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<br />Flood Warning System consisting of observers and automatic alarm <br />systems (Reference.4)" <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 thi" study. . Flood events of a magnitude which <br />are expected to be equaled ocexceededonce on the average during any <br />10-, 50-, 100-, or 500-year ?eriod (r~currence interval) have been selected <br />as having special significance for flOod plain management and for flood <br />insurance rates. These events, commohly termed the 10-, 50-, 100-, and <br />500-year 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 t.erm 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 flood which equals or exceeds the 100-year flood (1 percent <br />chance 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 reflect <br />flooding potentials based on conditiOnS existing in the community at the <br />time of completion of this study, Maps and flood elevations will be <br />amended periodically to reflGct 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 community. <br /> <br />Peak flow rates for Jimmy Camp Creek are designated by the CWCB, <br />and were developed by the COE (Reference 5), Synthetic unit <br />hydrographs and rainfall data from Precipitation-Frequency Atlas <br />of the Western United States (R~ference 6) were used to develop <br />the peak flows, <br /> <br />Flow rates for Fountain, Security, and Widefield Creeks were adopted <br />from the COE hydrology report o~ 1976 (Reference 5), In rural <br />areas with natural flow streams~ the peak flow rates were obtained <br />using the U.S, Geological Survey Technical Memorandum No.1, prepared <br />by the CWCB (Reference 7). In the urban basins and subbasins, <br />synthetic unit hydrographs were' used to develop peak flow rates <br />for the 10-, 50~, and 100-year recurrence intervals from S curves <br />that used rainfall data (Referepce 8), SCS soil survey information <br />for El Paso County (Re!:erence 9) were also used. The 500-year <br />flood peak discharges were obta~ned through extrapolation of the <br />10- through 100-year flood resu~ts on log-probability paper. <br /> <br />5 <br />