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Dowe Flats Special Use Permit Amendment Application May 2, 2022 <br /> highwall stability. The figure below shows the "L"-shaped benches'up dip of the mined <br /> limestone (HiCao and shale beds (2"d, 3'd, and 4t' ridges). <br /> Quarry to Excavate Quarry to Excavate <br /> Quarry to Excavate 3rd Ridge 4th Ridge <br /> Hical(includes 2rd Ridge (includes 34IR and <br /> and Interndge materials) 4th Ridge materials) <br /> To increase safety and stability, when mining, CEMEX, Inc. creates 12- to 22-foot-wide catch <br /> benches near the mining limit. This creates a 2:1 (horizontal: vertical) slope so if rocks fall, they <br /> land on the bench and cannot reach the pit floor. As material is mined and the rocks are piled <br /> up, CEMEX, Inc.'s operating procedures ensure that rock dumps do not exceed 35-degree <br /> slopes. CEMEX, Inc.'s proper mine planning and best management practices further assure no <br /> rockslides occur within the quarry. <br /> As noted in the figure above, limestone and shale outcrop near the surface, covered by a thin <br /> veneer of sandy-clay, rock fragments and topsoil. Both the limestone and shale units are <br /> competent and not subject to expansion, subsidence, or soil creep, and do not diminish the <br /> ground's integrity. Since commencement of the mine, there have been no signs of karst features <br /> or subsidence, landslides, mudslides, mudfalls, debris fans, unstable slopes, or rockfalls. The <br /> quarry is outside of mapped floodplains and flood-prone areas and was not impacted by the <br /> 2013 flood. The quarry is also not situated on an alluvial fan or in an avalanche corridor. <br /> CEMEX, Inc. owns its own drone and regularly flies the property to obtain up-to-date <br /> topographic mapping of the mine. This data aids CEMEX, Inc. with mine planning to keep the <br /> operation safe and in compliance with their permits and best management practices. <br /> 13. The proposed use shall not alter historic drainage patterns and/or flow rates unless the <br /> associated development includes acceptable mitigation measures to compensate for <br /> anticipated drainage impacts. The best available information should be used to evaluate <br /> these impacts, including without limitation the Boulder County Storm Drainage Criteria <br /> Manual, hydrologic evaluations to determine peak flows, floodplain mapping studies, <br /> updated topographic data, Colorado Geologic Survey landslide, earth/debris flow data, <br /> and creek planning studies, all as applicable given the context of the subject property <br /> and the application. <br /> A Drainage Report has been prepared for Dowe Flats, which shows drainage patterns and flow <br /> rates for the pre-mining versus reclamation plan conditions at the site. The entire Dowe Flats <br /> mining/quarry site is within one drainage basin that is not intersected by exterior drainage <br /> basins. Furthermore, all stormwater drainage after final reclamation will flow in the same <br /> direction (from north to south) as before mining, any minor amounts of runoff not infiltrating <br /> within the permit area will exit the site at the south end in the same manner as before mining. <br /> No increase in peak runoff rates is expected when comparing pre-mining to reclaimed <br /> conditions. <br /> Review Criteria Statement-Page 11 D TETRA TECH <br />