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Last modified
1/26/2010 10:09:41 AM
Creation date
10/5/2006 4:27:49 AM
Metadata
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Template:
Floodplain Documents
County
Larimer
Community
Fort Collins
Basin
South Platte
Title
Flood Insurance Study - Town of Evans
Date
3/18/1996
Prepared For
Fort Collins
Prepared By
FEMA
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />hanks with minimum freehoard. Because of the potemial hazard if these hanks fail. these <br />areas arc shown as subject to shallow nooding. <br /> <br />Floodplain boundaries were delineated using topographic work maps at a scale of I : I ,200, <br />with 2-foot contour intervals, for the Cache La Poudre River - Interstate Highway 25 <br />Divided Flow (Reference 41). <br /> <br />For Boxelder Creek Overflow, the 100-year flood was delineated using topographic work <br />maps at a scale of 1: I ,200, with 2-foot contour intervals (Reference 42). <br /> <br />The 100- and 5oo-year floodplain boundaries are shown on the Flood Insurance Rate Map <br />(Exhibit 2). On this map, the loo-year floodplain boundary corresponds to the boundary <br />of the areas of special flood hazards (Zones A, AE, AH, and AO), and the SOD-year <br />floodplain boundary corresponds to the boundary of the areas of moderate flood hazards. <br />In cases where the 100- and 5oo-year floodplain boundaries are close together, only the <br />loo-year floodplain boundary has been shown. Small areas within the floodplain <br />boundaries may lie above the flood elevations but cannot be shown due to Iimilations of <br />the map scale and/or lack of detailed topographic data. <br /> <br />For the streams studied by approximate methods, only the loo-year floodplain boundary <br />is shown on the Flood Insurance Rate Map (Exhibit 2). <br /> <br />4.2 Floodways <br /> <br />Encroachment on the floodplain, such as structures and fill, reduces flood-carrying <br />capacity, increases flood heights and velocities, and increases flood hazards in areas <br />beyond the encroachment itself. One aspect of floodplain management involves balancing <br />the economic gain from floodplain development against the resulting increase in flood <br />hazard. For purposes of the NFIP, a floodway is used as a tool to assist local <br />communities in this aspect of floodplain management. Under this concept, the area of the <br />loo-year floodplain is divided into a floodway and a floodway fringe. The flood way is <br />the channel of a stream, plus any adjacent floodplain areas, that must be kept free of <br />encroachment so that the loo-year flood can be carried without substantial increases in <br />flood heights. Minimum Federal standards limit such increases to 1 foot, provided that <br />hazardous velocities are not produced. The flood ways in this study are presented to local <br />agencies as minimum standards that can be adopted directly or that can be used as a basis <br />for additional flood way studies. <br /> <br />The floodways presented in this study were computed for certain stream segments on the <br />basis of equal conveyance reduction from each side of the floodplain. Floodway widths <br />were computed at cross sections. Between cross sections, the f100dway boundaries were <br />interpolated. The results of the floodway computations are tabulated for selected cross <br />sections (Table 2). In cases where the floodway and the 100-year floodplain boundaries <br />are either close together or collinear, only the flood way boundary is shown. <br /> <br />The area between the floodway and 100-year floodplain boundaries is termed the <br />floodway fringe. The floodway fringe encompasses the portion of the floodplain that <br />could be completely obstructed without increasing the water-surface elevation of the 100- <br />year flood more than I foot at any point. Typical relationships between the floodway and <br />the floodway fringe and their significance to floodplain development are shown in <br />Figure 2. <br /> <br />19 <br />
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