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.; ~II ~I~~I~~I~~~~~~I~ 999 TR9PP8R MINING INC. January 22, 1997 Mr. Joe Dudash Environmental Protection Specialist Colorado Division of Minerals and Geology 1313 Sherman Street -Room 215 Denver, CO 80203 Subject: Use of Parahafl Flumes at Trapper Mine Dear ]oe: During the Colorado Division of Minerals and Geology (CDMG) and the Office of Surface Miaing (OSM) Special Focus Lzspection of September 3 and 4, 1996, questions were mired concerning the use of par;hall flumes at Trapper. The CDMG and OSM questioned the sizing of the parshall flumes. The par,hall flumes used at Trapper are sized to measure calculated peak discharges wiWin the drainage area. Due to the intermittent nature of the drainages with the permit area, flumes must be capable of measuring the high, short- term nmoff usually experienced in the spring months. The 12-inch throat parshall flumes are best suited to record these levels, without the possibility of the flume capacity being exceeded. The attached table, from COIVTECH Constroction Products, Inc. presents the capacities of available par,hafl flumes. As shown, if a smaller 9" throat flume was used, a lower minimum flow maybe more accurately mad, but the maximum accw-ale flow capacity is reduced. Duriug 1995 spring runoff, Trapper measured runoff exceeding 3600 gpm a[ various NPDES points. This exceeded the rocommended maximum capacity of the l2-inch flumes but did not overflow the flume. If a smaller 9-inch throat parshall had been in place, it would have overflowed. A review of Trapper's Auuual Reports shows normal peak discharges vary from 122 to 602 gpm. This is well within the rated capacity of the 12-inch tluoat flume. A smaller 9-inch throat flume could meet Wis same range, but would not be able to measure the extremely high flows as experienced in 1995. Smaller-throat parshall flumes more accurately measure low flow rotes. Due to We ephemeral nature of the drainage areas at Trapper, peak flow readings are emphasized. Low flaw rotes are recorded, but may not be as accurate as those obtained from smaller flumes. However for [he purpose of monitoring the seasonal variations of flows, the parshall flumes in use at Trapper aze more Wan adequate for this task. For the 1997 field observations, a smaller 2-inch throat Parshall has been purchased for use at Trapper. This flume will be used during low flow conditions [o "calibrate" existing flumes at their lower stages. Asite-specific low flow rating curve for the 12-inch flumes will enable the low flow rotes to be measured with more accurecy. Please contact me if you have any further questions wnceming this issue. ~6AMv~?1'~~~i giuision,a i~p~~~taa a,~eulog~ Sincerely~,~~~ Forrest V. Luke Environmental Manager FVULWH/DMEFLUME enc c file P.O. Box 187 Craig, Colorado 81626 (303) 824-4401