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<br />expected to be equalled or exceeded once on the average during any 10-, <br />50-, 100-, and 500-year floods, haveaTO, 2, 1, and 0.2 percent chance, <br />respectively, of being equalled or exceeded during any year. Although the <br />recurrence interval represents the long term average peri od between floods <br />of a specific magnitude, rare floods could occur at short interval s or even <br />within the same year. The risk of experiencing a rare flood increases when <br />periods greater than 1 year are considered. For example, the risk of <br />having a flood which equals or exceeds the 100-year flood (1 percent chance <br />of annual occurrence) in any 50-year period is approximately 40 percent (4 <br />in 10), and, for any 90-year period, the risk increases to approximately 60 <br />percent (6 in 10). The analyses reported here reflect flooding potential s <br />based on conditions existing in the county at the time of completion of <br />this study. Maps and flood elevations will be amended periodically to <br />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 />val s for each stream studied in detail and to compare with results of <br />previous studies for the flooding sources studied here. <br /> <br />SCS performed a frequency analysis of streamgage records that was made <br />in accordance with Water Resources Council Bulletin 17. These values <br />were then compared with data from Colorado Water Conservation Board <br />Technical Manual No.1. Technical Manual No. 1 discharges were <br />slightly higher, however considering it accounted for affects of mixed <br />snowmelt and rainfall data, Technical Manual No.1 discharges were <br />selected for use in their study. <br /> <br />The procedure provided necessary peak discharge frequency val ues at <br />the streamgage site. Values at other locations along the stream were <br />determined from a procedure in the Colorado Water Conservation Board <br />Technical Manual No.1, Page 4 - Flood Information Near Gaged Sites on <br />the Same Stream. <br /> <br />The results of a regional analysis performed by COM were compared to <br />the results of the Flood Hazard Analyses for Cottonwood Creek (Refer- <br />ence 1). The 100-year discharge agreed within 5 percent of Reference <br />1, and therefore the hydrologic analysis as presented in the previous <br />study was adopted for use in estimating the flood boundaries for <br />Cottonwood Creek. The peak discharges are presented in Table 1. <br /> <br />6 <br />