2018-02-06_REVISION - M1982090 - Laserfiche WebLink

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Phase 1 Task 3: Stabilize the upstream historic tailings cell. Description: The upstream tailings cell is uncapped and located east and adjacent to Imogene Creek, and southeast and adjacent to Canyon Creek. Regrade and stabilize slopes of the tailings in accordance with an engineered, geotechnical stability analysis. Construct appropriate erosion control features and cap all areas with inert cap rock, hydro mulch, topsoil and/or compost. Establish vegetation as needed to stabilize areas where the matrix of the cover material is prone to erosion from storm water runoff. Timeline: Geotechnical analysis is anticipated to be completed during 2017 construction season. Grading, capping and revegetation is anticipated to be initiated during the 2017 construction season and completed during the 2018 construction season. Notes: A portion of this area underlies active powerlines operated by San Miguel Power Association, Inc. (SMPA). Depending on the design criteria for this task,the powerlines may need to be reconfigured,which may need to be completed by SMPA before and during this task. Deliverables Due Dates DRAFT design of excavation, grading, erosion control and restoration features. 30 days after ratification FINAL design of excavation,grading, erosion control and restoration features. 45 days after ratification Construction completion. October 1, 2018 Phase 1 Task 4: Enhance and reinforce streams adjacent to historic tailings. Description: The toes of the tailings cells are susceptible to erosion from Imogene,Sneffels, and Canyon Creeks during flood events. Establish stream restoration segments along all 3 creeks. In tandem with the regrading in Task 3, excavate, widen, and armor select portions of the banks of each Imogene Creek stream segment from the pipe bridge downstream to the confluence with Sneffels Creek. And in tandem with Tasks 2 and 3 and Phase 2, excavate, widen, and armor select portions of each Sneffels Creek and Canyon Creek stream segment from the County Road 361 bridge through the confluence of Sneffels and Imogene Creeks, and downstream to the disturbed area in Task 2. Excavation,widening, and armoring of the streambanks shall be completed to the extent necessary to prevent erosion of tailings from 100-year flood events. Remove,the large block of San Juan Tuff resting in/near the floodplain below the historic tailings. Amend uncovered areas of the streambanks and stream channel with a combination of rip-rap, erosion control features, hydro mulch, topsoil and/or compost. Establish vegetation as needed to stabilize amendments. Timeline: By proximity, this task will begin coincident with Tasks 2 and 3 during the 2017 construction season and the segments in Sneffels Creek will begin coincident with Phase 2 during the 2018 construction season. This task is anticipated to be completed during the 2018 construction season. Deliverables Due Dates DRAFT map of stream segments and design guidelines for excavation, grading, 30 days after ratification erosion control and stream restoration features. FINAL map of stream segments and design guidelines for excavation, grading, 45 days after ratification erosion control and stream restoration features. Construction completion. October 1, 2018 C. PHASE 2: PREVENT EROSION OF MODERN TAILINGS Description: As delineated in the AOC, the design and implementation of this phase may be completed under the authority and oversight of DRMS. The work will include regrading and stabilizing the slopes of the modern tailings in accordance with an engineered, geotechnical stability analysis, and further enhancing the left banks of Sneffels and Canyon Creeks to prevent erosion of the modern tailings from 100-year flood events. Timeline: The Permit will be amended by within 210 days after ratification and the implementation of the task will begin during the 2018 construction season. Page 4 of 5