2015-08-31_PERMIT FILE - C1981010 (59)

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EXECUTIVE SUMMARY This study was initially commissioned to develop an analytical (hydrological) model that demonstrates that Trapper Mine's existing postmining channel design and construction methods will meet bond release criteria. This would supplement the consistently observed successful erosional stability of Trapper Mine's permanent drainage channels at the time of bond release, incorporating such features as broad channel bottom width and a series of in -stream check dams that are used to construct the channels. During the course of the study it was discovered that for the case in which the majority of the watershed is at 2 years or more vegetative growth, the grass -lined channel alone is found to be stable without the need for erosion check dams. It appears this results from the generally wide trapezoidal ditch bottom employed by Trapper Mine for permanent channels in the final reclamation surface. These result in shallow flow depths for the design storm, thereby producing a small hydraulic radius which in turn results in mild flow velocities, lower than the erosion threshold in the grass channel lining. This finding is reported here as Part One of this study. That said, the series of regularly spaced check dams used by Trapper Mine provides significant benefits leading up to bond release. They produce at least four favorable effects in the establishment of a permanent reclamation condition: 1) they prevent erosion that may develop at one location from proceeding further back upstream, thereby minimizing stream repair costs in the time prior to bond release, 2) they reduce the flow velocity during the early stages of reclamation when runoff flows are higher, to allow a permanent and self-sustaining grass cover to establish in the channel, 3) they reduce the sediment load to the downstream sediment ponds during active mining and reclamation, thereby required pond clean out efforts, and 4) directionally, by reducing the sediment load to the ponds, they also enhance the water quality of the effluent released from the ponds during the mining and reclamation phases. Accordingly, the second and third parts of this study was redirected from using the check dams as part of the post -mining model, where it has been shown in Part One that they are not needed, to establishing via hydraulic modeling a rational model to set the spacing of the check dams. The basis for that model is to look at an intermediate reclamation stage in a typical generic watershed at Trapper by assuming that during the active mining stage approximately one third of the watershed tributary to the check dam would be at a freshly topsoiled condition (CN 86), one third at 1 year's growth (CN 80), and one third at 2 year's growth (CN 67), all the while the grass lining in the channel is still immature. The object of this modeling is to find a ditch check spacing that slows the flow enough to not present erosive velocities in a bare earth channel lining, modeled as "coarse gravel, non colloidal" in SEDCAD. The findings of this portion of the study are presented as Part Two. Part 3 provides recommendations for controlling sedimentation and runoff from recent reclamation using dozer basins.