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7 60 riprap protection should be provided adjacent to

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7-60.Riprap protection should be provided adjacent to hydraulic structures (drop structures, energydissipaters). When placed on erodible surfaces, it prevents scour at the ends of the structure. This protectionis required on the bed and banks for a sufficient distance to establish velocity gradients and turbulence levelsat the end of the riprap. The expansion allows the flow to expand and dissipate its excess energy in turbulencerather than directly on the channel bottom and sides. Riprap has been known to fail from—Movement of the individual stones from a combination of velocity and turbulence.Movement of the natural bed material through the riprap, resulting in slumping of the blanket.Undercutting and leveling of the riprap from scour at the end of the blanket.7-61.Consideration must be given to the selection of an adequate size of stone, use of adequately gradedriprap, provision of a filter blanket, and proper treatment of the end of the riprap blanket. The stone sizeselection required for protection with Froude numbers (the ratio of inertial forces to gravity forces) and depthsof flow in the channel is shown in figure 7-12, page 7-22. Two curves are given. One is for riprap subjectedto direct flow or adjacent to hydraulic structures (side inlets, confluences, energy dissipaters) whereturbulence levels are high. The other is for riprap on the banks of a straight channel where flows are relativelyquiet and parallel to the banks.
Chapter 77-22TM 3-34.48-129 February 2016Legend:ddepth of flowDdiameterFflowVvelocityFigure 7-12. Recommended riprap sizes for open channels7-62.With the depth of flow and average velocity in the channel known, the Froude number can be computedfrom the following equation:𝐹𝐹=0.716V√𝑑𝑑where—D=depth of flowF=flowV=velocityNote.The D50value can be determined from the appropriate curve.7-63.Curves for determining the riprap size required to prevent scour downstream from culvert outlets withscour holes of various depths are shown in figure 7-13. Make the thickness of the riprap blanket equal to thelongest dimension of the maximum size of stone or1.5 ×𝐷𝐷50, whichever is greater.7-64.When the use of large rock is desirable but impractical, substituting a grouted reach of smaller rock inhigh velocity areas or turbulence may be appropriate. An ungrouted reach should follow grouted riprap.
Open-Channel Design29 February 2016TM 3-34.48-17-23Legend:ddepth of flowDdiameterFflowVvelocityWwidthFigure 7-13. Recommended riprap sizes for culvert outlets7-65.A well-graded mixture of stone sizes is preferred to a relatively uniform size of riprap. A recommendedgradation is shown in figure 7-14, page 7-24. In certain locations, the available material may dictate thegradation of riprap to be used. The gradation should resemble the recommended mixture as closely aspossible. Consider increasing the thickness of the riprap blanket when locality dictates using gradations witha larger percentage of small stone than shown by the recommended plot. If the gradation of the availableriprap is such that movement of natural material through the riprap blanket would be likely, place a filterblanket of sand, crushed rock, gravel, or synthetic cloth under the riprap. The usual blanket thickness is 6inches, although a greater thickness is sometimes necessary.

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Term
Fall
Professor
NoProfessor
Tags
United States Navy, United States Air Force, United States Army, Reconnaissance, Culvert

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