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<br />.' <br /> <br />I <br />i --- <br /> <br />--- <br /> <br />'-- <br /> <br />for agriculture but now must serve to restrict flooding due <br />to greater run off as a result of urban development. This <br />combination of flash flooding and dam failure is precisely <br />the situation that resulted in the Rapid City disaster in <br />1972. Danger is greatest where a relatively steep gradient <br />exists between the dam and the settlement pattern. <br /> <br />Another hazard associated with dams in Colorado is <br />related to seismic activity. When many of Colorado's dams <br />were originally constructed, seismic activity was not <br />considered to be as great a risk as it is today, (for the <br />latest analysis of seismic risk see the analysis of poten- <br />tial effects of intensity discussed at pages 6 and 21). As <br />a result, structural allowances were not fully considered at <br />the time of construction. Today, a reasonably high level of <br />risk exists downstream from these dams, There is a possi- <br />bility that an earthquake (or minor seismic activity) could <br />shake the foundation of a dam causing the structure to <br />weaken and fail. <br /> <br />At least 130 dams are known to have failed in Colorado <br />since 1890.* <br /> <br />Tornados <br /> <br />The occurrence of tornados is confined primarily to <br />the Eastern Slope of Colorado where they occur with consid- <br />erable frequency. During the spring and summer months, <br />tornados are likely around Denver, and contiguous metropoli- <br />tan areas, the foothills of the Front Range, and in the <br />plains counties. Because of the meteorological conditions <br />found in Colorado, tornados often develop, move across the <br />ground, and then rise rapidly so that they do not continue <br />on a long, destructive storm path. Tornados therefore tend <br />to have a relatively short duration in Colorado, despite <br />their frequent occurrence. <br /> <br />When a tornado does occur, it is an immediate threat to <br />lives and property. Since tornados occur on the eastern <br />plains where population densities are very low, few lives <br />have been lost and relatively little property has been <br />damaged in recent years. But recently, it appears that many <br />more tornados occured every year than had been previously <br />recognized as developing on the eastern plains. They have <br />also been observed developing over the Front Range and have <br />impacted the growing urban areas: Manitou Springs, Thornton <br />(this tornado also impacted portions of Denver and Lakewood) <br />are two recent <br /> <br />* From the Flood Hazard Mitigation Plan for Colorado, Lawn <br />Lake Disaster. <br /> <br />7 <br />