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<br />or. <br /> <br />(sharp-edged or curved and smooth-surfaced), and (c) the position and spacing <br />of obstructions in the reach. Cowan developed a table presenting four dif- <br />ferent degrees of obstruction. <br /> <br />In judging the retarding effect of vegetation to determine a modifying <br />value (n4), consideration should be given to the following: height in <br />relation to depth of flow; capacity to resist bending; growing-season <br />condition versus dormant-season condition; the degree to which the cross <br />section is occupied or blocked out: the distribution of vegetation of differ- <br />ent types; and densities and heights in the reach under consideration. Cowan <br />also developed a table giving different degrees of vegetation and the range <br />of nq for these different degrees. <br /> <br />In selecting the modifying value for meandering (m), the degree of <br />meandering depends on the ratio of the total length of the meandering channel <br />reach to the straight length of channel reach. The meandering is considered <br />minor for ratios of 1.0 to 1.2, appreciable for ratios of 1.2 to 1.5, and <br />severe for ratios of 1.5 and greater. Cowan gives modifying values for each <br />degree of meander. <br /> <br />Re9reSSJon Analvsis <br /> <br />Garton (1970) conducted hydraulic studies using a smooth flume in which <br />cylindrical retardance elements were inserted at various regular spacings. <br />The effects of the spacing pattern, diameter of the elements, spacing of the <br />elements, slope, and flow rate on Manning's coefficient were determined. <br /> <br />The test procedure consisted of passing five measured flows down the test <br />channel and making all observations needed to compute the hydraulic charac- <br />teristics of the channel. Gradually varied flow was assumed. A 44-ft by 18- <br />in. aluminum-lined flume was used. The channel was fitted with round alu- <br />minum pegs that served as roughness elements. Two sizes of elements were <br />used, 3/32-in. and 9/32-in. diameter pegs about 3-1/2 in. long. Specific <br />longitudinal and transverse spacing was made to form patterns known as <br />diagonal-grid and square-grid systems. <br /> <br />Linear, quadratic, and exponential models were developed using dimen- <br />sional analysis. Multiple-correlation coefficients that generally were <br />greater than 0.97 were obtained. The linear-variable model and the exponen- <br />tial model gave slightly improved estimates, but resulted in a more complex <br />equation to solve. <br /> <br />j <br />* <br /> <br />9 <br />