2005-10-24_REVISION - M1987028 - Laserfiche WebLink

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EI~IIBTT G-- WATERSHED MAP AND STORM VOLUME PROJECTIOIHS A map showing the size and extent of the Taylor Gulch watershed is submitted with this exhibit. The Taylor Gulch watershed covers app. 390 acres of high mountain terrain, reflecting an overall elevation range of 12880' (highest) to 10560'(lowest) at the southern portion of the affected land. With an overall lateral distance of 5850' of run, this corresponds to an overall slope of 2320'elevaion change/ 5850' run = .397 average slope. Diversion ditches above the west of the mining azea and the mining area itself will reduce the overall acreage of the basin draining into Taylor Gulch to less than 150 acres, mostly those slopes lying east of Taylor Gulch and the greater part of the basin lying north of the affected land's northern extent. This limited drainage basin area results in low to moderate storm discharge expectations to Taylor Gulch. The slopes east as well as west of Taylor Gulch aze also cut by numerous mine workings, road and bulldozer cuts, which tend to break sheet washing affects of surface storm flows and re-directs surface flows along road ditches. Estimated projection for 100 year 24 hour rainfall event at this elevation is 5 inches. A conservative estimate of 150 acres of drainage basin at 40% slope and Cn Number of 92 was used in the storm estimate calculations. 100 Yeaz, 24 hour rainfall event, total discharge to Taylor Gulch 60 acre-feet Maximum Discharge Rate 175 CFS Basic Channel Cross Section Shape Trapezoid Minimum bottom channel dimension 4 feet Minimum channel depth 4 feet Channel side slopes 2:1, H:V Minimum Rip-Rap Dimension 8" diameter Minimum Rip-Rap Placement Depth 18" Rip-Rapped channels will be located along the eastern portions of the affected land boundary to isolate Taylor Gulch storm flows from mine drainage, which will be directed into the lower pit area. Armored channels will be maintains at points where Taylor Gulch crosses County Road 228 and where past disturbance of Taylor Gulch by mining operations have resulted in the need to re-construct a defmiive drainage channel separated from mine site drainage patterns by rock and earth berms along the west side of the Taylor Gulch drainage. Berms will be constructed of waste rock capped with topsoil and planted to native grasses and forbes. The berms will be no less than 5' higher than the bottom of the channel and will be constructed with side slopes no greater than 2:1 and a flat top at the crest of the berm at least 2' wide. 18