2001-05-16_REVISION - M2000158 (2)

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Appertdu D - Potential Hydrologic Structures ro Minimize lmpacrs and Mining Methods 7 BANK PROTECTION Bank protection may be required during operations, following reclamation, or both. Typically, armoring is performed to reduce bank erosion, but may be used to protect an operation's facilities or materials. Armoring of the mid-channel bar was identifted in split channel mining on Figure 8. Bank Armoring Armoring of a bank is generally installed through a zone that is likely to experience lateral migration, typically, the outside bank of a curve. Bank protection is a form of channelization and should be undertaken only when absolutely necessary. Frequently the impacts from such stabilization will create scour and erosion at other points in the river system. The design of dikes or revetements should take into consideration the design flood event for which protection is needed, end protection of the dike, toe protection of the dike, and the material available for construction. Engineered designs are based on site-specific information, but the -1 armoring is only as good as its installation. The failure to follow the design or the use of „~. Operators can install armoring with rip-rap, a combination of vegetation and rip-rap, soil cement, or wired gabion baskets. Rip-rap is a layer of angular, durable rock for which the median diameter and a size gradation have been specified. The median diameter of rip-rap is its Dso, 50 percent of the rip-rap is lazger and 50 percent is smaller. Rip-rap ranges in size from 0.2 x ~o to 2 x Dso. Standard rip-rap classification types are described in Table D-1. The thickness of the installation ranges from 1.5 x Dso to 1.75 x Dso. Rip-rap requires placement on a graded earthen material filter blanket or geotextile to prevent the loss of fine-grained bank material. Side slopes for rip-rapped banks, should be no steeper than 2.5:1 (H:V). General specifications can be reviewed on Figures D-5 and D-6. End protection can be achieved by extending the armoring into anon-erosive area within the floodplain or by doubling the thickness of the armoring at each end (Figure D-7). Similazly, toe protection can be achieved by burial of the toe of the rip-rap at least five feet, or through the extension of thicker rip-rap into the channel bed (Figure D-8.) Photographic examples of bank armoring are shown in Figure D-9. Several of the photographs are from non-mining applications. Biorevetements Geotextiles in fluvial settings provide temporary protection prior to the establishment of vegetation. Geotextiles are woven or non-woven fabrics that promote bank stabilization through a mulching capacity or as a strengthener. These synthetic fabrics may be a coarse netted fabric sandwiching a straw, coconut hull, or synthetic blanket that are used as mulches. Synthetic, non- woven or finely woven fabrics can be employed as replacements for graded gravel filter blankets below rip-rap. The use of geotextiles is considered a biorevetement when seeding is incorporated n.sseu~~~r3, iwa D-3 inappropriate or undersized materials will result in only temporary bank protection