Flood maps developed by the Federal Emergency Management Agency (FEMA) have long recognized two main types of flood zone. The first, known as the “V zone,” is where storm surges are likely to occur and waves are likely to reach at least three feet in height. In these areas, buildings face severe destructive forces not just from the surging water but also from the battering waves, which may carry debris. Even an East Coast Nor’easter of lower-than-hurricane strength can result in battering wave action able to knock apart wood frame walls. In Katrina, entire square miles of Mississippi coastal towns were swept clean of all structures by the deepwater currents and high, pounding waves.
Given such forces, house foundations built in the V zone must be open pilings, supported by deep piers. The lowest structural member has to be above the base flood elevation, plus a locally determined “freeboard” that reflects expected wave heights, and may include an additional factor of safety for good measure.
The “A zone” is found near rivers and streams, where rising water presents the primary flood risk. In A zones, occupied space or mechanicals can’t be placed below the base flood elevation level, but foundations can be enclosed. Crawlspace or basement foundations, however, must be equipped with flood vents within one foot of grade on at least two sides. In the event of a flood, this allows water to flow in and out of the house, relieving the water pressure.
In terms of technical information, builders who construct homes in either the V or A zones will find an indispensable reference in FEMA’s Coastal Construction Manual (FEMA 55), which is available at www.fema.gov/rebuild/mat/fema55.shtm.
Useful information can also be found at the “Raised Floor Living” Web site (www.raisedfloorliving.com) of the Southern Forest Products Association.
Coastal A Zone Considerations
Builders who work in a coastal region also should be aware of a new zone definition: the “Coastal A” Zone. First introduced by FEMA in 2000, the zone’s guideline are now included in “Recommended Residential Construction for the Gulf Coast” (FEMA 550), which can be found at www.fema.gov/library/viewRecord.do?id=1853.