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Ms. Amy Eschberger <br /> RE:Irwin/Thomas Mine M-2016-054 <br /> July 30, 2018 <br /> Mining area MA4 is furthest from St. Vrain Creek and has very limited potential for <br /> severe scouring. Flood flows entering MA4 would be expected to be shallow and of low <br /> duration during a flood event. It is unlikely that significant damaging scour would occur <br /> from flood flows entering or exiting MA4. <br /> ✓ Flood Mitigation Recommendations <br /> Flood impacts from overbank flows entering the active or reclaimed mining cells can be <br /> minimized by adjusting mine phasing and reclamation setbacks. It is recommended that <br /> the proposed mining areas closest to the creek be mined and backfilled before other <br /> portions of the mining areas. If possible, the initial mining and backfilling should also <br /> occur during non-flood prone seasons (October through April). For MA1, this means <br /> mining a portion of Cell 6 in conjunction with Cell lb. Early mining of Cell 6 should <br /> include the sediment pond area, extend at least 500 feet from the St. Vrain Creek top of <br /> bank, and be backfilled with a compactable overburden type material. With this <br /> recommendation, as the larger area of MA is mined, a safe setback from St.Vrain Creek <br /> will have already been established. <br /> The same mine phasing and reclamation recommendations apply to the mine cells in <br /> MA2 and MA3. The 200 feet of the mine cells closest to the creek should be mined first <br /> and backfilled with overburden type compactable material during non-flood prone <br /> seasons if possible. To achieve reclamation goals, backfilling of MA2 and MA3 mine <br /> cells closest to the creek has been planned to be as much as 5 feet below grade to <br /> accommodate future wetland areas. However, to protect the mining and reclaimed area <br /> from overtopping flood flows, the first 100 feet of backfill closest to the creek should <br /> extend to pre-existing grade prior to reducing the backfill elevation for wetland <br /> development. <br /> As mining progresses, the additional setback area created with the initial backfilling will <br /> provide enough material so that scour created from potential flood flows entering the <br /> mine cells (or reclaimed cells) should not be able to headcut back to the creek channel <br /> and, as flood flows recede, flows will return to the existing creek channel. Mining the <br /> areas closest to the creek first ensures that the active mine cells are relatively small when <br /> they are at the greatest risk of overtopping during flood flow events. If overtopping does <br /> occur, the smaller mining cells will fill with flood waters quickly, not allowing time for <br /> scour and headcutting to catastrophically damage the mining cell and/or creek. <br /> Additionally, in the event of flood waters damaging property during the active DRMS <br /> permit within the proposed permit boundary, or property outside the permit boundary <br /> caused exclusively by the operations within the active permit boundary, the applicant <br /> agrees to correct the damage in a timely manner. <br /> Page 3 of 5 <br />