2006-10-19_REVISION - M1995035 - Laserfiche WebLink

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Ms. Erica Crosby ~ . RE: Connell Resources, ]nc. Wellington Downs Pit Technical Revision . October.l3, 2006 _ Page 3 of 4 ' Wellington_Downs site. ,A smal_I area of the southern bank had been regraded and lowered during the Stute Pit reclamation and dewatering operations in 2005 acid eazly 2006 to allow excess water to drain from the, lake: The elevation of the. low point on the southern bank is 4833.5 feet and it can be, assumed that the groundwater lake surface elevation was. at a similaz elevation when flows out of the lake were observed: Under normal conditions, based on local groundwater well data and observations of historic water levels in the Weitzel,Pit groundwater lakes, it appeazs that the water surface elevation of the southeast groundwater lake typically fluctuates between 4,831-4,834 feet. Survey results indicate that the water surface elevation was 4,832.5 feet in July 2006. _ The lake observations and available data suggest that surface water will not typically flow over the southern bank of the southeast groundwater lake on the Weitzel Pit site (at elevation 4,833.5 feet). Flows over the bank could potentially occur temporarily during seasonally high groundwater conditions, but more likely, only during stormwater events. Since the local historic surface drainage generally conveyed stormwater flows from northwest to southeast across the Stute and Weitzel sites, it is very likely that future stormwater flows will pass through the Weitzel Pit southeast_groundwater lake, over the southern bank, and onto the Wellington Downs site. In order to address the drainage, an analysis was performed using the SEDCAD computer softwaze program (see attached results). The drainage azea for the Stute and Weitzel sites was conservatively estimated to be approximately 118 acres. For the drainage analysis, it was. assumed that an entire 100-yeaz storm event would flow off of the drainage area and into the southeast groundwater lake on the Weitzel Pit site. The low spot in the southern bank, at an elevation of 4,833.5 feet, is approximately 5 feet wide with 3H:1 V side slopes. The drainage.analysis indicates that the 100-year storm event would generate a flow of approximately 13.5 cubic feet per second over the low spot in.the southern bank of the lake. The storm event would increase the level in the lake approximately two feet and flows would last approximately two days until the lake level dropped below the elevation of the low spot in the bank. Once on the Wellington Downs site, flows would continue to be carried south in the existing ditch east of the reservoir. The ditch is approximately three feet deep with a narrow flat bottom and 3H:1V, side slopes: -Based om the ditch size and slope, and the anticipated flow rate, the velocity of the flow would be approximately 1-2 feet per second at a depth of about Z.5 feet. Flow velocities over the southern bank of the Weitzel Pit lake and through the ditch on the Wellington Downs site indicate that the structures can be adequately stabilized and protected from erosion by establishing native grass growth. Native grasses aze currently being established within and around the ditch on the Wellington Downs site. For added stability, the low spot on the southern bank of the Weitzel Pit groundwater lake will be covered with no less than 3-inches of '/<-inch gravel. The gravel will be placed onto the- bank, covering azeas where flow is anticipated. Surrounding azeas will be stabilized with native grasses.