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LAWRENCE G. W~FITE P.E. III~I ~ ~~II~II_____ _: _ __ 1650 M. STREET PENROSE, COLORADO 81240 i'=-, J2-649 August 24, 1992 Catherine W. Begej ;~~~~r~ AUG 2 51992 Reclamation Specialist ;!I~Li~~. ~~c Division of Minerals & Geology ~:1!NEF.,L`.~.,_. ~~ 1313 Sherman Street Denver, Colorado 60203 RE: twin Pines No. 2 Mine, C-83-058; Mine Site Drainage; Major Storm Event, August 17, 1992 Dear Catherine: At approximately 5:30 P,M. on August 17, 1442 the foot hills in the vicinity of the mine site e:<perienced a very major storm event. Unofficially it was reported that 2.62" cf rain fell from 5:20 PM to S:vS PM at Energy Fuels South Field Mine. South Oak Creek, which runs adjacent to the Twin Pines Coai Sale=_ Site, was at extreme flood stage. At the rail road crossing, aporoximately 1.25 miles downstream from the Coal Sales Site. the water ponded to 32.8 feet above the invert of the 12 ft. high x 9.5 ft. aide concrete box cul•iert through the fill. The calculated flow through the bo:< culvert at this head is 2,850 cfs. it is estimated that ll?0 to 150 acre feet e-r water was ponded behind the fill. At the recently constructed twin 10 ft. r. 10 ft. bo:< culverts crossing South Oak Creek in Rec'.;vale the water reached a depth of 13.5 feet above the uu stream invert. about 3 ft. helow the road ele~iatio~~~ ar~d l.b ft. below the design high water elevation for the box, The depth of flow within the boy: was approximately 2 ft, below the top dropping to 3.5 ft. at the downstream end. The boy: is 60 ft. lone. The calculatec flow for the twin bor, at the depth reached during the storm is 2,640 cfs. The design fcr th.e culvert indicated a !00 yr, ~Fiow of 3.300cfs and a 25 yr, flow of 1,vLb cfs. Coasidering the water detained at the railroad fiii it is estimated that the flaw in South Oak Creek exceeded 3.bOG cfs at the railroad fill. The estimated flow at Twin Pine=_ Coal Sales is 2,90i_~ cfs. T~n ere was nc ev:denr_e of obstructions wnicn could have partially 'o locked the 12'x.9.5' boa culvert, On Oak Creek [ did no: find a lccaticn where the flcw from the storm could be estimated. The stage of the creek was well above the structure on the Mine Acczss Road. Cbservino the Oak reek Channel down=-tream of Rockvale flocdinq was evident. J.F. Sato and Associates in their anal~~~sis of Surface Water Hydrology for the Cline calculated 100 yr. riows for South Oak Creek which varied from 3,087 cfs to 705 cr=_. (See pane JFSA-55 in the Permit Application).