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<br />0024: 3 q COASTAL FLOOD PROBLEMS <br /> <br />7 <br /> <br />6-132. <br /> <br />the gravitational attraction of the sun and moon and under- <br /> <br /> <br />water topography of a region. 7 Normal tide levels are' <br /> <br /> <br />subject to seasonal changes due to shifts in the orient a- <br /> <br />~~~ <br /> <br />tion of the earth and moon and slight changes due to water <br /> <br />temperature, prevailing winds, and atmospheric pressure, <br /> <br />Although gravitation factors can be computed and predicted <br /> <br />in advance,s wide variation in specific factors at a site <br /> <br />complicate prediction of peak flood elevations. Contrary <br /> <br />to popular belief, ocean levels are not constant. Flood <br /> <br /> <br />elevations depend upon the additive effect of ~mal tides, <br /> <br /> <br />~~ surges,~ action, and sometimes other factors. <br /> <br /> <br />In,an intense storm, pressure differentials and <br /> <br />winds significantly increase normal tide levels, This <br /> <br />difference between the nOJ:'mal. tide and storm tide is called <br /> <br />. . <br />the ~torm.-"'-tl.rge.- <br /> <br /> <br />While normal tide levels arc quite predictable, storm <br /> <br /> <br />surges vary greatly, Tropical storms which barely qualify <br /> <br /> <br />as a hurricane may elevate sea level no more than four <br /> <br /> <br />feet, but a hurricane may elevate sea level more than 15 feet. <br /> <br />above normal tide..ID Intense extratropicaJ. storms m5Y <br /> <br />cause rises'. of fi V(~ to six feet ctb:yvc nO)~Ti1al. 11 <br /> <br />Usually <br /> <br />the soa level rises in less than six hou.-cs. I? 'l'he fall m,:iY <br /> <br />bci even more rapid. <br /> <br />.';-;.". <br />"-..:-:<.; <br />\:):~/ <br />