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<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 equalled or exceeded ~ on the average during any <br />10-, 50-, 100-, or SOO-year period (recurrence interval) have been <br />selected as having special significance for flood plain management and <br />for flood insurance premium rates. These events, co~monly termed the <br />10-, 50-, 100-, and SOa-year floods, have a 10, 2, I, and 0.2 percent <br />chance, respectively, of being equalled or exceeded during any year. <br />Although the recurrence interval represents the long term average period <br />between floods of a specific magnitude, rare floods could occur at short <br />intervals or even within the same year. The risk of experiencing a rare <br />flood increases when periods greater than 1 year are considered. For <br />example, the risk of having a flood which equals or exceeds the 100-year <br />flood (1 percent chance of annual occurrence) in any 50-year period is <br />approximately 40 percent (4 in 10), and, for any 90-year period, the risk <br />increases to approximately 60 percent (6 in 10). The analyses reported <br />here 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 peak discharge- <br />frequency relationships for floods of the selected recurrence inter- <br />vals for each stream studied in detail in the community. <br /> <br />Discharges for the Cache La poudre River were provided by the u.s. <br />Army Corps of Engineers for the 10-, 50-, and lOa-year floods <br />(Reference 4). <br /> <br />Generally, data were taken from several gages located in surrounding <br />areas and analyzed statistically using the log-Pearson Type III <br />method (Reference 5). This information was then routed to the study <br />area. <br /> <br />The SOQ-year flood discharge was obtained from a straight-line <br />extrapolation of these values on log-probability paper. <br /> <br />A very wide flood plain, along with water-retaining channel storage <br />areas, affected the recorded discharges of the four gaging stations, <br />all located outside the corporate limits. <br /> <br />Peak discharge-drainage area relationships for the Cache La poudre <br />River are shown in Table 1. <br /> <br />4 <br />