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CM0001762 <br />Page 8 <br />correspondence provided in Appendix A. <br />The alignment was chosen based on existing <br />topography to provide the shortest length of Dam to minimize Dam construction material <br />requirements and associated costs and meet a CMC requirement that existing power poles <br />located near the alignment be left undisturbed. <br />The Dam's height and cross-section were developed based on a three foot elevation difference <br />between the maximum water storage elevation of Robinson Lake Dam (i.e., set at 10,892 MSL) <br />and the crest of the No.l Dam (i.e., set at 10,895 MSL). The Dam's spillway crest elevation was <br />set equal to Robinson Lake's maximum water storage elevation of 10,892 ft MSL (i.e., as <br />reported by Wheeler). The Dam crest was set three feet higher allowing for a two foot spillway <br />flow depth and one foot of freeboard. The cross-section zones and slope components of the Dam <br />were configured consistent with the configuration of a Class IV small dam. <br />4.3 Dam Foundation <br />The existing soil conditions under the footprint of the Dam's alignment will require the removal <br />of unsuitable soils and lake bottom sludges and placement of suitable material to provide a stable <br />foundation. A well compacted, unyielding foundation is required to maintain the stability of the <br />Dam. Control of moisture in the foundation backfill is critical for placement of an engineered <br />backfill that will provide a stable Dam foundation. Therefore, foundation and impoundment area <br />dewatering will be a continuous requirement during the Dam's construction. <br />Upstream of the Dam, the Robinson Tailings Pond's seep water will flow continuously during <br />the project. The seep water may be overland flow and/or seepage through existing soils and <br />fractured bedrock. This water must be diverted, collected and/or pumped from the construction <br />area. <br />Downstream of the Dam, water levels in Robinson Lake historically rise with the snow melt in <br />the late spring/early summer and peak in late June or early July as shown on Table 6. CMC <br />indicated that the preferred operational plan is to keep the Robinson Lake water level elevations <br />in the elevation range of 10,860. However, this can not be guaranteed due to uncertainty in <br />weather conditions. Dewatering of the downstream side of the Dam may be necessary. <br />• <br />• <br />• <br />c: ynn ypm Amax o M rep . cc <br />Environmental <br />--M__-