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<br />National Weather Service Study <br />When Cherry Creek Dam was built, the method used by the Army Corps of <br />Engineers to determine dam safety was based on the ability of a dam structure <br />to withstand a 500-year to 1,OOO-year flood. The Corps has now adopted a <br />method that calculates the Probable Maximum Precipitation (PMP), or the <br />worst storm that could ever occur in the Cherry Creek Drainage Basin. This <br />ultra-conservative method of estimating dam safety is subject to considerable <br />debate. <br /> <br />The use of the terms, Probable Maximum Precipitation and Probable <br />Maximum Flood, are misleading in that they both are based on extremely <br />improbable events, According to the U. S. Bureau of Reclamation, the <br />Probable Maximum Precipitation is an event that has somewhere in the range <br />of a one-in-a-million chance of occurring in any single year. <br /> <br />To make matters worse, the National Weather Service study that produced the <br />estimates for the PMP is replete with inconsistencies, Unfortunately, the <br />Weather Service historically has not been accountable for the completeness <br />and accuracy of such studies. Rather than obtaining an independent review <br />of the study, the Weather Service has merely relied on internal reviews. <br /> <br />As a result, the National Weather Service study produced numerous dubious <br />conclusions and findings, including: <br /> <br />· The study calculated that the Probable Maximum Precipitation for <br />Cherry Creek is approximately seven times the amount ofthe 100- <br />year rainfall. This is significantly higher than similar studies <br />prepared by dam safety officials in the Western United States, It is <br />a generally accepted rule ofthumb that the PMF should be about <br />three to four times as large as a 100-year rainfall. A ratio of7-to-1 <br />is usually sufficient cause to reevaluate the study, particularly when <br />such significant financial and social impacts are involved. <br /> <br />· In making its rainfall calculations, the Weather Service used data <br />from storms occurring in the Midwest, Gulf states and Eastern <br />Seaboard, not Colorado. The Weather Service ignored its own <br />guidelines not to use such a procedure for a region close to the <br />foothills of the Rockies without additional study. <br /> <br />2 <br />