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Ms.Amy Eschberger <br /> RE:Irwin/Thomas Mine M-2016-054 <br /> July 30, 2018 <br /> ✓ The St. Vrain Creek channel through the project reach is stable and will require minimal <br /> maintenance during mining and following reclamation. Infrastructure within the project <br /> reach including stormsewer outlets, improved trails, and road bridges maintain the <br /> creek's vertical and lateral stability. Historical photography shows the creek channel has <br /> maintained it's present location for decades, even following the September 2013 flooding <br /> event. <br /> ✓ Flood flows from St. Vrain Creek have the potential to affect all mining areas. All four <br /> mining phases are within the St. Vrain Creek 100-year floodplain. Although the <br /> jurisdictional floodplain within the project reach will not change, new modeling <br /> information produced in response to the September 2013 flooding event has provided <br /> updated hydrology and creek hydraulics through the project reach. The updated <br /> information was used for all project flood impact and mitigation evaluations. <br /> ✓ Although flood flows will likely enter mining and reclaimed areas,the potential for creek <br /> flows re-routing through the areas is extremely low. With the recommended mine <br /> operation phasing and backfilling (see below), scour potential from overbank flows into <br /> the mining areas will be limit and/or mitigated by setback distances and will minimize the <br /> potential for scour to headcut into the creek channel. The mining areas may temporarily <br /> capture flood flows and fill with flood waters, but when the flood waters recede the creek <br /> flows are expected to return to the existing creek channel. <br /> Mining area MA1 is bound upstream and downstream by State Highway 119 and North <br /> 119th Street, respectively. During the 100-year flooding event, flood flows are <br /> anticipated to overtop both roads. Overtopping of State Highway 119, however, is <br /> predicted to be shallow and of short duration during a flood event. Even with the <br /> addition of the MA1 mine cell(s) downgradient, scour potential across the highway is <br /> low. Overtopping of North I I9th Street is expected to be substantial. Flood flows across <br /> the road are predicted to be widespread across, and downgradient, of the road with <br /> relatively low flow velocities. The generally flat area, with little to inhibit or concentrate <br /> flows, downgradient of the road would not be expected to produce a high scour potential. <br /> Mining areas MA2 and MA3 are located immediately downgradient from recent <br /> St. Vrain Creek floodplain improvements. Although not reflected in the modeling <br /> information used (there was no "Proposed Condition" model for the project reach), the <br /> physical improvements limit the 100-year flood event to the creek channel and <br /> immediately adjacent floodplain. Although still flooded by overbank flows, the effect on <br /> MA2 and MA3 from the improvements would be expected to include minimal upgradient <br /> overbank flows entering the mine cells and reduced velocities of overbank flows <br /> downstream. Additionally, the smaller size of the mining areas and shallower depths of <br /> mining (aggregate depths are predicted to be less than 15 feet from existing ground <br /> surface) will limit the scour potential from flood flows entering the mine cells. With the <br /> recommended mine operation phasing and backfilling (see below), scour potential from <br /> overbank flows into the mining areas will be limit and/or mitigated by setback distances <br /> and will minimize the potential for scour to headcut into the creek channel. <br /> Page 2 of 5 <br />