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I I I I I I I I I I I I I I I I I I I INTERPRETATION AND USE OF REPORT DATA 100-Year Flood Event-- The 100-year flood event may also be used for engineering design purposes where a lower risk of failure than the 10- or 50-year flood is desired. However, the most important use of the 100-year flood event lies in floodplain designation and land use regulation as set forth in the state statutes. The State of Colorado considers the 100-year frequency flood as the flood event to be used in designing and protecting structures and dwellings for human occupation. Therefore, all floodplain regulations are based upon the 100-year flood. Also, the area which would be inundated by the 100-year flood may be delineated as an area of state interest as set forth in H.B. 1041 -- the state's land use bill. Additional uses include emergency preparedness planning and flood travel time analysis. Flood Frequency and Discharge The 10-, 50-, 100-, and 500-year flood events were used as the flood frequencies for this floodplain analysis. Thus, the data developed in this report will be compatible not only for regulation purposes, and H.B. 1041 designation, but also for Federal Emergency Management Agency flood insurance rate studies. These various fiood events have an average occurrence of once in the number of years as indicated. For example, the 100-year flood occurs, on the average, once in a 100-year period, and has a one-percent chance of being equalled or exceeded in any given year. Stated in another manner, the one-percent flood has a 26 percent chance of occurrence over the period of a 30-year mortgage. The particular uses for the various flood events, in addition to those stated above, are as follows: 10-Year and 50-Year Events-- Information regarding these lower frequency floods is especially useful for future engineering studies and land use planning purposes related to minor road systems, minor channel improvements, the location of parks and recreational facilities, agricultural lands, and appurtenant structures. For structures and uses of this type on the smaller tribu- taries and in areas where the high risk of structural failure is economi- cally feasible and the hazard of life and property nonexistent, the use of the lower frequency floods may be considered. 500-Year Flood Event-- The 500-year flood event is useful in making the public aware that floods larger than the 100-year flood can and do occur. Because an indi- vidual is living outside the 100-year flood boundary does not mean that he is completely safe from flooding. The 500-year flood event can also be used for regulating high risk developments within the floodplain such as nuclear power plants, or the storage or manufacture of toxic or explo- sive materials. Table 6 lists the discharges and water surface elevations for the 10-, 50-, 100-, and 500-year flood events at selected reference points. 10