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<br />Colorado State University (CSU) has under development a riprap <br />design method which applies not only to channel linings but <br />also to riprap design at bridge abutments and other channel,. <br />discontinuities (8). However, from preliminary evaluation ' <br />it does not appear that results from the CSU method will <br />differ appreciably from the methods of this circular, <br />assuming uniform, developed channel flow. When the flow is <br />rapidly varied, as when discontinuities such as bridge <br />abutments disrupt the flow, the methods of this circular <br />should not' be ,'used. <br /> <br />e <br /> <br />it <br /> <br />III. Design Concepts <br /> <br />The basic design method presented in this circular is based on <br />the concept of maximum permissible depth of flow, coupled with <br />the hydraulic resistance of the particular lining material. <br />In all cases, the lining material defines the hydraulic <br />resistance of the channel while providing its own peculiar <br />degree of erosion protection. <br /> <br />Erosion Prevention <br /> <br />The dmax charts are used to define the maximum permissible <br />depth of flow for a particular lining, based on channel slope, <br />So' and the erodibility of the underlying soil. The maximum <br />permissible depth concept is based on the tractive force <br />theory of channel lining design. Tractive force, basically, <br />is the shear stress exerted by the flow on the channel perimeter. <br />For wide channels of any shape, and for a given channel slope, <br />depth, and lining, the vertical velocity distribution in the <br />central and deepest section, where wall effects are negligible, <br />should be identical. Also, the first scour occurs at the <br />deepest portion of the channel, since the wall or bottom shear <br />stress is greatest in that portion. <br /> <br />e <br /> <br />If ,the depth of flow, channel slope, lining, and soil are the <br />same in all channels shown in Figure 1, then the flow rate <br />and the mean channel velocity for the three channels will be <br />different, but in the central section of the channels, <br />represented by AX, the vertical velocity distribution and <br />bottom shear stress will be nearly identical. Therefore, in <br />these channels, there exists a limiting depth of flow above <br />which scour will occur, and this depth, dmax, is the same <br />for all wide channels of the same longitudinal slope, lining, <br />and underlying soil. Of course, any depth of flow less than <br />dmax is noneroding. <br /> <br />. <br /> <br />.. <br /> <br />e <br /> <br />6 <br />