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<br />3.0 ENGINEERING METHODS <br /> <br />For flooding sources studied in detail in the community, standard <br />hydrologic and hydrau1 ic study methods were used to detenw;ne the flood <br />hazard data required for this study. Flood events of the magnitude Which <br />are expected to be equalled or exceeded once on the average during any <br />10-, 50-, 100-, and 500-year fl oods, havea 10, 2, I, and 0.2 percent <br />chance, respectively, of being equal] ed or exceeded during any year. <br />Although the recurrence interval represents the long tenn 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 />county 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 <br />discharge-frequency relationships for floods of the selected <br />recurrence intervals for each stream studied in deta;l and to <br />compare with results of previous studies for the flooding sources <br />studi ed here. <br /> <br />Peak discharges for the various recurrence interval s for the Rio <br />Grande were perfonned by the study contractor for the Rio Grande <br />Basin at Del Norte, Colorado. <br /> <br />Streamflow data from the Del Norte measuring station, located <br />within the Rio Grande basin, was used in this analysis. This data <br />is contained in U.S. Geological Survey Water Supply papers for the <br />Rio Grande Basin (Reference 3,4,5). <br /> <br />The peak flow figures for each frequency used in the hydrau1 ic <br />analysis of this study were estimated using a Regional Log-Pearson <br />Type III Analysis (Reference 6). The peak discharge is presented <br />in Table 1. <br /> <br />Table 1 - Summary of Discharges <br /> <br />Flood Source and Location <br /> <br />Drainage Area Peak DischarHe (cfs) <br />(sq. mi.) la-year 50-year 10 -year 500-year <br /> <br />Rio Grande <br /> <br />At Del Norte <br /> <br />1,320 <br /> <br />7,701 <br /> <br />9,879 <br /> <br />10,734 <br /> <br />12,60B <br /> <br />6 <br />