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<br />~ <br />. . <br /> <br />. <br /> <br />~ <br /> <br />miles, a storage capacity of nearly 1000 acre-feet, and a spillway <br />design capacity of 5000 cubic feet per second. The reservoir is <br />important in reducing the peak discharge of Blue River due to rain- <br />fall. The reduction, however, is only marginal for runoff due to <br />snowmelt (Reference 5). <br /> <br />3.0 <br /> <br />ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the county, standard hydro- <br />logic and hydraulic study methods were used to determine the flood hazard <br />data required for this study. Flood events of a magnitude which are <br />expected to be equalled or exceeded ~ on the average during any 10-, <br />50-, 100-, or SOO-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 SOO-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 <br />increases to approximately 60 percent (6 in 10). The analyses reported <br />here reflect flooding potentials based on conditions existing in the <br />county at the time of completion of this study. Maps and flood elevations <br />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 inter- <br />vals for each stream studied in detail in the county. <br /> <br />The discharge-frequency data for Blue River below Dillon Dam were <br />developed in a hydrology report prepared previously by the study <br />contractor (Reference 10). In brief, the approach used considers <br />the total inflow to Dillon Reservoir and its routing through the <br />reservoir to obtain the 10-, 50-, 100-, and SOO-year discharges on <br />Blue River downstream of Dillon Dam. The contribution of the local <br />inflow or the tributaries flow to the Blue River discharges is also <br />described. <br /> <br />Flood discharges for the 10-, 50-, 100-, and SOO-year recurrence <br />events on Willow Creek, Straight Creek, Tenmile Creek, West Tenmile <br />Creek, Snake River, Soda creek, Meadow Creek, and South Barton Gulch <br />were calculated using regression equations based on a regional <br />analysis of all the gages in Summit County. At each gaging station, <br />the annual flood peaks due to snowmelt events and rainfall events <br />were separated. Using log-Pearson Type III distribution, the 10-, <br />50-, 100-, and SOO-year recurrence-interval discharges were obtained <br /> <br />9 <br />