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<br /> <br /> <br /> <br />9 <br />Bedding and Filters <br />Once the gradation of the riprap is determined, the <br />gradation and thickness of the bedding layer should be <br />determined. In principal, the bedding layer provides a <br />foundation for the riprap placement and also provides <br />a filter-compatible transition layer to finer, underlying <br />embankment materials. The finer embankment <br />material underlying the riprap could be washed out <br />through the rock particles during reservoir fluctuations <br />and wave action. Retention of the underlying <br />embankment materials is attained by placing a finer- <br />grained layer of bedding under the riprap. Where very <br />large riprap is used, a progressively finer two-stage <br />bedding/filter layer may be required. The bedding <br />layer needs to be filter-compatible with both the <br />underlying embankment material and overlying riprap <br />to limit the potential of erosion and washout of both <br />embankment and bedding material between the voids <br />of the riprap. <br />Generally bedding should be a well-graded mixture of <br />gravel and sand that is filter-compatible with both the <br />riprap and the embankment materials. There is some <br />general guidance on developing the filter-compatible <br />gradation and the recommended thickness provided in <br />Chapter 7 of Embankment Dams (Reclamation 1992) <br />and in Design of Riprap Revetment (FHWA 1989). The <br />general guidance for bedding thickness is summarized <br />in Table 1. <br />Table 1: Bedding layer thickness according to riprap <br />layer thickness. <br />Riprap Layer <br />Thickness <br />Bedding Layer <br />Thickness <br />12-24” 9” <br />27-36” 12” <br />Over 36” 15” <br /> <br />Limit and Layout of Riprap Protection <br />According to TR-69, the lower limit of the riprap <br />protection should be 1.5 times the significant wave <br />height (Hs) below the reservoir normal water level at <br />the lowest ungated opening, or below the lowest <br />controlled outlet. <br />The upper limit of riprap is described by TR-69 as the <br />vertical distance above the reservoir still water flood <br />pool level equal to the sum of the wave runup (R) and <br />wind setup (S). This can be calculated as described in <br />the previous article. The lower limit of riprap is <br />determined by the lower of either the (a) vertical <br />distance of 1.5 times Hs below the still water flood <br />pool, or (b) lowest controlled outlet elevation. <br />The upper and lower limits of riprap are shown on <br />Figure 4. <br /> <br />Figure 4: Typical upper and lower limits for riprap <br />placement. <br /> <br />For owners of existing small dams the extent of a <br />riprap revetment may be limited by the budget <br />available to complete the project. When this is the <br />case the owner and designer should carefully consider <br />where the riprap can offer best value from a dam <br />safety and operational perspective. Priorities could <br />include, but may not be limited to, providing riprap on <br />sections of the embankment where erosion has <br />previously occurred, is deemed likely to occur (i.e., <br />adjacent to concrete structures and other <br />infrastructure), and or in horizontal bands at the <br />reservoir normal water level or normal operational <br />water level. Experience has shown that dam <br />embankments built with interior or exterior bends or <br />at angles that are perpendicular to prevailing winds, <br />can be more susceptible to erosion. Armoring of these <br />areas should be a priority. <br />Placement <br />According to TR-69, for dumped rock, the placement of <br />bedding and riprap on a dam embankment should be <br />as shown on Figure 5. This figure shows the riprap <br />supported by a level berm (also refer to Figure 4), <br />which facilitates placement.