Ord 1990-037 - Laserfiche WebLink

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3.0 ENGINEERING METHODS For the flooding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-,50-,100-, or SOO-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10,2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the lOO-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each flooding source studied in detail affecting the community. Flow frequencies for the streams studied by detailed methods were developed using the computer program NUDALLAS (Reference 4). The watershed was divided into sub-basins, and synthetic unit and flood hydrographs were developed at selected locations. National Weather Service Technical Paper No. 40, National Oceanic and Atmospheric Administration (NO~~) Technical Memorandum NWS Hydro-3s, and COE Civil Engineer Bulletin No. EM 1110-2-1411 were used in developing the 10-,50-, and 100-year frequen€y storms (References 5, 6, and 7). The SOO-year storm was based on extrapolated data from the previously mentioned sources. Peak discharge-frequency values were computed for selected locations. Routing of the flood hydrographs through each sub-basin was accomplished using a modified PULS reservoir routing. The HEC-2 backwater model provided the elevation-discharge storage relationships for each stream studied by detailed methods (Reference 8). A summary of the drainage area-peak discharge relationships for the streams studied by detailed methods is shown in Table 1, "Summary of Discharges." 6