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8-45 <br /> 804.3 Open Channels(cont.) <br /> C. Revetment Design ing from 2 inch through 24 inch riprap.It <br /> is not applicable for non-uniform flow. <br /> •� When the erosive forces of moving <br /> water are too great for an unlined earth The riprap selected for design must be of <br /> channel, a protective lining such as rip- sufficient stone size so that the resistive <br /> rap is required. Discharge, slope, size, shear,'CR,is greater than.The Division's <br /> shape and roughness of the channel are Standard Specifications' sizes of riprap <br /> considered in riprap lined channel de- are: 9",12",18" and 24". <br /> sign. The following design procedure, <br /> based on tractive force, is condensed The total thickness of the riprap layers <br /> from the Highway Research Board Re- should be at least 1-1h times the selected <br /> port 108.12 riprap size. When the flow will be of <br /> extended duration, or a greater safety <br /> Erosion occurs when the dynamic forces factor is desired, the total thickness <br /> of the water acting on the channel should be 2 to 2-1/z times the riprap size. <br /> boundary exceed the resistive forces of See Figure 804-3C for typical riprap geo- <br /> the channel lining.The unit shear acting metry. <br /> on the channel by uniform flow is ex- <br /> pressed as, Other design procedures described in <br /> reference No.4 may be more applicable <br /> 'r =62.4 CRS to certain situations. <br /> where, Z is the unit shear(lbs/sq.ft.) Plastic filter cloth is used to prevent <br /> along the channel perimeter leaching of finer material underlying the <br /> riprap. Leaching results when the total <br /> R is the hydraulic radius (area in sq. ft. layer thickness is inadequate and the <br /> divided by wetted perimeter in ft.) underlying material is fine, or when <br /> � — there is considerable wave action. <br /> S is the slope of the channel (ft/ft) <br /> The value of C is a function of the <br /> channel shape, alignment, and velocity <br /> distribution and is determined from Fig- <br /> ure 804-3D.The unit shear is determined <br /> separately for the sides and bottom.For <br /> determining shear stress in bends, the <br /> bend coefficient Cc is multiplied by Cb <br /> to determine shear stress on the bottom, <br /> and by Cs for the side shear stress(i.e.C= <br /> CcX Cb for bottom and C=Cc X Cs for <br /> side). <br /> The resistive unit shear of riprap is the <br /> value of the shear at which there is no <br /> movement of the stones. This is ex- <br /> pressed as <br /> T R= K4d50 <br /> where,d50(ft)is the mean stone size for <br /> which 50% of the stone mass consists of <br /> larger particles and 50% are smaller. <br /> The reduction factor of the resistive unit <br /> boundary shear,K,for riprap placed on <br /> side slopes is given in Figure 804-3E.This <br /> equation is valid for particle sizes rang- <br />