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<br />:For the other' channel linings, Manning n values-were found to <br />vary with slope and hydraulic radius; therefore, empirical <br />curves were developed to represent the data. Equations are <br />shown on the'respective charts for the unlined channel and <br />the temporary channel linings. For the vegetative linings <br />of various retardances, curves were taken directly from the <br />SCS Handbook (5). Retardance is the hydraulic resistance <br />relationship for a certain group of grasses of given lengths <br />as defined by the SCS (See Appendix C)." Retardance A refers <br />to grasses of high hydraulic resistance, such as 3D-inch <br />Weeping lovegrass, while Retardance E refers to grasses of <br />very low hydraulic resistance, such as 1.5-inch Bermuda grass. <br /> <br />e <br /> <br />;t <br /> <br />, <br /> <br />Channel Geometry <br /> <br />After the maximum permissible depth of flow has been defined, <br />it is necessary to relate that depth to the area and hydraulic <br />radius of the flow prism for a specific channel geometry. <br />There are a variety of methods of defining those relationships, <br />ranging from direct computation to channel geometry plots, <br />such as Chart 1 of this circular. Chart 1 was developed for <br />trapezoidal channels, but similar graphs could be developed <br />for other geometries, such as parabolic channels. Tables of <br />geometry and the appropriate equations for a variety of <br />channel shapes are given in the MSU report (3). Equations <br />from the MSU report for channels of various shapes are <br />included in Appendix B. <br /> <br />e <br /> <br />Rigid Channel Linings <br /> <br />For rigid channel linings, such as concrete or soil cement, <br />there is no maximum permissible depth for the flow velocities <br />normally encountered in highway drainage work, since no <br />erosion can occur. Thus, the maximum flow depth is based only <br />on the freeboard requirement for the channel. The Manning <br />equation may be solved by trial and error for designing these <br />channels or charts similar to those in HDS No. 3 (1) can be <br />used. One such chart is shown as Chart 35, developed for a <br />trapezoidal concrete channel with a bottom width of 4 feet, <br />4:1 side slopes, and a Manning n of 0.013. <br /> <br />~ <br /> <br />:0 <br /> <br />e <br /> <br />10 <br />