What is a Floodway and What Does it Mean
FEMA Flood Maps Show Areas Where Development is Restricted
Aug 14, 2009
As discussed in a previous article, the Federal Emergency Management Agency (FEMA) has defined many different flood zones, what they call “special flood hazard areas.” Part of some zones include an area called the floodway, which carries special regulations and restrictions.
Definition of a FEMA Floodway
According to FEMA’s frequently asked questions web page, the floodway is “where the water is likely to be deepest and fastest…the area of the floodplain that should be reserved (kept free of obstructions) to allow floodwaters to move downstream.”
A floodway is first determined by engineering analysis. Using a computer program such as the HEC-RAS program from the US Army Corps of Engineers, the central part of the floodplain is determined by reducing the floodplain width (in the calculations) until the computed base flood elevation (BFE) rises by a designated amount, typically 1.0 feet or less.
Floodways are shown on the flood maps by a special graphical treatment. See Figure 1, which is a portion of a flood map enlarged, and with special annotations. Since a detailed analysis is required before a floodway can be established, floodways are only shown on Zones AE or similar zones that have had a detailed analysis performed. Only when a floodway is designated within a flood plain do special building and development restrictions apply.
Development is Restricted Within the Floodway
Since preservation of the floodway is critical to continued passage of flood waters, FEMA puts additional restrictions on construction in the floodway. Any alteration of the floodway through placement of fill or adding a building requires two separate approvals.
- A conditional letter of map revision (CLOMR) issued by FEMA.
- A floodplain development permit (FPDP) issued by the local floodplain administrator.
The CLOMR comes first, followed by the FPDP. FEMA will only issue a CLOMR after rigorous and detailed engineering analysis. This analysis must demonstrate how the proposed construction will cause the floodway and floodplain to change, and that any increase in BFE will be within acceptable limits. While the Federal limit, which in general is 1.0 feet but which for any given waterway will have been defined by a prior flood insurance study,, many communities adopt the more restrictive limit of 0.0 feet. This further restricts development in the floodway.
Another requirement prior to issuing a CLOMR is that every property owner who will be affected by the change be notified individually, and that a legal advertisement be placed in the newspaper. In general, approval of the proposed changes must be obtained from the owner of any property that will be affected by the proposed changes.
Communities May Adopt Floodway Limits That Conform to Land Use Plans
While engineering analysis is the initial basis for determining the floodway limits, a local community may request that a different basis be used for establishing floodway limits. The local community may have adopted a land use plan, or zoning ordinances, that require a floodway more restrictive than that determined by engineering analysis.
Or, the community’s knowledge of future work to be done to the waterway, or of future development in the drainage basin, may cause them to have good reason to request that a different floodway be shown on the map. In these cases, the floodway becomes not only a flood protection tool but also a planning tool for the community.
The floodway as a center core of the floodplain has been part of FEMA regulations for years. Slowly, with each revision of a flood map, more and more streams are having a floodway designated. With each new floodway designated, more and more control is exerted over streams, and the flood protection for America’s communities becomes greater.
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