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<br />I- <br />W <br />W <br />U. <br />z <br />:::.60 <br />EO <br />~ <br />...J <br />W <br />Z <br />zso <br /><( <br />:r <br />o <br />u. <br />o <br />:r <br />li:40 <br />w <br />o <br />:::; <br />~ <br />:::; <br />X 30 <br /><( <br />:::; <br /> <br />As shown in figure 3, hYGraulically best channel cross sections are not <br />commonly found in natural channels. Instead, natural channels usually have a <br />cross-sectional shape between' trapezoid and a triangle. The relationship of <br />flow depths for natural channds (fig. 3) indicates that in the design of a <br />trapezoidal channel bed with bank protection, allowances should be made for a <br /> <br />1. 55- 1.:13 ) d <br />= 17 percent in maximum epth by erosion. <br />1.33 <br /> <br />17 percent increase ( <br /> <br />90 <br /> <br /> <br />80 <br /> <br />70 <br /> <br />TRIANGULAR CHANNEL <br />dm <br />- =2 <br />d. <br /> <br />HYDRAULICALLY BEST TRAPEZOIDAL CHANNEL <br />(Z = 0_577 teet) <br />dm B + 2Zdm <br />-a;- B + Zdm <br /> <br />dm <br />;: 1.33 <br />0~75 <br /> <br />20 <br /> <br />.RECT ANGULAR CHANNEL <br /> <br />dm <br />d =1.0 <br />. <br /> <br />da = Mean depth <br /> <br />dm = Maximum depth <br /> <br />B = Width of channel bottom <br /> <br />10 <br /> <br />z = Side slope, ratio of horizontal to <br />vertical <br /> <br />o <br />o <br /> <br />40 <br /> <br />50 <br /> <br />60 <br /> <br />70 <br /> <br />80 <br /> <br />90 <br /> <br />10 <br /> <br />20 <br /> <br />30 <br /> <br />MEAN DE PTH OF CHANNEL (dal,IN FEET <br /> <br />nGURE 3~ Carr parison of flow depths for various channel shapes. <br /> <br />17 <br />