Flood: Know Your Terms

Familiarize yourself with these terms to help identify a flood hazard:

Warnings

Flood Watch:
Flooding is possible. Tune in to NOAA Weather Radio, commercial radio, or television for information.

Flash Flood Watch:
Flash flooding is possible. Be prepared to move to higher ground; listen to NOAA Weather Radio, commercial radio, or television for information.

Flood Warning:
Flooding is occurring or will occur soon; if advised to evacuate, do so immediately.

Flash Flood Warning:
A flash flood is occurring; seek higher ground on foot immediately.

Surface-water hydrology is the study of the origin and processes of water in streams and lakes, in nature, and as modified by man. It includes such subjects as infiltration, channel storage, floods and droughts, direct runoff, and base flow. Surface-water hydrology shares with meteorology the study of precipitation and evaporation. Also, surface-water hydrology shares with geomorphology the study of the shape, size, and number of river channels, because river channels are formed as a consequence of the rates and quantities of water they must carry. Some of the tools used in the study and application of surface-water hydrology are unit hydrographs, flow-duration curves, flood-frequency curves, and correlation, all of which are defined in this report.

100-year flood: The flood having a 1% or greater annual probability of occurring.

500-year flood: The flood having a 0.2% or greater annual probability of occurring.

Base Flood: A flood having a 1-percent probability of being equaled or exceeded in any given year; also referred to as the 100-year flood.

Base Flood Elevation (BFE): Defined by FEMA as the elevation of the crest of the base or 100-year flood relative to mean sea level. BFE is not depth of flooding. To determine depth of flooding, you would need to subtract the lowest elevation of a particular property from the BFE.

Confluence, in geography, describes the meeting of two or more bodies of water. It usually refers to the point where a tributary joins a more major river, called the mainstem, when that major river is also the highest order stream in the drainage basin.

Floodplain: Any land area susceptible to being inundated by floodwaters from any source.

Flood Any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream and rising above the flood stage.

Flood crest The highest value of the stage or streamflow attained by a flood; it is the top of the flood wave.

Flood of record The highest observed river stage or discharge at a given location during the period of record keeping. (Not necessarily the highest known stage.)  

Flood plain The lowland which borders a river, usually dry but subject to flooding. Also the portion of a river valley which has been inundated by the river during historic floods.

Flood stage The stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which the elevation is measured.The stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which the elevation is measured.

Floodway. A part of the flood plain otherwise leveed, reserved for emergency diversion of water during floods. A part of the flood plain which, to facilitate the passage of floodwater, is kept clear of encumbrances.

Freeboard: A margin of safety added to the base flood elevation to account for waves, debris, miscalculations, or lack of data.

Hydrograph. A graph showing stage, flow, velocity, or other property of water with respect to time.

Repetitive Loss Property: A property for which two or more National Flood Insurance Program losses of at least $1,000 each have been paid within any 10 year period since 1978.

Riparian. Pertaining to the banks of a stream.