1988-09-26_ENFORCEMENT - M1977378 (2)

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SECTION II - HYDROLOGICAL DESIGN DATA SUMMARY OF CONCLUSIONS AND RECOMMENDATIONS 1 . Design runoff peak discharges from the drainage areas above tailings ponds No. 1 , No. 2, and No. 3 for a thunderstorm less than the probable maximum, which yields 2. 3 inches of rainfall , are 175, 29, and 265 cfs. 2. The design runoff peak discharge from the drainage area above proposed tailings pond No. 4 for a probable maximum thunderstorm of 8. 5 inches, is 1 ,380 cfs. 3. Trapezoidal ditches with side slopes of 1. 5 horizontal to 1 vertical are recommended for the diversion of surface runoff around the tailings i ponds. Riprap should be placed in the invert and along the side slopes of ditches having a gradient in exces-s of .01 ft/ft to prevent undue i scour and erosion. 4. Three36-inch diameter culverts will be required to pass diverted runoff �! under State Highway 110. 6 5. The decant system for proposed pond No. 4, consisting of two 10-inch diameter steel pipes, will be sufficient to evacuate, within ten days, 90% of the volume of water stored during the design storm, provided that the majority of the surface runoff is intercepted by a diversion ditch system above the pond. 6. The calculated peak flood stage along the Animas River flood plain adjacent to the proposed new alignment for State Highway 110, resulting from a 100-year frequency flood, is approximately 2 feet. Riprap should be placed along the proposed highway embankment to prevent undue scour ; and erosion. 7. Other hydrologic design and construction details are presented in this section. GENERAL In accordance with the "Design Guidelines for Mine Waste Piles" estab- lished by the Mining Enforcement and Safety Administration (MESA) , a hydro- logic investigation was conducted to provide maximum flood protection for the tailing ponds. The following hydrologic and hydraulic evaluations are presented in this section: I ' 18 ''