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<br />rainstorms typically create severe erosion damages in a short <br />period of time due to the relatively large peak discharge and <br />high velocity of the floodwater. Erosion damages from snowmelt <br />or general rainstorm floods usually result from prolonged <br />exposure to the floodwater because of the long duration typical <br />of these floods. <br /> <br />2.4 Flood Protection Measures <br /> <br />Three reservoirs in the South Platte River Basin located upstream <br />of Sterling were built primarily for flood control. These <br />reservoi rs, Cherry Creek and Chat field Dam, both near Denver, <br />Colorado, and Bear Creek near Morrison, Colorado, have drainage <br />areas of 385, 3,020, and 260 square miles, respectively. No <br />other flood protection measures are located within Logan County. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, <br />standard hydrologic and hydraulic study methods were used to determine <br />the flood hazard data required for this study. Flood events of a <br />magni tude which are expec ted to be equaled or exceeded once on the <br />average during any 10-, 50-, 100-, and 500-year floods, have a 10-, <br />2-, 1-, and 0.2-percent chance, respectively, of being equaled or <br />exceeded during any year. Although the recurrence interval represents <br />the long-term average period between floods of a specific magnitude, <br />rare floods could occur at short interval s or even wi thin the same <br />year. The risk of experiencing a rare flood increases when periods <br />greater than one year are considered. For example, the risk of having <br />a flood which equals or exceeds the lOa-year flood (I-percent chance <br />of annual exceedence) in any 50-year period is approximately 40 <br />percent (4 in 10), and, for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein <br />reflect flooding potentials based on conditions existing in the <br />community at the time of completion of this study. Maps and flood <br />elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the <br />discharge-frequency relationships for each flooding <br />studied by detailed methods affecting the community. <br /> <br />peak <br />source <br /> <br />Discharge magnitudes for floods on the South Platte River were <br />based on data collected at U.S. Geological Survey stream gaging <br />stations located at Denver, Henderson, Kersey, Weldona, Balzac, <br />and Julesburg, Colorado. Discharge values developed at Sterling <br />were part of an overall South Platte River study (Reference 1). <br />Peak discharge-probability relationships were developed from <br />these records. A basin model was developed to determine <br />discharge probability relationships for locations other than <br />those at the stream gaging station locations. <br /> <br />5 <br />