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<br />... J, <br />I <br />=~ <br />~ <br />, <br />~, <br /> <br />" <br />, <br /> <br /> <br />velocity is required. The flow velocity must be a representative veloc- <br /> <br /> <br />ity for each cross section. Both friction loss and the contraction and <br /> <br /> <br />expansion losses are calculated in terms of this representative velocity. <br /> <br /> <br />The following figure illustrates a case where the average velocity in <br /> <br /> <br />the cross section is not sufficiently representative to serve in the <br /> <br /> <br />energy calculations. Consider the distribution of discharge and flow <br /> <br />area shown. <br /> <br />LEFT .OVERllANK <br />(LOB) <br /> <br />MAIN CHANNEL <br /> <br />\MCHl <br />WS <br /> <br />RIGHI OVERBANK <br /> <br />,....\ 1/ ( <br />\'\"\~'\"'\,~.' I <br />b% of I./t /, 86% of Of <br />25.% of At I 40 "0 of At <br />1'/1 'I;" f, <br /> <br />I <br />, <br />,Iff <br /> <br />"/> I <br />8% Mat" <br />35 "0 of At <br /> <br /> <br />(ROB' <br /> <br />. \ \ '\ \ <br /> <br /> <br />at = Total discharge <br />At = Total cross section area <br /> <br />Fig. 5.01. Distribution of discharge and <br />flow area at a cross section <br /> <br />The average velocity at this cross section VA by definition, is <br />equal to Qt/At. However, the main channel conveys 86 percent of the <br />total discharge and therefore has an average channel velocity of <br />2.15 . VA' Neither the average velocity nor the average channel <br />velocity is sufficiently representative to be used for calculations <br />for this case. Moreover, the percent of total flow conveyed by the <br />channel changes with discharge. The velocity distribution factor, a, <br />is used to overcome this problem. <br />Another important distributed property is the distance between cross <br />sections. Except in the very special case of a straight channel, cross <br /> <br />5.03 <br />