In the
United States, flood openings are used to provide for the automatic equalization of
hydrostatic pressure on either side of a wall.
Building codes usually require the installation of flood openings in the walls of structures located in
A-type flood zones recognized by the
Federal Emergency Management Agency. Various agencies in the United States define necessary characteristics for flood openings. The NFIP Regulations and Building Codes require that any residential building constructed in Flood Zone Type A have the lowest floor, including basements, elevated to or above the Base Flood Elevation (BFE). Enclosed areas are permitted under elevated buildings provided that they meet certain use restrictions and construction requirements such as the installation of flood vents to allow for the automatic entry and exit of flood waters. The wet floodproofing technique is required for residential buildings.
Engineered vs. non-engineered openings Most regulatory authorities in the United States that offer requirements for flood openings define two major classes of opening: engineered, and non-engineered. The requirements for non-engineered openings are typically stricter, defining necessary characteristics for aspects ranging from overall size of each opening, to allowable screening or other coverage options, to number and placement of openings. Engineered openings ignore many of the requirements, depending on the particular regulatory authority. To qualify as an engineered opening, testing and/or certification by a qualified agency (varying from regulator to regulator, and indicated below where appropriate) is required.
American Society of Civil Engineers definition The
American Society of Civil Engineers (ASCE) requirements apply to any structure that is not
dry flood-proofed and which is in the
mapped flood zone. It calls for openings in
load-bearing foundation walls located below the mapped
flood elevation. Where non-engineered openings are used, each opening must be at least three inches in diameter, and have no screen or other cover that interferes with the transition of water between interior and exterior. The total net open area of all flood openings in the structure must be equal to or greater than one square inch, per square foot of footprint of the enclosed area—though no fewer than two openings, total, which must be located on different walls. Openings must be placed such that the bottom of each opening is no more than one
foot above the adjacent ground level. In lieu of these requirements, engineered openings must conform to a performance standard: during a flood with a rate of rise/fall of five feet per hour, the difference between interior and exterior flood water levels in an enclosure using the engineered openings must not be greater than one foot. These requirements apply for both non-engineered and engineered flood openings.
FEMA While the
Federal Emergency Management Agency does not have
de jure authority over the building code, it maintains crucial influence over flood opening standards through its administration of the
National Flood Insurance Program (NFIP). By controlling the standards for nearly all
flood insurance in the United States, the NFIP exerts exceptional
de facto authority over many aspects of floodplain construction. The FEMA (and, thus, NFIP) requirements for non-engineered openings are similar to requirements from the American Society of Civil Engineers. Unlike the ASCE, FEMA requires the placement of openings such that the bottom of each opening is no more than one foot above the higher of the adjacent ground level, or the interior foundation slab height. For engineered openings, FEMA offers two subclassifications: individual certification openings, and openings with
International Code Council Evaluation Service (ICC-ES) Evaluation Reports. Individual certification openings are offered for use when, "[f]or architectural or other reasons, building designers or owners may prefer to use unique or individually designed openings or devices". ==See also==