2001-05-16_REVISION - M2000158 (2)

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Potential NYdroloRic Strucntru to MitimiZe /nWaar and MitinR Mtthadr 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~hannel baz was identified in split channel mining on Figure D-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 revetments should take into consideration the design flood event for which protection is needed, end protection of the dike, tce protection of the dike, and the material available for construction. Conservative engineered designs are important to provide site-specific information, but the armoring is only as good as its installation. The failure to follow the design or the use of inappropriate or undersized materials will result in only temporary bank protection. 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 angulaz, durable rock for which the median diameter and a size gradation have been specified. The median diameter of rip-rap is its I~, and 50 percent of the rip-rap is larger and 50 percent is smaller. Rip-rap ranges in size from 0.2 x D~ to 2 x Dso . The thickness of the installation ranges from 1.5 x D~ to 1.75 x Dom. Rip-rap requires placement on a graded earthen material filter blanket or geotextile to prevent the loss of fine-grained bank material. Side slopes should be no steeper than 2.5:1 (H:~. General specifications can be reviewed on Figures D-5 and D-6. End protection can be achieved by extending the armoring into a non~rosive area within the floodplam or by doubling the thickness of the armoring at each end (Figure D-'n. Similarly, tce protection can be achieved by burial of the tce of the rip-rap at least five feet, or through the extension of thicker rip-rap into the channel bed (Figure D-8.) Jetties Jetties consist of dike-type structures which may be used for bank protection (Figure D-9). jetties project from the toe of a graded 4:1(H:~ channel bank 15-25 feet. Jetties are installed either perpendicular to a stream's centerline or with a 45 degree orientation downstream. Jetty spacing should be placed at a ratio of centerline spacing to jetty projection less than or equal to three. Thus, if the jetty extends 25 feet from the tce of the baak, jetty installation should occur R1.6561kbn,vY Z6, Isve D-5