2020-06-16_REVISION - C1981010 (17)

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Performance of the Trapper Mine No-Name Sediment Control System During the Worst-Case Year of Development of I-Pit and J-Pit The No-Name drainage and sediment control impoundments were constructed during the period from 1980 through 1987 to manage runoff from A Pit and D Pit in accordance with applicable regulations. The worst-case disturbance year that was the basis of the original impoundment designs has passed and much of the lands affected by A Pit and D Pit have been reclaimed for several years. The yet-to-be reclaimed D/E final cut area is a part of the No-Name watershed and consists mostly of spoils. As part of an updated mine plan, Trapper intends to re-disturb a portion of the reclaimed areas and create new disturbance in other areas in order to develop I-Pit and J-Pit. The affected sub- watersheds have been reconfigured with updated disturbance and sub-watershed boundaries and all other parameters needed to re-evaluate the performance of the No-Name sediment control system under the new plan. SEDCAD software was used to model the results. The year 2020 has been determined as the worst-case year as all associated additional topsoil stripping in the No-Name watershed is scheduled for this year. After 2020, a pit will exist that will intercept much of the runoff. The full watershed is comprised of 1,389.9 acres of which 142.7 acres are undisturbed (curve number 47), 255.5 acres are active and consists of roads/road corridors and newly stripped topsoil areas (curve number 87), 180.3 acres include spoils and final cut areas in D/E pit (curve number 75 in recognition of the fact that these previously mined areas will have a deep, loose soil structure with a high infiltration rate) and 811.4 acres of reclaimed lands (curve number 67). For modeling purposes, the permit stage storage information and spillway configurations were utilized. The model was run for a 10-year, 24-hour rainfall event and a 25-year, 24-hour rainfall event. A total of three diversions are a part of the drainage plan. The No-Name East and No-Name West Diversions route disturbed runoff into No-Name #4. The Coyote Diversion routes a small portion of the Coyote drainage runoff into No-Name #2. The areas draining into these diversions are relatively small: No-Name East — 14.0 acres, No-Name West — 27.3 acres and Coyote —44.4 acres. SEDCAD modeling was performed for each diversion for a 10-year, 24-hour rainfall event. The impoundment modeling outputs of interest include the peak settleable solids in the effluent coming from the last pond in the series, No-Name #5R. The standard requires a value not to exceed 0.5 ml/I. In addition, the freeboard resulting from the peak stage of a 25-year, 24- hour event on all three impoundments must be at least one foot. A summary of the relevant results is shown below: