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November 1993 Raton Creek ' Smal! Ana Exemption • 1 • for deterioration and evidence of effective sediment control. If there is evidence of significan[ sediment transpor[ through or around the sediment traps, installations will be modified. The amount of sediment fence surface area required to safely pass the peak discharge from a 10- year, 24-hour event was determined as follows: Sediment Fence Permissivity: 15 gal/min/ft= or 0.033 cfs/ftz Required Surface Area: Upper Fence Lower Fence 0.6 cfs / 0.033 cfs/EtZ = 18 ft' 1.8 cfs / 0.033 cfs/ftt = 54 ft' The flow rates used for the fence design was based on flows calculated. The ditch design (see Addendum D). The required length of fences should be calculated assuming a Flow depth of 1 foot. Attual fence installation should be a minimum of 1.5 feet to allow free board and a factor of safety on the permissivity. The silt fence should be installed in accordance with the manufactures recommendations. Designs assume regular inspections and main[enance. Given the limited size of the disturbed area, [he previously discussed drainage and sediment control measures (designed to effectively prevent significant adverse downstream water quality impacts), and the lack of any underground mine water discharge source, EFMC requests DMG approval of small area exemptions for [his site. Ditch TS-2 will extend from the center of the west side of the topsoil stockpile, around the north side of the pile, joining Ditch TS-1 at the lower end of the middle segmen[. Flow velocities for all designed segments of both Ditches TS-1 and TS-2 are below critical velocities so no supplemental erosion control will be required. The access road for the trailer area slopes to the east toward Ditch TS-1, so the majority of [he drainage from both the road and trailer pad will flow to this ditch following existing gradients. In order, however, to prevent any drainage from the trailer pad from discharging to the west, a 2-foot high berm, approximately 100 feet long will be established and maintained on the west edge of the trailer pad as shown on Map 12, Mine Site Water Systems and Refuse Area. Design calculations for Watershed Diversion 5 and Topsoil Containment Ditches TS-1 and TS-2 follow [his small area exemption request and all existing and proposed drainage struc[ures are shown on Map 12, Mine Site Water Systems and Refuse Area. Given the limited size of the trailer/stockpile disturbance area (total disturbance of approximately 0.95 acres), the previously discussed drainage and sediment control measures which will effecively prevent significant adverse downstream water quality impacts, and the lack . of any underground mine water discharge sources in the immediate vicinity, EFMC respectfully requests DMG approval of a small azea exemption for this area. A:~856BTRO7.pGS n/:v9s to,m TerraMatrix/ACZ lnc. ' P.O. Bos 7740!8 • Steamboat Springx Colorado 80477 • (303J 8796160