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<br />500-year Flood Event <br /> <br />The 500-year flood event is usefu I In mak i ng the pub Ii c <br />aware that floods larger than the 100-year flood can and do <br />occur. Just because a person is I i v i n9 above the 100'-year' <br />flood boundry does not mean that he is completely safe from <br />flooding. The 500-year flood event can also be used for <br />regulating high risk developments within the flood plain <br />such as nuclear power plants, or the storage or manufacture <br />of toxic or explosive materials. <br /> <br />B. Flood Elevation <br /> <br />Water surface e I evat ions for the 10-', 50-, 100-, and !500,- <br />year floods, as determined at each cross section, may be <br />found in Table 1 "Flood Frequency-Elevation and Discharge <br />Data". The flood profile, figures (fig 3-8) show" graphiGal <br />relationship of water surface elevations along the stream <br />reaches for the given frequencies. Figures 9-11 show <br />selected typical cross sections from different reaches <br />within the study area. <br /> <br />The flood profi les may be used in ar'eas where controversy <br />arises over the 100-year flood boundary shown on the Flood <br />Plain Maps. Since the flood profile, exhibits give the wa1;er <br />surface elevation at a s~,ecific point on the reference line, <br />the flood elevations can be surveyed on the ground to <br />alleviate any discrepancies on the base map. <br /> <br />C. Flooded Areas <br /> <br />Flood plain maps, sheets 1 and 2, show the boundary of the <br />100-year flood plain. T~e 500-year boundary is also shown <br />on the maps where there is a significant difference with the <br />100-year boundary. The flood plain boundary was plot1;ed <br />using flood contour elevations and stationing from the <br />plotted flood profiles. This was done at elevation <br />intervals compatable with 1;he map con1;our intervals for that <br />part of the study area ccvered by a set of 1982 Co II bran <br />Sewer District Topographic Maps. A set of 1984 Aerial <br />Photos of the Town of Co II bran was Llsed to upda1;e channel <br />changes not shown on the 1982 maps. Channe I chanSles have <br />occurred between cross sections 337 and 342, and at cross <br />sect i on 348 dur i ng the 2 year i nter'va I. That por't i on of the <br />study area not covered by the topographic maps was plotted <br />on the 1984 Aerial Photos directly using cross section data <br />and interpolations between crOSS sections. Flood contours <br />are shown as wiggly I ines at 5 ft intervals perpendicular to <br />the direction of flow. <br /> <br />9 <br />