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Last modified
1/25/2010 7:11:54 PM
Creation date
10/5/2006 3:01:32 AM
Metadata
Fields
Template:
Floodplain Documents
County
Statewide
Basin
Statewide
Title
Appeals, Revisions, and Amendments to Flood Insurance Maps - A Guide for Community Officials
Date
1/1/1990
Prepared By
FEMA
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />Chapter 7 <br />Floodway Revisions <br /> <br />The floodways shown on NFIP maps are <br />developed as part of detailed FISs and are <br />adopted by communities for use in establishing <br />sound floodplain management programs. By <br />restricting development in the floodway, a <br />community can preserve the conveyance area <br />necessary for the passage of floodwaters and <br />avoid significant increases in flood elevations. <br /> <br />The boundaries of the floodway are intended to <br />remain unchanged once they are established <br />and adopted by the community. In response to <br />extraordinary circumstances, however, a <br />community may find it necessary to change the <br />configuration of the floodway. The purpose of <br />this chapter is to discuss FEMA policy concerning <br />floodway revisions and to describe the <br />information that must be submitted in support <br />of a request for a floodway revision. <br /> <br />Background <br /> <br />Even though fill may be placed or construction <br />carried out in floodplains in such a way that the <br />flood hazards to the new development will be <br />minimal, the effect of such activities on existing <br />development, both upstream and downstream <br />must be considered. Encroachments, such as <br />construction and the placement of fill, within <br />the lOa-year floodplain can increase flood levels <br />by blocki ng areas of the floodplain that would <br />otherwise be open and able to convey <br />floodwaters. <br /> <br />To achieve a balance between any benefits to be <br />gained from floodplain development and the <br />resulting increase in the flood hazard, and to <br />help communities regulate such development <br />and avoid significant increases in BFEs, FEMA <br />establishes floodways. A floodway identifies the <br />portion of the lOa-year floodplain that must be <br />kept free of encroachment. The limits of the <br />floodway are determined through a hydraulic <br />analysis in which the rise in the BFE (the <br />surcharge) due to encroachment within the 100- <br />year floodplain is calculated. <br /> <br />FEMA has established as a standard a maximum <br />allowable surcharge of 1.0 foot. Because the <br />surcharge <br /> <br />generally increases as the amount of <br />encroachment increases, setting a limit on the <br />magnitude of the surcharge sets limits on the <br />amount of encroachment that may take place. <br />A floodway based on a maximum allowable <br />surcharge of 1.0 foot is therefore the channel of <br />a stream plus the portion of the floodplain <br />adjacent to it that must be kept free of <br />encroachment so that the entire 100-year flood <br />discharge can be passed with no greater than a <br />1.0 foot increase in the BFE. <br /> <br />The portions of the floodplain outside the <br />floodway are referred to as the floodway fringe. <br />Once a floodway has been established, the <br />community may allow development in the <br />floodway fringe with the assurance that flood <br />hazards will not be increased significantly. <br />However, all such development must meet the <br />minimum floodplain management standards <br />required for participation in the NFIP. <br /> <br />Several States have adopted requirements that <br />limit the allowable surcharge to less than 1.0 <br />foot. For States that have adopted more <br />stringent standards by legally enforceable <br />statutes or regulations, FEMA computes <br />floodways using those standards. In addition, <br />some individual communities have established <br />and enforce more stringent standards. <br />Although the NFIP maps for such communities <br />usually depict floodways based on a 1.0-foot <br />surcharge, FEMA encourages the adoption of <br />more stringent standards. Once a floodway has <br />been adopted by the community, any <br />encroachments within the floodway that would <br />increase the BFEs during the 100-year flood are <br />prohibited. Such encroachments could include <br />fill, new construction, substantial improvements, <br />and other types of development. <br /> <br />Hereafter in this Guide, when a 1.0-foot <br />surcharge is referred to, the assumption has <br />been made that there is no more stringent State <br />or local surcharge standard. Where such a <br />standard exists, the allowable surcharge is <br />limited to that standard. In addition, where a <br />stream with a regulatory floodway forms the <br /> <br />39 <br />
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