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DESIGN MEMORANDUM <br /> Aggregate Industries, Hazeltine Gravel Resource Spillway, Adams <br /> County, Colorado <br /> PREPARED FOR: Aggregate Industries, West Central Region, Inc. <br /> 1707 Cole Boulevard, Suite 100 <br /> Golden, Colorado 80401 <br /> PREPARED BY: Tetra Tech <br /> 1900 South Sunset Street, Suite 1-F <br /> Longmont, Colorado 80501 <br /> Tetra Tech Job No. 133-3919.018.85 <br /> DATE: January 21, 2008 <br /> This memorandum has been prepared to summarize analyses and design recommendations for <br /> the Hazeltine Spillway to be located on the northwest side of the Hazeltine gravel mine in <br /> Adams County, Colorado. The spillway will be between the reclaimed gravel mine and the <br /> South Platte River. The spillway location is on the east bank of the river, approximately 0.4 <br /> miles north of 1041h Avenue. <br /> BACKGROUND <br /> Aggregate Industries (AI), is mining sand and gravel at the Hazeltine Pit. Al plans to reclaim the <br /> pit as a below grade lined reservoir meeting seepage requirements of the State Engineer of <br /> Colorado (SEO). Previously, a slurry wall was constructed around the mine perimeter to <br /> minimize groundwater seepage into the mine. To date, mining has been limited to the east and <br /> north parts of the site. Per Adams County permit agreements, mining can not encroach within <br /> 400 feet of the South Platte Riverbank unless a side channel spillway or pit side slope <br /> protection plans are approved by the Urban Drainage and Flood Control District(UDFCD). The <br /> spillway, coupled with riverside reinforcement to be installed this winter by UDFCD, will allow Al <br /> to mine within 100 feet of the riverbank as shown on the Al mine permit. <br /> Al contracted with Tetra Tech to design the spillway. We have developed plans and <br /> specifications for the project. The original flood plain permit called for pitside reinforcement at <br /> the site. Construction of a spillway will require an amendment to the Adams County Flood Plain <br /> permit. <br /> The UDFCD plans to construct riverside stabilization opposite the southwest part of the <br /> Hazeltine mine this winter. Some riverside reinforcement consisting of Type M buried riprap is <br /> already present near the northeast corner of the mine. This reinforcement was placed during <br /> construction of a reaeration weir built by Metro Wastewater. <br /> - 1 - February 2008 <br /> F:�3919_018_061SpillwayASpillway_Design_Memora nd um.doc <br />