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<br />" . <br /> <br />--- <br /> <br />riverine floods), earthquakes, dam failure flooding, and <br />tornados are considered to be Colorado's highest risks. * <br />These major threats are often interrelated and one may <br />compound another: (1) a flash flood may cause a dam to fail <br />or (2) a relatively small earthquake may also lead to the <br />same result. This identification of very high risk threats <br />has been developed from historical analysis of the growth of <br />Colorado's population through the mining era into modern <br />times and the losses which have been recorded. <br /> <br />. <br /> <br />Flash Flooding <br /> <br />In most of the mountainous areas of Colorado, flash <br />flooding usually occurs in spring and summer. In southwest- <br />ern Colorado flash flooding occurs most frequently in <br />September and October. Heavy rainfall, possibly combined <br />with snowmelt in the intermountane canyons, threatens <br />population living along the stream bed or near outwash <br />areas, Very significant landslide hazards often accompany <br />flash floods in canyons and river headwaters. <br /> <br />"--' <br /> <br />Flash flooding danger is of greatest significance along <br />floodplains located in the mountainous areas and in parti- <br />cular in the Front Range where population is concentrated, <br />where gradients are steep, where large drainage areas can <br />focus considerable quantities of water and where very heavy <br />rainfall can occur. Of these components of a hazardous <br />flash flood, amounts of rainfall and the time over which <br />rainfall extends are the most difficult to predict and <br />prepare for. A historical analysis of rainfall patterns <br />along the Front Range by Professor Koel zer, Colorado State <br />University, has shown that probable maximum amounts of 20 <br />inches of rainfall can occur in a given 24 hour period. <br />This varies considerably from expectations held by the <br />public as to likely amounts of rainfall. <br /> <br />. <br /> <br />It is significant to note that the Big Thompson flood <br />resulted from a maximum rainfall of 12 inches. The maximum <br />rainfall leading to the 1965 flood was 14 inches. We have <br />only minimal data concerning frequencies and amounts of <br />rainfall to accurately predict flash flood intensities. In <br />another example, in 1935, Fountain Creek had rainfall <br />amounts of 18 inches occurring in 3-4 hours. It has been <br />documented that a large flood -- a flood greater than a 100 <br />year flood -- has occurred on every large Colorado stream <br />basin. <br /> <br />*Urban fires arid wildfires, serious threats in Colorado, <br />have not been considered here since they do not represent <br />the degree of risk to human life that the extreme events <br />listed above do. <br /> <br />. <br /> <br />'-' <br /> <br />4 <br />