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<br />-- <br /> <br />In equation 16, the expression NWeY is the total area of the roughness <br />elements under water, and B6X is the area of channel bed in the reach Ax. <br />The ratio of these two is defined as the concentration of roughness elements, <br /> <br />and <br /> <br />a <br /> <br />NWeY <br />8aX ' <br /> <br />(17) <br /> <br />Cf <br /> <br />oc. <br /> <br />(18) <br /> <br />Tests were perfor.med on various types of roughness elements with <br />different combinations of patterns and spacings. The various combinations <br />were selected to ensure a broad range of values for channel roughness. As <br />the experiments were intended to simulate the roughness characteristics of <br />forested flood plains, all elements were arranged to protrude through the <br />water surface. <br /> <br />Three basic patterns of roughness elements were used: random, <br />rectangular, and diamond. For each pattern, both the longitudinal and <br />lateral spacing was varied to reflect the concentration of elements along the <br />channel bottom. <br /> <br />Tseng examined the five variables listed below (eq. 19) to find their <br />significance to flow resistance in a forested flood plain. <br /> <br />where <br /> <br />f <br /> <br />- (F, Re, d/8, a, 0) <br /> <br />(19) <br /> <br />F <br />Re <br />d <br />8 <br />a <br />o <br /> <br />Froude number, <br />Reynolds number, <br />mean depth, <br />~ width of flume, <br />roughness concentration, and <br />- roughness element pattern. <br /> <br />The results showed that for a given type of roughness-element pattern, <br />0, the flow resistance, i, is a unique function of the roughness <br />concentration, o. <br /> <br />The functional expression used by Tseng for any roughness pattern is <br /> <br />or <br /> <br />aaE <br /> <br />(20) <br /> <br />f <br /> <br />n <br /> <br />~ ai<fl <br /> <br />(21) <br /> <br />where ex, E, (Xl, and El are constants for different roughness pat terns. <br /> <br />7 <br />