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Geoteehnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey 8 Whitney, LLP <br />Section 2 -Condition of the Dam <br />2.1 Description <br />Monument Dam and Reservoir is an on-channel reservoir located on the Dry Fork Minnesota <br />Creek approximately 8 miles upstream of Paonia, Colorado. It is located in Section 32, <br />Township 13 South, Range 90 West of the 6a` Principal Meridian in Water District 40 of <br />Division 4, see Figure 2.1. The dam and reservoir are owned by the Minnesota Ditch and <br />Reservoir Company for the purpose of supplying water for irrigation. <br />The dam was originally constructed in about 1899. It was raised twice, first in 1910 and <br />again in about 1933 (Wright Water Engineers, 1994). In 1980, the Colorado State Engineer's <br />Office (SEO) placed a restriction on the reservoir at 10 feet below the dam crest. In 1985, <br />under order from the SEO, the spillway was excavated to Elevation 7330.2 to provide 10 feet <br />of freeboard below the dam crest. Also in 1985, a crack in the left side of the downstream <br />embankment was repaired, and a trench drain was constructed under the left portion of the <br />dam. <br />. According to the SEO, Dam Safety Branch inspection records, the dam has a height of 72 <br />feet, a crest length of 422 feet, and a crest width of 11 feet. The SEO has classified <br />Monument Dam as an Intermediate-size Hazard Class II Dam (Significant Hazard). Hazard <br />Class II is defined by SEO Rules and Regulations for Dam Safety and Construction (SEO, <br />1988) as "a dam for which significant damage is expected to occur, but no loss of human life <br />is expected in the event of failure of the dam. Significant damage is defined as damage to <br />structures where people generally live, work, or recreate, or public or private facilities <br />exclusive of unpaved roads and picnic areas. Damage means rendering the structures <br />uninhabitable or inoperable." <br />The dam and reservoir are currently operated under restrictive orders by the SEO that have <br />been in place since 1980 because of an inadequate spillway capacity, seepage exiting at high <br />elevation on the downstream face of the dam and associated slumps, and cracks observed in <br />the dam crest and downstream embankment. The most recent restriction was ordered in 1987 <br />and requires the reservoir be maintained 10 feet below the elevation of the emergency <br />spillway (El. 7330.2). The restriction has been modified to allow increased storage up to the <br />emergency spillway elevation (conditional full storage) for a few months during the summer, <br />subject to site inspection and verification of instrument data by the SEO and in accordance <br />with increased monitoring requirements. An additional dam safety concern is the cast-iron <br />outlet conduit, which is subject to the reservoir water pressure head along its full length <br />through the embankment because flow is controlled by a valve at the downstream end of the <br />i <br />GEI ~ 3 06035006-03-16MonummtDemGmteduvcalEvil <br />March 2006 <br />