2001-05-16_REVISION - M2000158 - Laserfiche WebLink

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Memo to Erica Crosby 7 May I5, 2001 must be in place (DMG. 1998, figure D-5, copy attached). Since the Applicant will be required to armor cut banks and other areas of potential instability as determined by the Division and the Applicant, and will be required to install the riprap prior [o mining within 400 feet, there is no need to bond for the riverbank riprap installations. This is because the permit requirements will dictate that the riprap be in place before it is actually needed. The Division has reservations about the Applicant's proposal is to install the pit slope riprap onto the l: l mined slopes, then fill and grade over the riprap to establish the finished 3:1 grade. The Division's guidance states that side slopes for riprapped banks should be no steeper than 2.5:1 (DMG, 1998, p. D- 3, copy attached). L` the riprap is installed at 1:1, and not immediately covered, floodwaters flowing over and through the riprap on the steep slope could result in failure of the armor and rapid down cutting. Even if soil cover is installed over the riprap, and used to create a 3:1 slope, the soil could be washed away during a flood and the over steepened riprap layer would be exposed. It is the Division's position that the pit banks where riprap will be installed must be graded to 2.5: l prior to placement of the graded filter (if required) and the riprap. A soil cover may then be placed over the riprap to improve the appearance and utility of the lakeshores, and to achieve the final 3: l slopes. Placement of Lakeshore riprap at a 2.5:1 slope rather than the Applicant's proposed 1:1 slope, will increase the tonnage of riprap required per linear foot to 1 l.4 tons and the tonnage of bedding to 2.9 tons per linear foot. The cost estimate for riprap per linear foot of Lakeshore [hen becomes: $10.5/Txl 1.4T//f~+~$8/Txl1.4T/lf~+~$1/Tx11.4T/[f~+~$3.5/Tx2.9T//f~+~$2/Tx2.9T/!f~ _ $238.25 per linear foot Unless the Applicant provides a technical demonstration that riprap installed on the 1:1 mined slope will be effective, placement on a 2.5:1 slope should be required and $238.25 per linear foot installation cost should be used to establish the amount of reclamation bond. ATTACHMENTS: Division of Minerals and Geology, 1998. In-stream aggregate extraction and reclamation guidance document. Prepared by Greystone Consultants. 46 pages plus appendices. (Selected pages attached). Urban Drainage and Flood Control District, 1987. Technical review guidelines for gravel mining activities within or adjacent to 100-year floodplains. Prepared Wright Water Engineers. 50 pages. United States Department of Agriculture, Soil Conservation Service. Rock Riprap for Streambank Protection on Smaller Streams. Pages 186-189.