Laserfiche WebLink
<br />Flood plain boundaries for Helmer and Ramsey Gulches were delineated <br />using the l:G,OOO topographic maps (Reference IG). <br /> <br />The 100- and SOO-year flood plain boundaries are shown on the Flood <br />Boundary and Floodway Map (Exhibit 2). In cases where the 100- <br />and SOO-year flood plain boundaries are close together, only the <br />lOa-year flood plain boundary has been shown. Small areas within <br />the flood plain boundaries may lie above the flood elevations but <br />cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />Approximate lOa-year flood plain boundaries were delineated using <br />topographic maps at scales of l:G,OOO and 1:24,000, with contour <br />intervals of S, 20, and 40 feet (References IG and 17). <br /> <br />For the streams studied by approximate methods, only the lOa-year <br />flood plain boundary is shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as structures and fill, reduces <br />flood-carrying capacity, increases flood heights and velocities, <br />and increases flood hazards in areas beyond the encroachment itself. <br />One aspect of flood plain management involves balancing the economic <br />gain from flood plain development against the resulting increase <br />in flood hazard. For purposes of the NFIP, a floodway is used as <br />a tool to assist local communities in this aspect of flood plain <br />management. Under this concept, the area of the lOa-year flood <br />plain is divided into a floodway and a floodway fringe. The <br />floodway is the channel of a stream, plus any adjacent flood plain <br />areas, that must be kept free of encroachment so that the lOa-year <br />flood can be carried without substantial increases in flood heights. <br />Minimum Federal standards limit such increases to 1.0 foot, provided <br />that hazardous velocities are not produced. The floodways in this <br />study are presented to local agencies as minimum standards trat <br />can be adopted directly or that can be used as a basis for additional <br />floodway studies. <br /> <br />The floodways presented in this study were computed on the basis <br />of equal conveyance reduction from each side of the flood plain. <br />The results of these computations are tabulated at selected cross <br />sections for each stream segment for which a floodway is computed <br />(Table 2). <br /> <br />A floodway is not appropriate on an alluvial fan. For this reason <br />no floodways are presented for Helmer and Ramsey Gulches. <br /> <br />No floodway was computed for Alkali Creek. <br /> <br />As shown on the Flood Boundary and Floodway Map (Exhibit 2), the <br />floodway boundaries were computed at cross sections. Between cross <br /> <br />J4 <br />