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<br />. <br /> <br />protection because the initial installation costs are often less than alternative methods for <br />preventing erosion. However, riprap installations require periodic inspection and maintenance. <br /> <br />Ordinary riprap refers to a protective blanket of large loose stones that are usually placed by <br />machine to achieve a desired configuration. Many factors govern the size of riprap required to <br />resist stream forces for a given situation. For the riprap itself, the variables include the size and <br />weight of the stones, the shape and gradation of the stones, the blanket thickness. the type of <br />bedding under the riprap, and the side slopes of the channel. <br /> <br />Hydraulic factors affecting riprap design include the flow velocity, flow depths, current vectors, <br />eddy action, and others parameters. Experience has shown that riprap failures result from <br />undersized stones, improper gradation of the stones (which reduces the interlocking action of the <br />material), and improper bedding below the riprap. Improper bedding allows the native channel <br />soils to leach through the stones thereby causing a potential bank failure. <br /> <br />Rock used for riprap should be angular in shape and free from cracks, overburden, shale, and <br />organic matter. The rock should also have a specific gravity of 2.65, but never less than 2.50. <br />The recommended gradation for installation at the subject property is presented in the table <br />below. <br /> <br />. <br /> <br />% Smaller Than <br />Given Size by Intermediate Rock <br />Weight Dimension (Inches) Dso (Inches) <br />100 42 <br />50-70 33 24 <br />35-50 24 <br />2-10 9 <br /> <br />Gradation of Riprap <br /> <br />Granular bedding for the riprap is required for proper installation. A single layer consisting of an <br />8 to 12 inch blanket of small granular rock is highly recommended for the subject property. The <br />gradation for the bedding is presented in the table below. <br /> <br />Gradation of Granular Bedding <br /> <br /> % by Weight <br />U.S. Standard Sieve Size Passing Sieves <br />'" 90-100 <br />J <br />I to 1.5" -- <br />3/4" 20-90 <br />3/8" -- <br />#4 0-20 <br /> <br />. A design drawing for riprap installation is included in Appendix A. <br /> <br />5 <br />