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MMRR quarry DMG Adequacy Review Response 3/15/2005 Page 2 3. Temporary diversion channels arc sized to convey the flows resulting from a 10-year recurrence interval storm. Permanent channels are sized to convey the flows resulting from a 100-vear recurrence interval s[onn. Drainaee Basins The portion of the site in which mining activities are proposed to be located is in a small ephemeral drainage basin with two tributaries. The two tributaries aze referred to as the west Basin (Basin W) and the east Basin (Basin E). The area draining to the main channel between the confluence and the property line is referred to as Basin S. The stream channel draining the basin crosses the Colorado Departrnent of Transportation (CDOT) Right of Way boundary and discharges beneath State Highway 119 through a 6-foot wide by 8-foot high concrete box culvert. Downstream of the culvert, the channel joins the North Fork of Clear Creek. The confluence of the west and east tributazies is about 750 feet upstream of the SH 119 right of way The total area of the drainage is about 171 acres. Basin W contains about 49.1 acres, Basin E about 102 acres and Basin S about 19.6 acres. The basins aze steep; the high elevation of Basin E is 8580 feet msl and the elevation at the culvert about 7205 feet msl. The length of the channel segment is about'S,140 feet; the average slope is about 28.5%. Much of the tributary azea slopes at about 40% to 60% toward the channels. Historic Flows Historic peak flows from Ute 100-yeaz storm event in each of the basins the watershed are shown below: West Basin 81 cfs East Basin 166 cfs ;, South Basin ~5 cfs Total 254 cfs Runoff in the ephemeral channels exits the property in a 6 foot by 8 foot concrete box culvert under SH 119. Although survey data was not available, the culvert appears to be capable of passing runoff from the 100-yeaz storm event without overtopping the road. Diversion Ditches The objective of the diversion ditches described in this report is to divert runoff from undisturbed areas around areas to be disturbed as part of the mining activities. The natural flows will then be returned to the watercourse once past the mining azea. In some cases where there is only a small undisturbed catchment tributary to an area to be disturbed by mining activities, the benefit of constructing a diversion ditch is limited. In such cases, the undisturbed area runoff will be allowed to commingle with runoff from the disturbed area. This nuroff will then be routed to a sediment pond. Diversion Ditch A captures undisturbed azea runoff in the West Basin and routes it around the waste rock pile to a ditch adjacent to the Haul Road (Exhibit C-2B). The Haul Road is graded to drain away from the ditch. The ditch discharges the water onto a ripmp pad in the main channel. downstream of the Process Area. Diversion Ditch B captures undisturbed area runoff in the East Basin and routes it across the Process Area. The ditch dischazges the water onto a riprap pad in the main channel. Both ditches are permanent and will be retained flowing reclamation (Exhibit