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DRAINAGE CRITERIA MANUAL <br />MAJOR DRAINAGE <br />Typical cross-sections suitable for grass lined channels are given in Figure <br />2-4. <br />2.3.4 Roughness Coefficients. The hydraulic roughness of man-made grass lined <br />channels depends on the length of cutting, if any, the type of grass, as well as <br />the depth of flow (11). Typical roughness coefficients are as follows: <br />TABLE 2-4 <br />MANNING ROUGHNESS COEFFICIENTS, n* <br />Depth of Depth of <br />Flow of Flow Greater <br />0.7-1.5 ft. Than 3.0 ft. <br />Bermuda grass, Buffalo grass, Kentucky Bluegrass <br />a. Mowed to 2 inches 0.035" 0.030 <br />• b. Length 4-6 inches 0.040 0.030 <br />Good stand any grass <br />a. Length of 12 inches 0.070 0.035 <br />b. Length of 24 inches 0.100 0.035 <br />Fair stand any grass <br />a. Length of 12 inches 0.060 0.035 <br />b. Length of 24 inches 0.070 0.035 <br />*For straight channels without shrubbery or trees <br />The 0.7 to 1.5 foot depth in Table 2-4 is generally suitable for computing the <br />wetted channel portion for the initial storm runoff, while the greater than 3 <br />• foot depth is suitable for the major runoff computations. A depth of flow of <br />2.0 feet or more will begin to lay the grass down to form a smoother bottom <br />surface. <br />l1DFCD_ 1990 <br />