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<br />"'":\ <br />. . I <br /> <br />Woodward.clyde <br />Consultants <br /> <br />1.0 <br />INTRODUCTION <br /> <br />Beaver Reservoir Dam, located just outside of Paonia, Colorado, has been placed on <br />restricted storage by the Colorado Office of the State Engineer due to the recent <br />discovery of new sinkholes in the reservoir bottom just upstream of the embankment and <br />possibly in the upstream shell ofthe embankment. Woodward-Clyde Consultants (WCC) <br />plans to conduct an investigation of selected portions of the embankment, foundation, <br />and impoundment to determine the probable causes and sustaining mechanisms of the <br />sinkholes. The results of this investigation will be used to evaluate remedial alternatives <br />and develop a preliminary remedial design for the preferred alternative which will <br />address the dam safety issues raised by the Office of the State Engineer. WCC will also <br />develop a construction cost estimate and a financial program for the repair work. The <br />results of these studies will be presented in a Feasibility Study report for submittal to the <br />Colorado Water Conservation Board (CWCB). <br /> <br />The causes of both the historic and existing sinkholes at Beaver Reservoir are not known <br />with reasonable engineering certainty at this time. Based on discussions with <br />representatives of the Beaver Reservoir Company (BRC) and a review of the Colorado <br />State Engineer's files, we understand that sinkholes started to appear approximately <br />twenty-eight years ago and have reappeared fairly regularly since. The sinkholes have <br />been backfilled with sandy materials and capped with clayey materials but continue to <br />reappear. <br /> <br />Embankment and foundation sinkholes are co=only the result of seepage and piping <br />of soil particles into and through coarse embankment zones, outlet conduits and/or <br />coarse or fractured foundation materials. Based on review of the historical record there <br />is no indication that sinkholes have developed solely through the embankment. Also, <br />there is no indication of sinkhole development associated with flow into the outlet <br />conduit from the surrounding embankment materials. The historic sinkholes have <br />appeared in the natural foundation materials just upstream of the embankment on both <br />abutments. These sinkholes were probably created in response to soil particles being <br />transported by seepage of reservoir water into the natural foundation materials. <br /> <br />Q1D2:7Pf~:7 06-2.5.Q2(IO; 17pm./PROJ54 <br /> <br />-1- <br />