Many of the problems engineers/surveyors encounter in the field when doing a FEMA elevation certificate are flood enclosures below the BFE. Many of the enclosures are constructed unintentionally by a homeowner because of either a lack of knowledge or changes to the flood zones subsequent to the original construction. Buildings in an AE Zone, for example, have the flexibility of adding vents as long as the usage of the enclosure is restricted to access, parking or storage. Fully enclosed areas below the lowest floor must be designed to automatically equalize hydrostatic flood forces on exterior walls by allowing the entry and exit of floodwaters. Designs must meet certain minimum criteria for openings in the enclosure walls or be certified by a registered design professional. For more information on the subject of flood enclosures, you can go to http://www.fema.gov/floodplain-management/enclosure#1
Crawlspace foundations are commonly used in some parts of the nation to elevate the lowest floors of residential buildings located in Special Flood Hazard Areas (SFHAs) above the Base Flood Elevation (BFE). Crawlspaces should be constructed so that the floor of the crawlspace is at or above the lowest grade adjacent to the building. Crawlspaces that have their floors below BFE must have openings to allow the equalization of flood forces.
Buildings that have below-grade crawlspaces will have higher flood insurance premiums than buildings that have the interior elevation of the crawlspace at or above the lowest adjacent exterior grade. For more information on the subject of flood enclosures, you can go to http://www.fema.gov/national-flood-insurance-program-2/crawlspace
In V Zones, the space below the lowest floor of a building shall be free of obstruction or constructed with non-supporting breakaway walls, open wood lattice work, or insect screening intended to collapse under wind and water loads without causing collapse, displacement, or other structural damage to the elevated portion of the building or supporting foundation system. Designs to meet this criteria must be certified by a registered design professional or meet certain minimum criteria for breakaway walls.
The graphic examples above are taken from the S.C. Department of Natural Resources’ “Flood Mitigation Program” at this link: http://www.dnr.sc.gov/flood/quickguide.html