Ord 1990-037 - Laserfiche WebLink

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are close together, only the lOa-year floodplain boundary has been shown. Small areas within the floodplain 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 floodplain boundary is shown on the Flood Insurance Rate Map (Exhibit 2). 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, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the lOa-year floodplain is divided into a floodway and a f100dway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the lOO-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 a minimum standard that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this study were computed for certain stream segments 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 (Table 2). The computed floodways are shown on the Flood Insurance Rate Map (Exhibit 2). In cases where the floodway and lOa-year floodplain boundaries are either close together or collinear, only the f100dway boundary is shown. Portions of the floodway widths for the San Marcos River, Bypass Creek, and the Blanco River extend beyond the corporate Hmi ts. The area between the floodway and lOa-year floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increaslng the water-surface elevation of the lOa-year flood by more than 1.0 foot at any point. Typical relationships between the floodway and the f1oodway fringe and their significance to floodplain development are shown in Figure 2. 12