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1.0 DESCRIPTION This final design report describes the design criteria and final design for management of surface water run-on, collection and diversion of surface water run-0ff and discharge of collected surface waters from the surtace area of the West Tailing Impoundment at the Camp Bird Mine. Detailed information pertaining to site topography and related factors is provided within Technical Revision No. 1 to MLRD Permit No. M-82-090 (January 25, 1999). The drainage area associated with the West Tailing Impoundment is defined as including the surface area of the tailing material which comprises approximately 7.15 acres. Additionally included is the immediate upslope sub-area circumscribed by the upgradient edge of the tailing material and the south (down gradient) shoulder of County Road 361 along the northerly extent of the tailing impoundment. This additions{ area comprises approximately 0.75 acres and results in a total drainage area of approximately 7.9 acres. The surface of County Road 361 and all areas upgradient are drained by a Ouray County maintained v-ditch system adjacent and parallel to the upslope shoulder of the roadway. The drainage features depicted in this final design report have been designed to facilitate drainage as associated with the 100-year/24-hour precipitation event. 2.0 DESIGN CRITERIA The Rational Method has been utilized to calculate run-off associated with the 100-year/24-hour precipitation event. Utilizing NOAA Atlas 2 - Volume III (Colorado), the 100-year/6-hour precipitation depth is indicated to be 2.4 inches and the 100-yeari24-hour precipitation depth is indicated to be 3.5 inches. Using a 1,000-foot travel distance and a nominal velocity of 1 foot/second (fps) yields a representative time of concentration of 16.7 minutes for the combined drainage sub-areas. Rainfall intensity for the time of concentration as derived from the intensity-duration curve is 3.75 inches/hour. Using arun-off coefficient of 0.4 for the tailing sand surface and arun-off coefficient of 0.65 for the grassy, forested slope immediately above, the total calculated run-off (0) is 12.56 cfs. This number was rounded up to 13.0 cfs for design purposes. A discharge pipeline was selected over open-channel flow due to the significant slopes and limited (constrained) surface area available to accommodate an open-channel conveyance feature. A further limiting factor is the inability to obtain authorization (from Ouray County) to construct the drainage feature within the defined Ouray County right-of- 2