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<br />M-2000-158 TR Memo 6 Mav 17, 2001 <br />is installed at ] : I, and not immediately covered, floodwaters flowing over and through the riprap on the steep <br />slope could result in failure of the armor and rapid down cutting. Even if soil cover is installed over the riprap, <br />and used to create a 3:1 slope, the soil could be washed away during a flood and the over steepened riprap layer <br />would be exposed. It is the Division's position that the pit banks where riprap will be installed must be graded to <br />2.5:1 prior to placement of the graded filter (if required) and the riprap. A soil cover may then be placed over the <br />riprap to improve the appearance and utility of the lakeshores, and to achieve the final 3:1 slopes. <br />Placement of lakeshore riprap at a 2.5:1 slope rather than the Applicant's proposed 1: I slope, will increase the <br />tonnage of riprap required per linear foot to 11.4 tons and the tonnage of bedding to 2.9 tons per linear foot. The <br />cost estimate for riprap per linear foot of lakeshore then becomes: <br />$10.5/Txll.4T/lf]+~$8/Txll.4T/If~+~$1/Txll.4T/lf~+~$3.5/Tx2.9T/lf~+~$2/Tx2.9T/lf~ <br />_ $238.25 per linear foot <br />Unless the Applicant provides a technical demonstration that riprap installed on the l :l mined slope will be <br />efTective, placement on a 2.5:1 slope should be required and $238.25 per linear foot installation cost should be <br />used to establish the amount of reclamation bond. <br />ATTACHMENTS: <br />Division of Minerals and Geology, 1998. In-stream aggregate extraction and reclamation guidance document. <br />Prepared by Greystone Consultants. 46 pages plus appendices. (Selected pages attached). <br />Urban Drainage and Flood Control District, 1987. Technical review guidelines for gravel mining activities within <br />or adjacent to 100-year floodplains. Prepared Wright Water Engineers. 50 pages. <br />United States Department of Agriculture, Soil Conservation Service. Rock Riprap for Streambank Protection on <br />Smaller Streams. Pages 186-189. <br />Please be aware this item has been scheduled for the May 24, 2001 (Thursday) Mined Land Reclamation Board <br />Hearing located at 1313 Sherman Street, Room 318 beginning at 9:00 a.m. Your response to this letter must be <br />submitted to the Division as soon as possible. If you have any questions, please do not hesitate to call me at 303- <br />866-4943. <br />Si r ly, c <br />J~ <br />Erica Crosby <br />Environmental Protec ~ pecialist <br />Enclosure <br />cc: Carl Mount; DMG <br />Cheryl Linden; AGO <br />Mark Held; AGO <br />Stevan O'Brian; Environment Inc. <br />Objectors & Interested Parties (via fax, a-mail or overnight mail) <br />