FLOOD02876 - Laserfiche WebLink

Home Browse Search

The study contractor has determined that some areas shown on the Federal Emergency Management Agency Flood Hazard Boundary Map (Reference 17) are areas of minimal flooding; therefore, these areas were not delineated on the maps. Flood boundaries for the 100- and 500-year floods are shown on the Flood Boundary and Floodway Map. Small areas within the flood boundaries may lie above the flood elevations and, therefore, not be subject to flooding; owing to limitations of the map scale, such areas are not shown. 4.2 Floodways Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of flood plain management involves balancing the economic gain from flood plain development against the resulting increase in flood hazard. For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to assist local communities in this aspect of flood plain management. Under this concept, the area of the lOa-year flood is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment in order that the lOa-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Emergency Manage- ment Agency limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodways presented in this study were computed on the basis of equal conveyance reduction from each side of the flood plain. The results of these computations are tabulated at selected cross sections for each stream segment for which a floodway is computed (Table 2). Because of the supercritical flow on Reach 2 of Brush Creek, the floodway was delineated to include all of the lOa-year flood limits except for areas of ineffective flow. As shown on the Flood Boundary and Floodway Map, the floodway bound- aries were determined at cross sections; between cross sections, the boundaries were interpolated. In cases where the floodway and lOa-year flood boundaries are close together, only the floodway boundary has been shown. The area between the floodway and the boundary of the lOa-year flood is termed the floodway fringe. The floodway fringe thus encompasses the portion of the flood plain that could be completely obstructed without increasing the water-surface elevation of the lOa-year flood more than 1.0 foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to flood plain development are shown in Figure 2. 13