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<br />":l <br /> <br />Ms. Honey R. Moschetti, President <br /> <br />-8- <br /> <br />December 19, 1994 <br /> <br />analyzed, with appropriate factors of safety. The computed stresses indicate that the upstream <br />face of the raise section would be in compression for reservoir water levels up to about the 100- <br />year runoff event (Loading Cases 1,2, and 3). For overtopping depths much greater than about <br />2 feet (Loading Cases 4 and 5) and for the ice loading case (Loading Case 6), the computed <br />stresses indicate that the concrete would be in tension at the upstream face. The calculated <br />tensile stresses at the base of the raise section indicate that it would not bear on the original dam <br />along the full base width of the raise section. This condition would reduce the sliding stability <br />of the raise section by reducing the bearing area contributing to the static frictional resistance at <br />the base. For simplicity, this reduction in sliding stability was not considered in our analyses. <br /> <br />CONCLUSIONS AND RECOMMENDATIONS <br /> <br />Conclusions <br /> <br />The overall condition of DeWeese Dam appears to be good, with the exception of the upper 12 <br />feet of the dam. The upper 12 feet of the dam, comprising the raise section completed in the <br />1930' s, appears to be in poor to very poor condition. This section of the dam is exhibiting <br />considerable weathering and erosion damage. Additionally, there is evidence on the surface of <br />the concrete of appreciable seepage through the section and at the base of the section. Results <br />of core testing performed in 1986 indicate that the concrete in the lower four feet of the raise <br />section has a relatively low compressive strength. <br /> <br />Based on the information presented in this report, the raise section of the dam appears to be <br />structurally stable for overtopping depths of at least two feet. This conclusion is consistent with <br />the results of previous simplified stability analyses performed by GEl in 1987. An overtopping <br />depth of 2 feet would result from a flood or runoff event having a recurrence interval of about <br />100 years. The raise section may be capable of withstanding overtopping depths of more than <br />two feet. The analyses presented in this report do not allow an accurate prediction of the actual <br />amount of overtopping for which the raise section is structurally stable. However, based on the <br />simplified analyses and assumptions presented in this report, we believe that it is unlikely that <br />the raise section would be structurally stable for overtopping depths resulting from the current <br />estimate of the 50 percent PMF. A flood of this magnitude would overtop the dam by more than <br />20 feet. <br /> <br />To satisfy current Colorado dam safety standards, DeWeese Dam must be capable of safely <br />withstanding passage of the 50 percent PMF. According to state regulations, this requirement <br />can be reduced if analyses demonstrate that no additional potential for loss of life or property <br />damage would result from failure of the dam at a lesser flood event. The small reservoir volume <br />in comparison to the volume and peak discharge of the 50 percent PMF suggests that a flood less <br />than the 50 percent PMF may be acceptable as the inflow design flood for DeWeese Dam. <br /> <br />94311\DDREPORT.L 1R <br />