FLOOD10349 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I A map index for the floodplain boundary maps is shown on Plate 4. Floodplain boundaries for the 100- and 500-year flood events are delineated on Plates 5 through 12. The corresponding flood profiles on plates 13 through 22 present the water surface elevations and water depths for the 10-, 50-, 100-, and 500-year flood events relative to the river stream bed. Representative cross-sections are shown on Plate 23. 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS 4.1 Floodplain Boundaries To provide a national standard without regional discrimination, the 1- percent-annual-chance (100-year) flood has been adopted by FEMA as the base flood for floodplain management purposes. The 0.2-percent-annual- chance (500-year) flood is employed to indicate additional areas of flood risk in the community. The 100- and 500-year floodplain boundaries have been delineated using the flood elevations determined at each cross section, Between cross sections, the boundaries were interpolated using the topographic mapping. 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the National Flood Insurance Program (NFIP), a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 100-year floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 100-year flood can be carried without substantial increases in flood heights. Minimum federal standards limit such increases to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this study are presented as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodway presented in this study was computed on the basis of equal- conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated, The results of the floodway computations are tabulated for selected cross sections, as shown on Plates 2 and 3. In ICON Engineering, Inc. May 2004 7