2001-05-22_REVISION - M2000158 - Laserfiche WebLink

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ENVIRONMENT, IN G. MAY 22, 2001 PAGE 4 will be doubled in accordance with the guidance document's recommenda- tions. River bank armoring The armoring will be done along the river channel side of the bank, but only where mining is within 400 feet of the river. A map will be prepared and submitted to the Division showing the areas of concern for review and approval prior mining within 400 feet of the river. in the areas where armoring is required, Platte Sand & Gravel will follow the guidelines published by the Division. In general, a DSO x 1.75 to 3.0 will be used depending on the site specific require- ments in each reach being armored. if an armored reach ends on a natural riverbank, the Division's published guidance for riprap end protection will be followed. This calls for doubling the riprap thickness at the ends of an armored reach that begin and end on natural banks or extending them to non erosive areas. The riverbank armor will be keyed five feet below the river bed or, alternatively, extension of a thicker (doubled) layer of riprap into the channel bed will be done as suggested in the guidance docu- ment. (Figures D-5 and D-B, DMG, 1998, copies attached). There may be areas where keying the toe into the river bottom is not practical so both methods may be used depending on what is encountered in a reach. A granular bedding and filter will be placed under the rip rap. As noted above the in situ material will be tested at each site to determine if it is acceptable. Prior to installing riprap into areas of a cut bank, it will be graded to no steeper than 2.5:1 and granular bedding and filter subgrade will be placed. On the areas of riprap above the normal highwater line the applicant will use a method of slope protection measures found in section 2.3.2 of the Urban Drainage and Flood Control guideline document. in general, a cover of dirt will be placed over the riprap and willow stakes and grass will be planted to lock the dirt in place. The willows will eventually spread and create a vegetation blanket that slows flood water along a bank reducing erosion. The timetable for riprap installation The timing for rip-rap placement depends on how fast the mine develops. The operator will make every reasonable effort to complete final lakeshore protection measures for all such areas prior to April 1st of each year. The lakeshore sensitive areas are those areas depicted in green on the map included with this response. Map Exhibit C-1 show how the mine will look approximately 15 years after mining