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-: l <br />~- ~ <br />NUMBERED RESPONSES TO QUESTIONS AND CONCERNS STATED IN MLR LETTER <br />DATED AUGUST 8, 1979: <br />1. (This response will also address the concern raised during the August 27, <br />meeting and listed as Item number 3 in the attached summary.) <br />Referring to the revised copy of Figure D.5-11, Cotter will construct post-mining <br />run-off (drainage) control ditches uphill from the backfilled pit and across the <br />waste dump area which will be capable of carrying the runoff generated by a ten- <br />year precipitation event. The cross-sectional area of these runoff control ditches <br />was determined using the method given in the Engineering Field Manual for Conserva- <br />tion Practices which was prepared by the Soil Conservation Service, dated 1969. <br />The assumption used for the purposes of determining channel flow are given below <br />Figure D.5-11 approximately locates the permanent (post reclamation) surface <br />diversion structures for the JD-7. The berms south of the pit will be constructed <br />during year one of the mining operation; whereas, the structures on the waste dump <br />will be finalized in conjunction with sequential reclamation operations. Vegetative <br />retardance will be used as the primary energy dissipator and the installation of <br />locally obtained rip-rap will assist with this function. The watershed was divided <br />into two segments; the first draining toward the pit and the second toward the waste <br />dump area. <br />The pit watershed comprises an area of about 214 acres. A ten-year precipitation <br />event was considered in accordance with the SCS Field Engineering Manual for <br />Conservation Practices. The following information was generated: <br />1.8" of rainfall - 10 year storm <br />21% slope - steep designation <br />Runoff curve number 70 - B retardance <br />Land use: pasture/range in poor condition <br />11 cfs - fair vegetative cover <br />3.0 ft./sec. - diversion velocity <br />Diversion structures will be located above the pit, feeding runoff to existing <br />arroyos with the aid of culverts at the Monogram Mesa Road. Section "A"will drop <br />30 feet over a 1400 run yielding a slope of 2%. The "B" retardance designation <br />limits flow velocity to 3.0 ft./sec. within the diversion. The actual incoming <br />flow will be 11 cfs, but 20 cfs has been assumed to allow for additional free board <br />The structure will be about 17 feet wide and 2.5 feet deep and will decrease <br />velocity to 1.5 ft./sec. well below the maximum estimated velocity. <br />Section "B" of the pit diversion drops about 50 feet over a 2300 foot run resulting <br />in a 2% grade. For simplicity, 20 cfs was also used yielding an idential cross <br />section to Section "A": 17 feet in width, 2.5 feet in depth, and a velocity of <br />1.5 cfs. <br />-2- <br />