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<br /> <br />B.4 <br /> <br />Relative effect of obstructions MOdif~~~~~ <br /> <br />Negligible <br />Minor <br />Appreciable <br />Severe <br /> <br />0.000 <br />0,010 to 0.015 <br />0.020 to 0,030 <br />0,040 to 0.060 <br /> <br />5th step. Selection of modifying value for vegetation. The re- <br />tarding effect of vegetation is probably due primarily to the turbulence <br />induced as the water flows around and' between the limbs, sterns and foliage, <br />and secondarily to reduction in cross section, As depth and velocity in- <br />crease; the force of the flowing water tends to bend the vegetation. <br />Therefore, the ability of vegetation to cause turbulence is partly relat- <br />ed to its resistance to bending force. Furthermore, the amount and <br />character of foliage; that is, the grpwing season condition versus dormant <br />season condition is important, In ju~ging the retarding effect of vegeta- <br />tion, critical consideration should be given to the following: the height <br />in relation to depth of flow; the capacity to resist bending; the degree <br />to which the cross section is occupied or blocked out; the transverse and <br />longitudinal distribution of vegetatipn of different types, densities and <br />heights in the reach under consideration, The following table may be used <br />as a guide to the selection: <br /> <br />Vegetation and flow conditions <br />comparable to: <br /> <br />Degree of <br />effect on n <br /> <br />Range in modify- <br />ing value <br /> <br />Dense growths of flexible turf grasses <br />or weeds, of which Bermuda and blue grasses <br />are examples, where the average depth. of <br />flow is 2 to 3 times the height of vegetation, Low <br /> <br />0.005 to 0,010 <br /> <br />Supple seedling tree switches such as,willow, <br />cottonwood or salt cedar where the average <br />depth of flow is 3 to 4 times the height of <br />the vegetation. <br /> <br />Turf grasses where the average depth Of <br />flow is 1 to 2 times the height of vegetation. <br /> <br />,.. <br /> <br />Stemmy grasses, weeds or tree seedlings with <br />moderate cover where the average depth of <br />flow is 2 to 3 times the height of vegetation, Medium <br /> <br />0,010 to 0.025 <br /> <br />Brushy growths, moderately dense, similar to <br />willows 1 to 2 years old, dormant sea~on, <br />along side slopes of channel with no Signif- <br />icant vegetation along the channel bottom, <br />where the hydraulic radius is greater than <br />2 feet, <br />