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<br />was designed to store the surplus flows of Plateau, Leon, and <br />Park Creeks; and to furnish supplemental irrigation water to 22,210 <br />acres of land and electrical energy for west-central Colorado <br />(Reference 6). Although Vega Dam was not intended to provide <br />flood protection, the flows for the 100- and 500-year frequency <br />floods at Collbran are estimated to be reduced by 12.5 percent <br />and 6 percent, respectively. <br /> <br />There is no flood plain management in effect within the Town of <br />Collbran. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equalled or exceeded once on the average during any <br />10-, 50-, 100-, or 500-year period (recurrence interval) have been selected <br />as having special significance for flood plain management and for flood <br />insurance premium rates. These events, commonly termed the 10-, 50-, <br />100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, <br />respectively, of being equalled or exceeded during any year. Although <br />the recurrence interval represents the long term average period between <br />floods of a specific magnitude, rare floods could occur at short intervals <br />or even within the same year. The risk of experiencing a rare flood <br />increases when periods greater than 1 year are considered. For example, <br />the risk of having a flood which equals or exceeds the 100-year flood <br />(1 percent chance of annual occurrence) in any 50-year period is approxi- <br />mately 40 percent (4 in 10), and, for any 90-year periOd, the risk in- <br />creases to approximately 60 percent (6 in 10). The analyses reported <br />here reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood eleva- <br />tions will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for floods of the selected recurrence <br />intervals for each flooding source studied in detail affecting <br />the community. <br /> <br />For Plateau, Grove, and Buzzard Creeks at Collbran, peak dis~harges <br />for the 10-, 50-, 100-, and 500-year floods were determined using <br />a regional floodflow frequency analysis (Reference 7). For the <br />regional study, U.S. Geological Survey stream gage records for <br />15 gaging stations in hydrologically similar watersheds were analyzed. <br />Lengths of record for these gages ranged from 9 to 53 years. <br />The station data were separated into rainfall and snowmelt peaks. <br />Frequency analyses were conducted on the rainfall and snowmelt <br />data (Reference 8). The results of the snowmelt and rainfall <br />frequency analysis were combined statistically to give a maximum <br />flow for both events. Regional curves of drainage area versus <br />discharge were computed for the four frequency floods. <br /> <br />6 <br />