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<br />Because streamflow records are far fro~ complete on Roan Creekj. a <br />number of unrecorded flood events have presumably occurred. <br /> <br />2.4 Flood Protection Measures <br /> <br />The Town of De Beque presently has no fidod protection measures <br />within the corporate limits. <br /> <br />.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a <br />magnitude which are expected to be equaled or exceeded once on the <br />average during any 10-, 50-, 100-, or 500-year period (recurrence <br />interval) have been selected as having special significance for <br />floodplain management and for flood insurance rates. These event s, <br />commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-. <br />2-, 1-, and O.2-percent chance, respectively, of being equaled or <br />exceeded during any year. Although the recurrence interval represents <br />the long term average period between floods of a specific magnitude, <br />rare floods could occur at short intervals or even within the same <br />year. The risk of experiencing a rare flood increases when periods <br />greater than one year are considered. For example, the risk of having <br />a flood which equals or exceeds the 100-year flood (1 percent chan~e <br />of annual exceedence) in any 50-year period is approximately 40 <br />percent (4 ih 10), and, for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein <br />reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this studYe Maps and flood <br />elevations will be amended periodically to reflect futu~e changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydeologic analyses were carried out to establish the peak <br />discharge-frequency relationships for each flooding source <br />studied by detailed methods affecting the community. <br /> <br />Tributaries to the Colorado River from the vicinity of De Beque <br />to Glenwood Springs, Colorado, are streams that flood from <br />snowmelt as well as from summer rain. The intent of this <br />analysis was to separate the annual peak discharges into rai~fall <br />events and snowmelt events. Separate frequency curves should be <br />combined statistically to produce a, final diSCharge-frequency <br />curve. Streamflow data of rainfall flood events were <br />insufficient to accomplish this, therefore the SCS TR-20 computer <br />program was used to simulate rainfall flood peaks. The model was <br />used on 16 watersheds of varying sizes. A regional curve of <br />drainage area versus peak discharge and frequency was d,eveloped <br />for rainfall flooding. The TR-20 analysis included the standard <br />SCS Type II (24-hour) rainfall distribution and curve numbers for <br /> <br />an <br /> <br />average <br /> <br />5 <br />