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DRAINAGE OVERVIEW As the overall run-off from this site will not be changed, no permanent stormwater management facilities are required by Douglas County. A pond will be constructed primarily for sedimentation purposes but will also provide storage for the total 100-year run-off from the disturbed areas on this site. The detention pond will release through a small diameter (6'~ rigid pipe which will outfall near the east property line which will be rip-rapped to resist erosion and the run-off will then flow overland to the Lemon Gulch. A diversion ditch will be constructed along the north property line to direct offsite flows away from this site and away from the disturbed areas. Most of the diverted water will flow the east, with a small portion flowing to the west. The diversion ditches have been conservatively designed to carry the entire offsite flows for the 100-year storm. The ends of these ditches will be rip-rapped to resist erosion. The site will be graded so that all disturbed areas will flow to the settling pond. Although stormwater management is not required for this site, it will be performed as a consequence of the settling pond as the historic run-off rates for the mining area (20.8 acres) is 51.0 cfs and after the pond is built, the release from this area will be 1.5 cfs. There is an additional volume provided by this pond (approximately 19,000 ft~ located below the invert of this pipe that will eventually seep from this pond. ^ DRAINAGE CRITERIA The drainage criteria used in this report was the Douglas County Storm Drainage Design and Technical Criteria Manual which was supplemented with the Urban Drainage and Flood Control District's Drainage Criteria Manual. All flows were calculated using the rational method with the run-off coefficients and rainfall values (Zone 1) taken from the Douglas County Manual. The detention volume for the pond was calculated assuming a triangular distribution. 1