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<br />CHAPTER 4. PROBABLE MAXIMUM FLOODS <br /> <br />Section 4.01. Definition <br /> <br />The probable maximum flood is that flood discharge which would <br />result from the most severe combination of critical meteorologic and <br />hydrologic conditions that are reasonably possible in the region. Since <br />there is great uncertainty in estimating potential hydrologic magnitudes, <br />extreme conservatism in estimating each variable would ordinarily result <br />in probably maximum floods that are unreasonable and of no practical <br />utility. Accordingly, a great amount of judgment is ordinarily required <br />in selecting rainfall, snowpack, loss rates, etc. A moderately conserva- <br />tive estimate of extreme values of each variable should be used, and <br />these should be consistent with the selected application as to be made. <br />The moderate, rather than extreme, degree of conservatism is the factor <br />that has led to the terminology probable maximum rather than maximum <br />possible. Nevertheless, the resulting flood must be one that the engineer <br />considers is virtually impossible of exceedence, because the flood is <br />ordinarily used to assure the integrity of a dam whose failure would <br />cause a great loss of life and major property damage that would not occur <br />under natural conditions. Probably maximum flood estimates are app1i- <br />able to projects where consideratiDn is to be given to virtually complete <br />security against potential floods. <br /> <br />Section 4.02. Storm maximization <br /> <br />There are two general factors that determine the severity of <br />precipitation during storms. One is the amount of moisture present in <br />the atmosphere and the second is the intensity of the precipitation- <br />forming mechanism. <br /> <br />4-01 <br />