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<br /> /' vI <br /> ,/" ./ vI <br /> ./ --- /" ../ vI <br /> ~ ,/" ./ /" ~ vI <br /> .--' <br /> .-"'" ;....- ../ /' ......... ,-'" ~' I <br /> /" V ./ ---: ......... .-' .~ --- l-- <br />-- 1--- - -- .:; ;:.. ...:. - ... l-- I <br /> ':/ /' --- i-- <br /> ~ ~ " "..- v - - - - - I <br /> A ~ :;..... ::::- --::: J..-- - - - <br /> i ~ - I <br /> ~ e - <br /> <br />3.4 <br /> <br />3.0 <br /> <br />2.6 <br /> <br />aZ2.2 <br /> <br />1.8 <br /> <br />14 <br /> <br />1.0 <br />o <br /> <br />0.3 <br /> <br />0.4 <br /> <br />0.1 <br /> <br />0.2 <br /> <br />3.4 <br /> <br />3.0 <br /> <br />2.6 <br /> <br />2.2 a, <br /> <br />1.8 <br /> <br />1.4 <br /> <br />0.5 <br />M <br /> <br />0,6 <br /> <br />0.9 <br /> <br />LO <br />1.0 <br /> <br />0.7 <br /> <br />0.8 <br /> <br />Figure 5.-Aid for estimating 0'2' <br /> <br />The method of computation will be further illus- <br />trated in the examples in chapter XII. <br />A second coefficient, Cl2, is required to correct the <br />velocity head for nonuniform velocity distribution <br />under the bridge, <br /> <br />0!2 = <br /> <br />};(qv') <br />QV,' <br /> <br />(3b) <br /> <br />where v, q and Q are defined as above but apply here <br />to the constricted cross section and <br /> <br />V. = average velocity in constriction = Q/ A" <br /> <br />The value of a, can be computed but a, is not <br />readily available for a proposed bridge. The best <br />that can be done in the case of the latter is to collect, <br />tabulate and compare values of a, from existing <br /> <br />bridges. This has been done and values of both a, <br />and ", are tabulated in columns 13 and 14 of table <br />B-2, The information for determining a, was ob- <br />tained from current meter traverses and soundings <br />taken from the downstream side of bridges by the <br />USGs. Figure 5, relating a, to "1 and the contraction <br />ratio, M, is supplied for estimating purposes only. <br />The value of a, is usually less than a, for a given <br />crossing but this is not always the case. Actually <br />there should be no definite relation between the two <br />but there is a trend, Local factors at the bridge <br />should also be considered such as asymmetry of flow, <br />variation in cross section and extent of vegetation <br />in the bridge opening. Perhaps the best advise for <br />estimating a, is to lean toward the generous side, <br />The construction of the chart shown on figure 5 is <br />described in section B.3, appendix B. <br /> <br />II <br />