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Lyons Quarry ��� <br /> 112d Reclamation Permit Operations <br /> Technical Revision <br /> operations. Without the available stockpiled overburden materials for placement and re-grading at <br /> the site, drill and blast excavation will be required in the overburden sedimentary rocks to achieve a <br /> balance of the cut and fill quantities to re-grade the mined areas. The mining operations at the site <br /> were further impacted by the 2013 flooding that destroyed access to the site. To support <br /> reclamation activities at the mine, HDR understands that Boulder County is to replace the site's main <br /> access point via a bridge over South St. Vrain Creek with an adequate tonnage capacity and vehicle <br /> clearance with respect to turn radius and overhead obstructions. <br /> 2 Project Location and Description <br /> 2. 1 Location <br /> Lyons Quarry is located southwest of Lyons, Colorado in Boulder County. Figure 1.1, Attachment <br /> A, shows the Lyons Quarry site just south of South St. Vrain Creek, along Colorado State Highway 7 <br /> in Sections 25 and 26 of Township 3 North, Range 71 West of the 6th Principle Meridian. <br /> 2.2 Recent Flooding <br /> The floods in September 2013 drastically changed the low-lying areas of the Lyons Quarry as up to <br /> 17 inches of rain fell over a three-day span raising the levels of the South St. Vrain Creek to 100 and <br /> even 500-year floodplains (Boulder County's annual precipitation is about 20.7 inches). The flood <br /> washed away much of the infrastructure and equipment that was located within the floodplain area <br /> as well as the two access bridges that crossed the South St. Vrain from Highway 7. As of 2016, the <br /> site remains inaccessible to motor vehicles. The flood also changed the channel alignment of the <br /> South St. Vrain as the new thalweg (lowest point of the channel) runs directly through the old scale <br /> house area. Figure 1.2 and Figure 1.3, Attachment A, display the pre- and post-flood conditions of <br /> the site as satellite images from October 7, 2012 and October 6, 2013, respectively. The extent of <br /> the flood destruction and change in creek flow especially at the east end of the site is shown on <br /> these images. <br /> 2.3 Lyons Quarry <br /> Lyons Quarry is currently inactive and is being prepared for closure and reclamation. The quarry <br /> consists of mined and leveled ground along the north end of the site, adjacent to South St. Vrain <br /> Creek, with mined highwalls and overburden waste piles extending less than 1500 feet to the south <br /> of the creek. Figure 1.4, Attachment A, shows the mining operations at the site in September 2002. <br /> As of September 2013, several one story buildings and processing equipment were present near the <br /> entrance of the site but most were destroyed or lost during the 2013 flood. Many of these structures <br /> were cleaned up, stockpiled away from the drainageway or removed from the site during the initial <br /> flood recovery operations in 2013-2014. <br /> The surface elevation across the site ranges from elevation (El.) 5,508 feet near the entrance of the <br /> site to above El. 6,000 feet at the south end of the site. Existing natural slopes in sedimentary rock <br /> strata are as steep as 1.75 horizontal to 1.0 vertical (1.75H:1.OV), where the sedimentary strata are <br /> capped by the Lyons Sandstone. Where the sedimentary rocks are not capped by the Lyons <br /> Sandstone, these slopes ranged from 2.OH:1.OV to 4.OH:1.OV. <br /> June 2016 1 2 <br />