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<br /> <br /> <br />- <br /> <br />EM 1110-2-1601 <br />1 July 70 <br /> <br />-, <br /> <br />. . <br /> <br /> <br />.eference 52 presents information Qn grass-lined channels. Departures from <br />uggested permissible velocity or shear values should be based on reliable <br />ield experience or laboratory tests. Channels whose velocities and/or shear <br />ceed permissible values will require paving or bank revetment. The per- <br />issible values of velocity and/or shear should be determined so that damage <br />ceeding normal maintenance will not result from any flood that could be <br />easclnably expected to occur during the service life of the channel. <br />4. Riprap Protection. a. General. The criteria presented herein may <br />e Classified into two general categories: those which apply to the physical <br />haracteristics of the riprap material and those which apply to the stability <br />of the riprap revetment. Criteria pertaining to the physical characteristics <br />of the riprap material are based on available field experience and past prac- <br />tice. Criteria for the stability of riprap revetment are based on the analyt- <br />ical determination of shear forces created by channel flow and the ability of <br />the riprap revetment to withstand those forces. Available laboratory data <br />were utilized in developing the analytical method for determining shear <br />forces. While the criteria yield definite sizes. and gradations of riprap, these <br />should be used for guidance purposes and revised as deemed practical to <br />provide for such other factora as are diacussed herein. Riprap protection for <br />flood control channels and appurtenant structures should be designed so that <br />damage exceeding nominal maintenance or replacement (see para 7v, <br />ER 1110-2-1150) will not result from any flood that could reasonably be ex- <br />pected to occur during the aervice life .of the channel or structure. <br />b. Factors Affecting Riprap Revetment Stability. The ability of riprap <br />revetment to resiat the erosive forces of channel flow depends on the inter- <br />relation of the following main factors: stone shape, size, weight, and grada- <br />tion; channel side slopes, roughness, shape, alignment, and invert slope; and <br />riprap layer thickness. In addition, the manner of placement and treatment <br />of e>c:tremities affect the stability of riprap revetment. <br />c. Stone Shape. Riprap stone should be blocky in shape rather than <br />elongated, as more nearly cubical stones "nest" together best and are most <br /> <br />r' ._~ <br /> <br />; <br /> <br />37 <br /> <br />\~,--_/ <br />