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<br />2.4 Flood Protection Measures <br /> <br />The principal flood protection measure within the Bear Creek Basin <br />which affects the City of Sheridan is the Bear Creek Dam just <br />downstream of the Town of MOrrison. Designed by the COE, the <br />dam has greatly affected the peak discharges of Bear Creek and <br />provides a flood ,control reservoir that intercepts flows from <br />areas in the upper and middle parts of the basin. At the Bear <br />Creek Reservoir, peak flows from the 100-year event have been <br />reduced from 30,000 cfs to approximately 1,000 cfs through storage <br />in the reservoir. There has been some straightening and enlargement <br />of the channel downstream of the dam to reduce flooding in the <br />lower, more developed areas of Bear Creek (References 1 and 2). <br /> <br />The construction of Chatfield Dam and Lake on the South Platte <br />River just downstream from the mouth of Plum Creek also provides <br />flood protection for the City of Sheridan. The reservoir has <br />a storage capacity of 385,000 acre-feet, with 215,000 acre-feet <br />for flood storage and 20,000 acre-feet for sediment control (Refer- <br />ence 3). <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 equaled 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 rates. These events, commonly termed the 10-, 50-, 100-, <br />and 500-ye,ar floods, have a 10, 2, 1, and 0.2 percent chance, respectively, <br />of being equaled or exceeded'during any year. Although the recurrence <br />interval represents the long term average period between floods of a <br />specific magnitude, rare floods could occur at short intervals or even <br />within the same year. The risk of experiencing a rare flood increases <br />when periods'greater than 1 year are considered. For example, the risk <br />of having a Ilobd which equals or exceeds the 100-year Ilood (1 percent <br />chance of annual exceedence) in any 50-year period is approximately <br />40 percent (4 in 10), and, for any 90-year period, the risk increases <br />to approximately 60 percent (6 in 10). The analyses reported herein <br />reflect flooding potentials based on conditions existing in the community <br />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 each flooding source studied in detail <br />affecting the:' communi ty. <br /> <br />5 <br />