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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />the grass cover is sparse owing to the inert nature of the sand and gravel. <br />Numerous erosion rivulets exist on the downstream face. <br /> <br />1.2.5 Seepage <br /> <br />Uncontrolled seepage is one of the primary concerns at Peterson Dam that has <br />led to the present restriction at elevation 9498 (G.H. 20). Seepage <br />measurement weirs have been in place for years and the City inspects the dam, <br />reads piezometers, and observes seepage on a daily basis when the reservoir is <br />being operated. Because the reservoir was drained, no seepage was observed <br />duri ng any of the i nspecti ons, al though the 1 ush grass cover on the lower <br />one-thi rd of the downstream slopes is evi dence of its occurrence. However, <br />review of previous inspection reports and piezometric data collected when the <br />reservoir was storing water indicates the serious nature of the seepage <br />condition. Seepage appears to be taking place through the embankment, <br />emerging on the downstream slope at a point about one-third the dam height <br />above the toe. The water level measUred in piezometers installed in the area <br />of seepage has been observed to be significantly higher than the downstream <br />slope of the embankment. Thi s condition coul d seri ously affect the stabil i ty <br />of the dam. It appears that the "free-draining" materi al that was to have <br />been placed at the toe during the 1972 repairs was actually less permeable <br />than the original fill, and thus blotked the path of seepage flow, driving it <br />up to the surface. <br /> <br />1.3 Outlet Works <br /> <br />The primary cause of the City-imposed zero storage restriction at Peterson <br />lake Dam is that during the 1983 and 1985 rehabilitation work, boreholes near <br />the inl et structure to the out1 et condui t detected voi ds in the backfi 11 <br />surrounding the drop inlet. These Voids were grouted, but continued concern <br />caused the City to restrict reservoir storage. The outlet works originally <br />consi sted of a 3.0 foot hi gh by 2.5-foot wi de arch-shaped concrete condui t <br />with a wet well near the center of the dam rising to the crest. The inlet to <br />the condui t was through a timber-cri b drop structure. No energy di ssi pator <br />was constructed at the outlet end. The outlet conduit and wet well have been <br /> <br />~38- <br />