a peak discharge at the dam of 126,000 cfs, with an overtopping depth of 17.9 feet over the dam crest. As a result of the significant level of overtopping during the IDF relative to the 12-foot-high crest section and overall height of the dam, the flood loading condition controlled the design of the alternatives presented in this report, 3.3 ALTERNATIVE NO.1 - CREST REPLACEMENT Alternative No, 1 consists of the complete removal of the existing 12-foot-high crest section and construction of a new crest section, The new crest section would have a structural height of 12 feet (consistent with the existing crest raise sections), a vertical upstream face, a 0.5H:IV (horizontal-to-vertical) downstream face, a crest width of 5 feet, and a base width of 11 feet. The new crest section would include reinforcing bar anchors drilled and grouted into the original dam, and a drainage/seepage control system, ~ GEl Consultants, Inc, 96006\REPORTS\FEAS\TEXT.NEW I I I I I I I I I I I I I I I I I I I