Flood risk definitions

Building envelope
The entire exterior surface of the building — including walls, doors, and windows – which encloses or envelopes the space within.

Elevation datum
Or datum plane, is an arbitrary surface that serves as a common reference for the elevations of points above or below it. Elevations are expressed in terms of feet, meters, or other units of measure and are identified as negative or positive depending on whether they are above or below the datum plane. Three common elevation datums are Mean Sea Level (MSL), NGVD, and NAVD.

Erosion
The removal of soil that lowers the ground surface across an area. Scour is the removal of soil around objects that obstruct flow, such as the foundation walls of houses and other buildings.

Flash flood
A flood that rises and falls very quickly and usually is characterized by high flow velocities. Flash floods often result from intense rainfall over a small area.

Hazard mitigation
Action taken to reduce or eliminate long-term risk to people and property from hazards such as floods, hurricanes, earthquakes, and fires.

Lowest adjacent grade (LAG)
The lowest ground surface that touches any of the exterior walls of a structure.

Lowest floor of a building
Under the National Flood Insurance Program, the floor of the lowest enclosed area within the building, including the basement. The only exception is an enclosed area below an elevated building, but only when the enclosed area is used solely for parking, storage, or building access. The elevation of the lowest floor can be very important in retrofitting.

Rainfall intensity
the amount of rain that falls during a given amount of time. It is usually expressed in inches of rainfall per hour. The higher the number of inches per hour, the greater the intensity.

Retrofitting
Making changes to an existing building to protect it from flooding or other hazards such as high winds and earthquakes.

Storm surge
The rise in the level of the ocean that results from the decrease in atmospheric pressure associated with hurricanes and other storms.

Substantial damage
Under the National Flood Insurance Program, if the cost of restoring the building to its before-damage condition would equal or exceed 50 percent of the market value of the structure before the damage occurred, it is considered substantial damage.

Watershed
The geographic area that contributes surface water, from rain or melting snow, to a stream, river or body of water.