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Check dams used by Trapper Mine provide significant benefits leading up to bond release. They produce <br />at least four favorable effects in the establishment of a permanent reclamation condition: <br />1) they prevent erosion that may develop at one location from proceeding further back upstream, <br />thereby minimizing stream repair costs in the time prior to bond release, <br />2) they reduce the flow velocity during the early stages of reclamation when runoff flows are higher, <br />to allow a permanent and self-sustaining grass cover to establish in the channel, <br />3) they reduce the sediment load to the downstream sediment ponds during active mining and <br />reclamation, thereby reducing pond clean out efforts, and <br />4) directionally, by reducing the sediment load to the ponds, they also enhance the water quality of <br />the effluent released from the ponds during the mining and reclamation phases. <br />Trapper commissioned Thomas C. Peterson, a registered professional engineer in the state of Colorado, <br />representing Environmental Solutions, Inc., (ESI) of Cheyenne, Wyoming to create a hydraulic model to <br />determine the minimum number of check dams within reconstructed drainageways on Trapper reclaimed <br />lands. The basis for the model was to look at an intermediate reclamation stage in a typical generic <br />watershed at Trapper by assuming that during the active mining stage approximately one third of the <br />watershed tributary to the check dam would be at a freshly topsoiled condition (CN 86), one third at 1 <br />year's growth (CN 80), and one third at 2 year's growth (CN 67), all the while the grass lining in the <br />channel is still immature. Hence, the runoff curve number selected for the model was 78. <br />The object of the modeling was to find the capacities of check dams that temporarily slows the flow <br />enough to not present erosive velocities in a bare earth channel lining, modeled as "coarse gravel, non - <br />colloidal" in SEDCAD. The model was applicable only to reclaimed lands and associated drainage <br />channels less than 640 acres in size. The 10 year, 24 hour rain storm event used was 1.40 inches, as <br />approved by the Division pursuant to RN -03 (July, 1992). The ESI report is presented in Appendix Q, <br />Section 37. <br />ESI evaluated various channel slopes to help arrive at the number of check dams required during the <br />initial period of reclamation. As the channel slopes increase, the temporary storage capacity decreases. <br />The storage capacities achieved for a 3 -foot high check dam constructed in a 20 foot wide channel vary <br />as shown below. <br />Channel slope <br />Storage Capacity <br />5% <br />0.0435 ac -ft <br />10% <br />0.0225 ac -ft <br />15% <br />0.0137 ac -ft <br />20% <br />0.0129 ac -ft <br />25% <br />0.0033 ac -ft <br />It can be seen that for a channel slope of 15 percent, the storage capacity for each check dam is only <br />about 31% of the storage available for a check dam constructed in a channel with a 5% slope. A three <br />foot high check dam in a 20% slope will only back water up about 15 feet upstream, while the same check <br />dam in a 5% slope will back water up about 60 feet. <br />Given the porous nature of the check dams, the dams do not actually result in water storage. However, it <br />is assumed that temporary storage is achieved under design storm conditions and that temporary in - <br />channel detention occurs. This assumption is further verified by the fact that Trapper has seen these <br />structures both overtop during large flow events as well as overtopping due to sediment accumulation. <br />4-183d <br />Revision: & —0 7 <br />APR 012015 Approved: <br />