What is the Limit of Moderate Wave Action line shown on FEMA Flood Maps and what does this mean?
The Limit of Moderate Wave Action (LiMWA), or projected hazard line for wave action is shown on FEMA’s preliminary, and newer adopted, flood maps and it’s not only for units right on the beach but also shown in places like Jupiter and adjacent to Lake Worth (the water, not the town) where the intracoastal waters pose a “danger of 1.5-foot or higher breaking waves during a 1%-annual-chance flood event.” If homes seaward of this line are damaged or “substantially” re-built they MAY have to conform to higher specifications as if they were on the coast. The line itself is shown as a dark black line with triangles on it. “The LiMWA area alerts property owners on the seaward side of the line that although their property is in say Zone AE, their property may be affected by 1.5-foot or higher breaking waves and may therefore be at significant risk during a 1-percent-annual chance flood event. While not formally defined in the NFIP regulations or mapped as a flood zone, the area between Zone VE and the LiMWA is called the Coastal A Zone (see Figure 2)”
FEMAs proposed flood maps depict two coastal flood hazard zones: Zone VE, where the flood elevation includes wave heights equal to or greater than 3 feet; and Zone AE, where the flood elevation includes wave heights less than 3 feet.
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At this time the local ordinance (which are implemented in compliance with the NFIP) do NOT differentiate construction in relation to the LiMWA. For instance Article 18 of the Unified Land Development Code adopted by Palm Beach County does not reference this line or construction seaward of it.
Currently this is an FYI line on many FEMA FIRM maps in Florida but as they are adopted it will become consequential. The Florida Building Code Chapter 16 incorporates by reference ACSE 24 for the design of homes in the flood zone. The classification system is based upon locally defined zones and since there is currently no LiMWA line then there is no Coastal A Zone. If however this line IS shown on the adopted FIRM locally then there would be a Coastal A Zone and that IS defined in ASCE 24 and treated, for design purposes, as a Zone V, for places like being right on the beach in Juno or a BIG area of West Palm Beach along the ICW. FEMA has this synopsis of ASCE 24.
Why would it be shown, or perhaps not, on the adopted FIRM? Because the community gets points toward something called the Community Rating System (CRS) for having a Zone A defined.
LiMWA QUICK FACTS
Waves of 1.5 feet or higher have been shown to cause significant damage to structures
A LiMWA line is shown on some FIRMs for areas along coastlines
Structural fill should not be used in the Coastal A Zone
International Codes® require Zone VE construction standards in identified Coastal A Zone areas
Structures in the Coastal A Zone should be built with piling or column foundations Enclosure under elevated structures should be limited to 299 square feet or less within the Coastal A Zone Elevation of the lowest horizontal structural member of the lowest floor should be at or above the base flood elevation (BFE)
(see http://www.fema.gov/medialibrary/assets/documents/3490?id=1718 for more information) in the Coastal A Zone Communities that adopt Zone VE standards in the Coastal A Zone and reference the LiMWA area receive Community Rating System (CRS) credits, which could lower flood insurance premiums for residents and business owners For additional background information on LiMWA, please refer to FEMA Procedure Memorandum 50, available at: http://www.fema.gov/media-librarydata/1388777384290.pdf
