2018-06-25_REVISION - M2011001 (2)

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April 5,2018 Page 2 BACKGROUND The Iverson Mine is located in Section 34, Township 1 North, Range 66 W 611, P.M. in Weld County, Colorado. The site is about 0.45 miles east of 59 h Avenue and 0.15 miles north of F Street at 5011 West F Street, Greeley, Colorado 80631. An aerial map of the mine is shown on Figure 1. The site covers an area of 66.5 acres and is located in an oxbow of the Cache la Poudre River. The site is located entirely in the 100-Year Flood Zone of the Cache La Poudre River. The FEMA FIRM Map of the site is shown in Figure 2. All elevations in this report are based on NGVD 1929 Datum as adjusted. Concerns regarding the flood impacts at the site were documented in the First Adequacy Review by DMRS dated 4-1-2011. The DRMS noted in Comment 16 that portions of the proposed mine site lie within the 100-year flood plain and stated that a flood event would likely cause the pit to be captured by the river. Comment 16 required that a flood analysis and flood control plan must be evaluated and submitted, including pit side armoring, river side armoring, inflow and outflow channels, or other appropriate measures. Comment 16 stated that the UDFCD Technical Review Guidelines for Gravel Mining&Water Storage Activities Within or Adjacent to 100-Year Floodplains (UDFCD Guidelines) will be used for the adequacy review. The DMRS also noted concerns in Comment 14 regarding the proposed 100-foot setback from the Cache La Poudre River and required plans for engineered inflow and outflow structures to be submitted. A review of the responses to the adequacy review dated June 6, 2011 and July 8, 2011 did not find a flood analysis and flood control plan based on hydraulic modeling. The setback was increased to 200 feet at potential erosion locations based on a site visit. There were no designs submitted for pit side armoring and river side armoring. The responses stated that the inflow and outflow channels would be constructed at the time of reclamation. Gravel mining operations were started by Martin Marietta in June 2012 under Permit No. M-2011-001 from the DMRS. The mining was interrupted in September 2013 when the riverside berm was breached and the mine flooded. The riverside berm was breached a second time on May 25,2014. Subsequently, Martin Marietta ended mining operations in June 2014. Reclamation work was conducted by Martin Marietta starting in February 2015 consisting of site grading and construction of a lake to reclaim the mine pit. The work included construction of inlet and outlet spillways to the lake. The lake was filled from March to May 2015. A flood washed out the spillways and breached the riverside berm at the northwest end of the lake on May 12,2015. The spillways were re-constructed in the spring of 2016 by Martin Marietta. The re-construction did net follow the notification and approval process of DMRS and a technical revision to the reclamation plan was submitted to DMRS in September 2017. The technical revision included revised grading plans and technical specifications. Construction activities on the revised plan started on October 25, 2017 and the landowner was notified on December 1, 2017 that all work was complete. The reclamation completed on December 1, 2017 includes an unlined lake with a surface area of 27.2 acres. Refer to Figure 1 for the location of the reclamation improvement components discussed in the following sections. The lake is separated from the river by a riverside berm about 5,400 feet in length. Trapezoid inlet and outlet spillways are located through the berm at the south side of the lake. The spillways are intended to equalize the water surface in the river and lake during flood events. The