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.. . Mountain towns, foothill settlements, and plains communities are iJffected by the ability of these creeks to carry stormwater and snow runoff adequately. During most years the creek banks contain those waters, yet in some seasons the combination of rapidly melting snow and constant heavy riJins in the spring or severe local thunderstorrls in the summer have wreaked havoc with desire of people to mine, farm, trade, and live in the County. Typically, the storm's intensities are such that the ground is unable to absorb the great quantities of rainfall. The result is, of cour e, flooding. The floods, which have occurred in nearly every town, are listed in Chapter III and Appendix III. FLOOD FREQUENCY Floods have been comr:lOnly categorized according to their recurrence interval as 10-, 50-, 100-, and SOO-year floods. Each recurrence interval is related to a probability factor, or percent chance, that a flood may occur in any given year. . These terms for floods describe the average time interval for the occurrence of a flood of a given magnitude. This has caused some confusion. A lOa-year flood, for example, does not happen with predictable regularity every hundred years. Although the average length of time between occurrences for a flood of that magnitude is one hundred years, a lOa-year flood has a 1 % chance of occurring randomly in any year. It may, given the right combination of meteorologic conditions, happen in succeeding years. Colorado state law (H, R. 1041) requires that the lOa-year floodplain be used for flood hazard land use regulation. It is important to remember, however, that floods with a lower frequency than 1 % can cause substantial damage and that floods greater than the lOa-year standard project flood may occur at any time and may exceed the estimates for such an event as well. The recurrence interval is inversely related to the percent chance of flooding as summarized in the chart below: la-Year Flood A flood that has a recurrence interval of about ten years, but has a 10% chance of occurring in any year. 50-Year Flood A flood that has a recurrence interval of about fifty years, but has a 2% chance of occurring in any year. lOa-Year Flood A flood that has a recurrence interval of about 100 years, but has a 1% chance of occurring in any year. SOO-Year Flood A flood that has a recurrence interval of about 500 years, but has a .2% chance of occurring in any year. .