FLOOD06991 - Laserfiche WebLink

Home Browse Search

Leland Creek shallow flood boundaries along the u.s. Highway 40 embankments, northwest of Leland Creek, were delineated using topo- graphic mapping described above. Approximate flood boundaries along Jim Creek have been delineated using topographic maps at a scale of 1:24,000 (Reference 13), and methods described in Section 3.2. The 100- and SOO-year flood plain boundaries are shown on the Flood BOundary and Floodway Map (Exhibit 2). In cases where the 100- and SO a-year flood plain boundaries are close together, only the lOa-year flood plain boundary has been shown. Small areas within the flood plain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. For the streams studied by approximate methods, only the lOa-year flood plain boundary is shown. 4. 2 Floodways Encroachment on flood plains, 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 flood plain management involves balancing the economic gain from flood plain development against the resulting increase in flood hazard. For purposes of the NFIP, 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 plain is divided into a floodway and a floodway fringe. The flood- way is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment so that the lOa-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 to local agencies as minimum standards that can be adopted directly or that can be used as a basis for addi- tional floodway studies. 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 4). No floodway has been presented for Leland Creek downstream of U.S. Highway 40 because all flooding is contained within the channel. As shown on the Flood Boundary and Floodway Map (Exhibit 2), the floodway boundaries were computed at cross sections. Between cross sections, the boundaries were interpolated. In cases where the 17