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FLOOD11039
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Last modified
1/26/2010 10:14:04 AM
Creation date
11/29/2007 11:00:02 AM
Metadata
Fields
Template:
Floodplain Documents
County
Grand
Community
Grand County and Incorporated Areas
Basin
Colorado Mainstem
Title
FIS - Grand County and Incorporated Areas
Date
1/2/2008
Prepared For
Grand County
Prepared By
FEMA
Floodplain - Doc Type
Current FEMA Regulatory Floodplain Information
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<br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the I-percent-annual-chance <br />(IOO-year) flood has been adopted by FEMA as the base flood for floodplain management <br />purposes. The O.2-percent-annual-chance (500-year) flood is employed to indicate additional <br />areas of flood risk in the community. For each stream studied by detailed methods, the <br />100- and 500-year floodplain boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the floodplain boundaries were <br />interpolated using topographic maps at a scale of 1 :6,000 and I :4,000, with contour intervals <br />of 10 feet and 2 feet, respectively (References 13 and 14). <br /> <br />The 100- and 500-year floodplain boundaries are shown on the FIRM. On this map, the <br />100-year floodplain boundary corresponds to the boundary of the areas of special flood <br />hazards (Zones A and AE), and the SOO-year floodplain boundary corresponds to the <br />boundary of areas of moderate flood hazards. In cases where the 100- and 500-year <br />floodplain boundaries are close together, only the 100-year floodplain boundary has been <br />shown. Small areas within the floodplain boundaries may lie above the flood elevations but <br />cannot be shown because oflimitations of the map scale and/or lack of detailed topographic <br />data. <br /> <br />For the streams studied by approximate methods, only the 100-year floodplain boundary is <br />shown on the FIRM. Approximate floodplain boundaries along Jim Creek have been <br />delineated using topographic maps at a scale of I :24,000 (Reference IS). Approximate <br />floodplain boundaries along the Fraser River and Tenmile Creek in the vicinity of Granby <br />have been delineated using two-foot contour mapping (Reference 16). <br /> <br />Approximate 100-year floodplain boundaries in some portions of the study area were taken <br />directly from the Flood Hazard Boundary Maps for the Towns of Grand Lake and <br />Hot Sulphur Springs (References 17 and 18). <br /> <br />4.2 Floodways <br /> <br />Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, <br />increases flood heights and velocities, and increases flood hazards in areas beyond the <br />encroachment itself. One aspect of floodplain management involves balancing the economic <br />gain from floodplain development against the resulting increase in flood hazard. For <br />purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect <br />of floodplain management. Under this concept, the area of the I OO-year floodplain is divided <br />into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any <br />adjacent floodplain areas, that must be kept free of encroachment so that the 1 OO-year flood <br />can be carried without substantial increases in flood heights. Minimum Federal standards <br />limit such increases to 1 foot, provided that hazardous velocities are not produced. The <br />floodways in this study are presented to local agencies as minimum standards that can be <br />adopted directly or that can be used as a basis for additional floodway 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 <br />computed at cross sections. Between cross sections, the floodway boundaries were <br />interpolated. The results of the floodway computations are tabulated for selected cross <br />sections (see Table 3, "Floodway Data"). In cases where the floodway and 100-year <br />floodplain boundaries are either close together or collinear, only the floodway boundary is <br />shown. <br /> <br />The area between the floodway and 100-year floodplain boundaries is termed the floodway <br />fringe. The floodway fringe encompasses the portion of the floodplain that could be <br />completely obstructed without increasing the WSEL ofthe I OO-year flood more than I foot <br /> <br />12 <br />
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