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:~ iii iiiiiiiiiiiiiiii 999 BLACK ROCK MINING COMPANY Decmber 16, 1980 Mr. Jim Herron Mined Land Reclamation 1313 Sherman St. Denver, Colo. Dear Mr. Herron: DEC 1 ~ 1980 ~A~M1ED LAND pKLAMA710N~~9A1~ COW DE-1. O_F L1AfURAL ILS01~~ Pursuant to our conversation concerning the new sedimentaion pond at the Sunlight Coal Mine in Garfield Co., I submit the following Calculations and Design Criteria. A 10-year 24-hour rainfall event at the mine site is 1.6 inches.. With a C type soil and some vegita- tion, 1.6 inches of percipitation will yield .52 inches of runoff. To accommodate this event for the 1.407 acre disturbed area, the sediment pond will require the following storage capacity: (1) Runoff storage - .061 acre Ft or 3657 Cu. Ft. (2) Sediment storage - .1407 acre "t. or 6129 Cu. Ft. (3) Total runoff and sediment storage 8785 Cu. Ft. The pond will be constructed 30 s' 90 feet on the surface and 12 X 72 feet on the bottom with lz:l side slope walls. P,4aximum depth will be 7.35 feet, but~.to the emergency spillway at 6 feet, the storage capacity of the pond iS 10,206 Cu. Ft. ~ursopss ~i« 3r a:/d,~~ The primary dewatering device will be a 4 inch pipe with gate or valve and will be located 2Z feet off the bottom of the pond. The emergency spillway will be 1 Ft. deep and 1 Ft. across the bottom with 2:1 side walls. This spillway will be located 6 feet from the bottom of the pond, and will have the capacity to discharge 15 C.F.S. or twice the amount needed for a 25-year 24-hour event. Both spillways will be rip-raped and will discharge in a natural diversion to the existing irrigation ditch. The sediment pond will be certified by a Certified Engineer. If any additional information is needed contact me. Sincerely bmitted / ~~ _ ~1~ 32950 Inverness Dr Evergreen, Colo., 80439