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Once the spillway plans are approved, UDFCD will allow mining within 100 feet of the riverbank <br /> on the northeast side of the mine because the UDFCD considers the reinforcement placed near <br /> the reaeration weir to be sufficient riverside protection. The UDFCD will not allow mining within <br /> 100 feet of the riverbank on the southwest part of the site until after the UDFCD has completed <br /> riverbank protection construction in this area. <br /> SITE DESCRIPTION <br /> The Hazeltine mine is located on an alluvial terrace adjacent to the east bank of the South <br /> Platte River. The mine is on the north side of 1041h Avenue in Adams County, Colorado. The <br /> Bull Seep and Fulton Ditch are immediately east of the mine and a levee is present along the <br /> river bank west of the mine. <br /> Pre-mining topography shows the natural ground at the site slopes very gently to the north- <br /> northeast from a high point of approximate elevation 5045 in the southwest to a low point of <br /> approximate elevation 5036 on the northeast part of the site. The levee along the riverbank <br /> immediately west of the mine has crest elevations ranging from 5052 on the south to 5046 on <br /> the north. <br /> SIDE CHANNEL SPILLWAY DESIGN <br /> The spillway was designed using criteria discussed in meetings with UDFCD as well as criteria <br /> outlined in the UDFCD Technical Review Guidelines for Gravel Mining Activities (1987, minor <br /> revisions April, 2004). The spillway is located on the north part of the site where the levee <br /> terminates. The spillway crest will be 200 feet long with 10:1 cut slopes between the crest and <br /> natural ground. Five foot deep concrete cutoff walls will be constructed along both the pit and <br /> riversides of the crest. The river and pit side cutoff walls will have top elevations of 5039.8 and <br /> 5038.0 respectively resulting in a sloping spillway crest allowing drainage towards the mine. <br /> Type M, buried riprap will be placed on the river side of the cutoff walls and Type VH, buried <br /> riprap will be placed on the pit side of the cutoff walls. The riprap will extend to respective <br /> depths of five and three feet below the river thalweg on the river and pit sides of the spillway. A <br /> maintenance access road will be constructed across the top of the crest within the UDFCD <br /> easement. Following are the criteria used in determining the spillway length and location and <br /> riprap size. <br /> Standard UDFCD Spillway Length <br /> The UDFCD Technical Guidelines Document (1987, revised 2004) provides a formula to <br /> determine the lengths of two equal length side channel spillways at gravel mines along the <br /> South Platte River. Based on our meetings with UDFCD, UDFCD now prefers one spillway <br /> equal to the calculated length of the two spillways combined. The resulting formula for the <br /> spillway length is: <br /> Length = 2 * (0.6Ap/12,000) <br /> Ap=Surface area of the mine at the high water line in square feet. <br /> For the Hazeltine mine the surface area at high water line elevation 5,034 is approximately <br /> 5,000,000 square feet. Thus the spillway length is calculated as follows: <br /> Length = 2 * (0.6(5,000,000/12,000) = 500 feet <br /> - 2- February 2008 <br /> F:\3919_018_06\Spi IRvay\S pi llway_Desi gn_Memora nd um.doc <br />