Flood Terminology

100-Year Flood

The 100-year flooding event is the flood having a 1 percent chance of being equaled or exceeded in magnitude in any given year. Contrary to popular belief, it is not a flood occurring once every 100 years.

100-Year Floodplain

The 100-year floodplain is the area adjoining a river, stream, or watercourse covered by water in the event of a 100-year flood.

Base Flood Elevation (BFE)

The term "Base Flood Elevation" refers to the elevation (measured in feet above sea level) that the 1% annual flood (100-year flood) is expected to reach.


For floodplain ordinance purposes, means any man-made change in improved and unimproved real estate, including but not limited to:

  • Buildings or other structures
  • Dredging
  • Excavation or drilling operations
  • Filling
  • Grading
  • Mining
  • Paving
  • Storage of equipment or materials

The definition of development for floodplain purposes is generally broader and includes more activities than the definition of development used in other sections of local land use ordinances.


The floodway is one of two main sections that make up the floodplain. Floodways are defined for regulatory purposes. The floodway is the channel of a river or other watercourse and adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. The floodway carries the bulk of the floodwater downstream and is usually the area where water velocities and forces are the greatest.

Flood Fringe

The flood fringe refers to the outer portions of the floodplain, beginning at the edge of the floodway and continuing outward. This is the area where development is most likely to occur, and where precautions to protect life and property need to be taken.


A floodplain is a land area that is subject to flooding, adjacent to a:

  • Estuary
  • Lake
  • River
  • Stream
  • Other water body

This area, if left undisturbed, acts to store excess floodwater.