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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 />Chapter III - Background Information <br /> <br />described below under the spillway observations. The left abutment is also overgrown with <br />small to medium pines. <br /> <br />An abandoned spillway is located in the right abutment of the embankment. The <br /> <br /> <br />abandoned spillway consists of a 36-inch metal pipe and what appeared to be a control gate stem <br /> <br /> <br />and masonry headwall on the upstream end. The approach channel of the spillway gate is not <br /> <br />buried. It is not known whether the pipe was plugged. Bedrock is exposed above the <br /> <br />embankment in the right abutment and the old discharge channel downstream of the spillway. <br /> <br /> <br />Bedrock consists of the same sandstone exposed in the left abutment. The right abutment is <br /> <br /> <br />uneven and appears to be lower than the embankment dam crest. No riprap surface protection <br /> <br /> <br />was observed in the upstream right abutment, and numerous small to large trees were observed. <br /> <br />The spillway for Monument Lake Dam is located in the left abutment. The spillway <br /> <br /> <br />consists of an unlined approach channel, an uncontrolled ogee-crest concrete structure with <br /> <br />concrete training walls, and a concrete stilling basin with baffle blocks. A detailed structural <br /> <br /> <br />inspection was not performed, but the structure overall appears to be in good condition. The <br /> <br />stilling basin is filled with rocks and soil debris. Some minor deterioration of the concrete chute <br /> <br />blocks and baffle blocks was observed. <br /> <br />Significant erosion of the channel downstream of the spillway structure was observed. <br /> <br /> <br />The precast concrete wall panels on the left side of the channel are in good condition. However, <br /> <br /> <br />downstream of the precast wall, the lower porti0l1 of the bedrock channel wall was eroded and <br /> <br /> <br />IO-foot-high overhangs were observed. Gabion wall on the right side of the spillway channel <br /> <br /> <br />suffered significant damage as a result of past discharge, especially the downstream half of the <br /> <br />wall. A large sinkhole, about 8-feet wide by 10 feet wide, was observed behind the gab ion wall <br /> <br /> <br />near the downstream end of the wall. A concrete slab and cutoff wall at the downstream end of <br /> <br /> <br />the gabion wall had been undercut, with large voids under the slab and cutoff wall. <br /> <br />Our observations are consistent with that reported in the 1997 SEO inspection report, <br /> <br /> <br />except the observation on seepage. The 1997 SEO inspection report indicated "seepage under <br /> <br /> <br />and through spillway structure". During our inspection, no uncontrolled seepage was observed in <br /> <br />the spillway. Based on these observations, the conditions of the spillway structure itself are <br /> <br />Monument Lake Dam Feasibility Study <br /> <br />]] <br /> <br />BOYLE <br />