2010-11-23_PERMIT FILE - M2009076

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Additionally, as each 40' high terrace is completed, the surface of the terrace will be re-vegetated as described above. Rip-rap will be installed for a channel width of 6 feet at each end of the impoundment, at the intersection with the natural grade, to provide a permanent stonnwater control. The existence of a natural diversion channel will also be acceptable for stormwater diversion as is found near the bottom half of the impoundment area. This natural channel is incorporated into the sedimentation pond. More reclamation details are to be found in Exhibit D and within the maps and drawings of Exhibit E. Specific details about the Tailings Impoundment Structures can be found above in 6.3.3(1)(e). 6.3.3(2)(c) Drainage control within the Tailings Impoundment Affected Area has been considered using the U.S. Soil Conservation Service, Technical Release 55: Urban Hydrology for Small Watersheds, USDA (U.S Department of Agriculture), June 1986 for design guidance. Additionally, the USDA Natural Resources Conservation Service has published software for analysis of systems covered by the Technical Release 55. The up-gradient native watershed from the Tailings Impoundment was analyzed using the following assumptions: • Drainage area of approximately 40 acres • "Group D" hydrologic soil classification, with relatively low infiltration (not because of swelling clays, but because of shallow nature of soil over rather impervious bedrock conditions). • Run Off Curve number, weighted = 73, within a meager rangeland (50-75% cover) with moderate brush and minimal forestation. • An average slope of 0.3 ft/ft. The watershed of the affected Tailings Impoundment was analyzed using the following assumptions. • Drainage area of approximately 8 acres, total buildout considered with phased reclamation • "Group C" hydrologic soil classification (sandy clay), a moderately low degree of rainwater infiltration at 0.05- 0.15 in/hr. • Run Off Curve number, weighted = 74, within a meager rangeland (50-75% cover) with minimal brush and no immediate forestation. • An average slope of 0.6 ft/ft on embankment faces. As reported by NOAA's Atlas 2 Vol. 1.11 precipitation maps for Colorado, the following data is relevant for this site: • 100 year/24-hour precipitation = 3.8" or 0.158"/hr., Type II distribution • 10 year/24-hour precipitation = 2.6" or 0.108"/lu., Type II distribution A 100 year/24-hour event will produce the following Peak Discharge and Runoff at the Tailings Impoundment storm management diversion structures from the upland area: • Peak Discharge within the diversion ditches = +/- 80.8 cfs. Each side of the complex will receive approximately one half of this flow, or 40.4 cfs. Design has been considered at total Bench 45 buildout with full perimeter ditch. • The Peak Discharge spikes for a two hour period, as illustrated below. Considering the above data and design guidance for riprap revetment provided by the Federal Highway Administration Hydraulic Engineering Circulars HEC-11 & HEC-15 and the Urban Drainage c& Flood Control Criteria, it has been detennined that a perimeter diversion ditch with the following parameters will be sufficiently adequate to safely pass a 100 year/24-hour storm event and limit damage to the Tailings Impoundment: • Diversion Ditch -A trapezoidal charnel with a bottom width of 6 feet, side slope of 1:1 and a minimum clear depth of 2 feet shall be constructed at each end of the impoundment bench as it is raised, at the intersection with the natural grade, to provide a permanent stormwater control. Type L riprap, sized nominally with a median diameter of Dso=9 inches, shall be hand placed at a depth of 18 inches. Rubble and/or slightly weathered stone sorted on-site from available material that meets this size specification shall be acceptable for use. This perimeter ditch will be constructed in phases as the impoundment expands. Refer to Exhibit E Drawing E7 for the Phase I ditch and Drawing E 10 for Phase 2 ditch. Interim diversion along the projected side slope line, Page 31 of 37