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2.4 Flood Protection Measures The Goose Pasture Tarn is located immediately upstream of the Town of Brceckenridge. The tarn has a drainage area of approximately 43.5 square miles, a storage capacity of nearly 1000 acre-feet, and a spillway design capacity of 5000 cfs. The reservoir is important in reducing the peak discharge of the Blue River due to rainfall. The effect, however, is only marginal for runoff due to snowmelt, which is normally the major cause of peak flows (Reference 1). Other reservoirs in the Blue River basin above Breckenridge provide only incidential flood protection (Reference 1). The recent changes that have been made in the Blue River channel through part of Breckenridge will significantly reduce flood potential in the town. The improvements were made with the intention of containing a IOO-year flood. During a IOO-year flood event, much of the flow, which previously would have spilled over the banks, will now be confined to the channel, particularly in the areas from the northern corporate limits to approximately 400 feet downstream of Watson Road; from approximately 600 feet downstream of Lincoln Avenue to the Washington Avenue footbridge; and, in the area of the Four Seasons shopping center. There is no flood plain management in the Town of Breckenridge. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equalled or exceeded once on the average during any 10-, ~O-, 100-, or SOO-year period (recurrence interv~l) have been selected as having special significance for flood plain management and for flood insurance premium rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equalled or exceeded during any year. Although the recurrence interval represents the long term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the IOO-year flood (1 percent chance of annual occurrence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported here reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 4