Design Guide for Improving Critical Facility Safety 
from Flooding and High Winds

FEMA 543 / January 2007 

Glossary of terms 

100-year flood. See “base flood.”

A

Alluvial Fan. A fan-shaped deposit of alluvium formed by a stream where its 
velocity is abruptly decreased, as at the mouth of a ravine or at the foot of a 
mountain.

Astragal. The center member of a double door, which is attached to the fixed or 
inactive door panel.

B

Base flood. The flood having a 1-percent chance of being equaled or exceeded in 
any given year, commonly referred to as the “100-year flood.” The base flood is 
the national standard used by the NFIP and all Federal agencies for the purposes 
of requiring the purchase of flood insurance and regulating new development. 

Base flood elevation (BFE). The height of the base (1 percent or 100-year) flood 
in relation to a specified datum, usually the National Geodetic Vertical Datum 
of 1929, or the North American Vertical Datum of 1988. 

Basic wind speed. A 3-second gust speed at 33 feet above the ground in Exposure 
C. (Exposure C is flat open terrain with scattered obstructions having heights 
generally less than 30 feet.) Note: Since 1995, ASCE 7 has used a 3-second peak 
gust measuring time. A 3-second peak gust is the maximum instantaneous speed 
with a duration of approximately 3 seconds. A 3-second peak gust speed could be 
associated with a given windstorm (e.g., a particular storm could have 
a 40-mph peak gust speed), or a 3-second peak gust speed could be associated 
with a design-level event (e.g., the basic wind speed prescribed in ASCE 7).


Building, enclosed. A building that does not comply with the requirements for 
open or partially enclosed buildings.

Building, open. A building having each wall at least 80 percent open. This 
condition is expressed by an equation in ASCE 7.

Building, partially enclosed. A building that complies with both of the 
following conditions:

1. The total area of openings in a wall that receives positive external pressure 
exceeds the sum of the areas of openings in the balance of the building envelope 
(walls and roof) by more than 10 percent.
2. The total area of openings in a wall that receives positive external pressure 
exceeds 4 square feet, or 1 percent of the area of that wall, whichever is 
smaller, and the percentage of openings in the balance of the building envelope 
does not exceed 20 percent.These conditions are expressed by equations in ASCE 
7.
Building, regularly shaped. A building having no unusual geometrical 
irregularity in spatial form.

Building, simple diaphragm. An enclosed or partially enclosed building in which 
wind loads are transmitted through floor and roof diaphragms to the vertical 
main wind-force resisting system.

C

Components and cladding (C&C). Elements of the building envelope that do not 
qualify as part of the main wind-force resisting system.

Coping. The cover piece on top of a wall exposed to the weather, usually made of 
metal, masonry, or stone, and sloped to carry off water.

D

Design flood. The greater of the following two flood events: (1) the base flood, 
affecting those areas identified as special flood hazard areas on a community’s 
Flood Insurance Rate Map (FIRM); or (2) the flood corresponding to the area 
designated as a flood hazard area on a community’s flood hazard map or otherwise 
legally designated.

Design flood elevation (DFE). The elevation of the design flood, including wave 
height, relative to the datum specified on a community’s flood hazard map.

Dry floodproofing. An adjustment, modification, or addition of a feature, or any 
combination thereof, that eliminates or reduces the potential for flood damage 
by sealing walls and closing openings to keep water from entering a building. 

E

Erodible soil. Soil subject to wearing away due to the effects of wind, water, 
or other geological processes during a flood or storm or long-term exposure. 

Escarpment. Also known as a scarp. With respect to topographic effects, a cliff 
or steep slope generally separating two levels or gently sloping areas. 

Exposure. The characteristics of the ground roughness and surface irregularities 
in the vicinity of a building. ASCE 7 defines three exposure categories—
Exposures B, C, and D. 

Extratropical storm. A cyclonic storm that forms outside of the tropical zone. 
Extratropical storms may be large, often 1,500 miles (2,400 kilometers) in 
diameter, and usually contain a cold front that extends toward the equator for 
hundreds of miles.

F

Federal Emergency Management Agency (FEMA). The Federal Emergency Management 
Agency is the Federal agency which administers the National Flood Insurance 
Program (NFIP).

Fetch. Distance over which wind acts on the water surface to generate waves.

Flashing. Any piece of material, usually metal or plastic, installed to prevent 
water from penetrating a structure.

Flood Insurance Rate Map (FIRM).   The official map of a community on which FEMA 
has delineated both the special hazard areas, and the risk premium zones 
applicable to the community.

Flood Insurance Study (FIS). An engineering study performed by FEMA to identify 
flood hazard areas, flood insurance risk zones, and other flood data in a 
community; used in the development of the FIRM.

Floodplain. Any land area, including the watercourse, that is susceptible to 
partial or complete inundation by water, from any source.

Floodplain management regulations. Zoning ordinances, subdivision regulations, 
building codes, or special-purpose ordinances that set flood-resistant standards 
for new construction, land use, and development.

Flood profile. A graph of computed flood elevations at points located along a 
riverine waterway. A flood profile typically is available for a waterway that 
has Base Flood Elevations (BFEs) shown on the Flood Insurance Rate Map (FIRM). 
Flood profiles are usually found in the Flood Insurance Study (FIS) report.

Floodway. The channel and that portion of the floodplain that is to be reserved 
to convey the base flood, without cumulatively increasing the water surface 
elevation more than a designated height.

Floodway fringe. The area of the floodplain outside of the floodway, where 
floodwaters may be shallower and slower. 

Freeboard. A factor of safety, usually expressed in feet above a flood level, 
for purposes of floodplain management. Freeboard also compensates for the many 
unknown factors that could contribute to flood heights greater than the height 
calculated for a selected size flood and floodway conditions, such as wave 
action, constricting bridge openings, and the hydrological effect of 
urbanization of the watershed. A freeboard of 1 to 3 feet is often applied to 
critical facilities.

Frontal dune. Ridge or mound of unconsolidated sandy soil, extending 
continuously alongshore landward of the sand beach and defined by relatively 
steep slopes abutting markedly flatter and lower regions on each side.

G

Glazing. Glass or a transparent or translucent plastic sheet used in windows, 
doors, and skylights.

Glazing, impact-resistant. Glazing that has been shown, by an approved test 
method, to withstand the impact of wind-borne missiles likely to be generated in 
wind-borne debris regions during design winds.

H

Hurricane-prone regions. Areas vulnerable to hurricanes; in the United States 
and its territories defined as:

1. The U.S. Atlantic Ocean and Gulf of Mexico coasts, where the basic wind speed 
is greater than 90 miles per hour.
2. Hawaii, Puerto Rico, Guam, U.S. Virgin Islands, and American Samoa.
Human intervention. The presence and active involvement of people necessary to 
enact or implement floodproofing measures prior to the onset of flooding.

Hydrodynamic load. Loads imposed by water flowing against and around an object 
or structure, including the impacts of debris and waves.

Hydrostatic load. Load (pressure) imposed on an object or structure by a 
standing mass of water; the deeper the water, the greater the load or pressure 
against the object or structure.

I

Impact-resistant covering. A covering designed to protect glazing, which has 
been shown by an approved test method to withstand the impact of wind-borne 
missiles likely to be generated in wind-borne debris regions during design 
winds.

Importance factor, I. A factor that accounts for the degree of hazard to human 
life and damage to property. Importance factors are given in ASCE 7.

L

Lowest floor. The lowest floor of the lowest enclosed area (including basement). 
An unfinished or flood resistant enclosure, usable solely for parking of 
vehicles, building access, or storage, in an area other than a basement area, is 
not considered a building's lowest floor, provided that the enclosure is 
compliant with flood-resistant requirements. 

M

Main wind-force resisting system. An assemblage of structural elements assigned 
to provide support and stability for the overall structure. The system generally 
receives wind loading from more than one surface.

Mean roof height, (h). The average of the roof eave height and the height to the 
highest point on the roof surface, except that, for roof angles of less than or 
equal to 10 degrees, the mean roof height shall be the roof eave height.

Missiles. Debris that could become propelled into the wind stream. 

N

National Flood Insurance Program (NFIP). A Federal program to identify flood-
prone areas nationwide, and make flood insurance available for properties in 
communities that participate in the program. 

Nor’easter. Nor’easters are non-tropical storms that typically occur in the 
eastern United States, any time between October and April, when moisture and 
cold air are plentiful. They are known for dumping heavy amounts of rain and 
snow, producing hurricane-force winds, and creating high surfs that cause severe 
beach erosion and coastal flooding. A nor’easter is named for the winds that 
blow in from the northeast and drive the storm up the east coast along the 
Gulf Stream, a band of warm water that lies off the Atlantic Coast. 



O

Openings. Apertures or holes in the building envelope that allow air to flow 
through the building envelope. A door that is intended to be in the closed 
position during a windstorm would not be considered an opening. Glazed openings 
are also not typically considered openings. However, if the building is located 
in a wind-borne debris region and the glazing is not impact-resistant or 
protected with an impact-resistant covering, the glazing is considered an 
opening.

R

Racking. Lateral deflection of a structure resulting from external forces, such 
as wind or lateral ground movement in an earthquake.

Ridge. With respect to topographic effects, an elongated crest of a hill 
characterized by strong relief in two directions.

S

Scour. Removal of soil or fill material by the flow of floodwaters. The term is 
frequently used to describe storm-induced, localized erosion around pilings at 
building corners and other foundation supports where the obstruction of flow 
increases turbulence. 

Seiche. A wave that oscillates in lakes, bays, or gulfs from a few minutes to a 
few hours as a result of seismic or atmospheric disturbances.

Sheetflow. Rainfall runoff that flows over relatively flat land without 
concentrating into streams or channels.

Stillwater elevation. The elevation that the surface of coastal flood waters 
would assume in the absence of waves, referenced to a datum.

Storm surge. Rise in the water surface above normal water level on the open 
coast due to the long-term action of wind and atmospheric pressure on the water 
surface.

Substantial damage. Damage of any origin sustained by a structure, whereby the 
cost of restoring the structure to its pre-damage condition equals or exceeds 50 
percent of the market value of the structure before the damage occurred (or 
smaller percentage if established by the authority having jurisdiction). 
Structures that are determined to be substantially damaged are considered to be 
substantial improvements, regardless of the actual repair work performed. 

Substantial improvement. Any reconstruction, rehabilitation, addition, or other 
improvement of a structure, the cost of which equals or exceeds 50 percent of 
the market value of the structure (or smaller percentage if established by the 
authority having jurisdiction) before the start of the improvement. 

T

Tsunami. An unusually large sea wave produced by submarine earth movement or a 
volcanic eruption.

W

Wave runup. Rush of wave water up a slope or structure. The additional height 
reached by waves above the stillwater elevation. 

Wet floodproofing. Permanent or contingent measures and construction techniques, 
applied to a structure or its contents, that prevent or provide resistance to 
damage from flooding while allowing floodwaters to enter the structure. 
Generally, this includes properly anchoring the structure, using flood-resistant 
materials below the BFE, protection of mechanical and utility equipment, and the 
use of openings or breakaway walls. 

Wind-borne debris regions. Areas within hurricane-prone regions located:

1. Within 1 mile of the coastal mean high water line where the basic wind speed 
is equal to or greater than 110 mph and in Hawaii.
2. In areas where the basic wind speed is equal to or greater than 120 mph.


[End of Appendix B]