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<br />design and construction of water-supply systems, waste-water systems, <br />and other public works; and design and construction of drainage <br />systems subjecting other property to floodwater damage. <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-, SO-, 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-, SO-, <br />100-, and SOO-year floods, have a 10, 2, I, and 0.2 percent chance, <br />respectively, of being equaled or exceeded during any year. Although <br />the recurrence interval represents the long term averaqe 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 SO-year period is approxi- <br />mately 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 here reflect <br />flooding potentials based on conditions existing in the community at <br />the time of completion of this study. Maps and flood elevations will <br />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 <br />intervals for each flooding source studied in detail affecting <br />the community. <br /> <br />The flood discharge values used for Clear Creek were obtained <br />from the U.S. Army Corps of Engineers, Omaha District (Reference 9). <br />The U.S. Army Corps of Engineers established peak discharge-frequency <br />relationships for floods of 10-, SO-, 100-, and SOO-year recurrence <br />intervals. A log-Pearson Type III analysis (Reference 10) was <br />conducted on the discharge records for the Clear Creek stream <br />gages at Golden (1911-1976) and Derby (1934-1976); however, the <br />statistical parameters computed by these methods were not sufficiently <br />reliable to predict the frequency of extreme events. In lieu <br />of a discharge-frequency analysis, a rainfall-runoff approach <br />was used. The Massachusetts Institute of Technology Catchment <br />Model (Reference 11) was constructed for the 400-square-mile <br />area above the Golden gage, and a storm water management <br />model (Reference 12) was constructed for the 17S-square-mile <br />area between the Golden and Derby gages. The rainfall depths <br />used in the analysis were based on data obtained from the 1973 <br /> <br />11 <br />