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<br />many large regions of the world slJggests that recorded stonns could have <br />occurred with about e~ual severity at any of many locations in the region <br />where they actually occurred. Thus, if a large flood occurs on one river <br />in a region, it is possible to estimate floods that could have resulted <br />on other streams in that region assuming the storm was centered over the <br />drainage area tributary to anyone of the other streams. This is done <br />by transposing the isohyetal pattern of total storm rainfall to the <br />drainage area in such a position that maximum rainfall will occur over <br />the drainage area. Historical events having a large areal coverage <br />usually have several centers of high rainfall. Hhen transposing this <br />type of storm to another river basin, it usually will re~uire several <br />trials before the most critical centering can be detennined. Figure <br />2.Q2 illustrates the centering of an isohyetal pattern over the drainage <br />basin be10\'1 a flood control reservoir and upstream of a local protection <br />project. This pattern is an hypothetical areal pattern but is reasonably <br />typical of severe historical occurrences to be valid. The centering il- <br />lustrated is centered to Maximize the stonn rainfall-runoff most adversely <br />for the local protection project. If the major concern were the flood <br />control storage requir~'ent at the reservoir, the stonn could be recenter- <br />ed so as to result in maximizing rainfall and runoff upstream of the dam. <br />Either centering is equally valid in most instances, and both centerings <br />would normally be investigated for purposes of sizing both the reservoir <br />and the local protection works. <br />^s stonns occur, they usually do not center over any particular <br />river basin. Consequently, even where maximum flood flows occur, some <br />transposition of the storm would ordinarily increase those flows. There <br />is some question as to whether the orientation of isohyets, or their <br />general shape, can reasonably be changed, because meteorological factors <br />that result in major storM precipitation might be associated closely <br />with the isohyetal patterns. Each storm must be considered individually <br />to evaluate the extent it can be moved and rotated. <br />If storms are transposed over long distances, the rainfall amounts <br />should be modified in accordance with the maximum precipitable moisture <br />variations that are characteristic of the respective large regions, If <br /> <br />2-06 <br />