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Flood Definitions

♠ Posted by amsran in at 10:19 PM
Flood Definitions Flood Hazard

Flood Definitions

Floods are caused by weather phenomena and events that deliver more precipitation to a drainage basin than can be readily absorbed or stored within the basin.
Volume of water required to cover 1 acre of land (43,560 square feet) to a depth of 1 foot; equivalent to 325,851 gallons.
Cubic feet per second (ft³/s).
A unit of measurement expressing rates of discharge. One cubic foot per second is equal to the discharge of a stream of rectangular cross section, 1 foot wide and 1 foot deep, flowing water an average velocity of 1 foot per second. Equivalent to 448.8 gallons per minute.
Rate of flow--a volume of fluid passing a point per unit time, commonly expressed in cubic feet per second, million gallons per day, or gallons per minute.
Drainage basin.
A part of the surface of the Earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water. Land area drained by a stream or river.
Flash flood.
The result of heavy or excessive amounts of rainfall within a short period of time, usually less than 6 hours, causing water to rise and fall quite rapidly.
An overflow or inundation that comes from a river or other body of water and causes or threatens damage. Any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream.
Flood frequency.
Refers to a flood level that has a specified percent chance of being equaled or exceeded in any given year. For example, a 100-year flood occurs on average once every 100 years and thus has a 1-percent chance of occurring in a given year.
Flood plain.
A strip of relatively flat-lying land that borders a stream and is underlain by sediment carried by the stream and dropped in the slack water beyond the influence of the swiftest current.
Flood stage.
The stage at which overflow of the natural streambanks begins to cause damage in the reach in which the elevation is measured. Flood stages for each USGS gaging station are usually provided by the National Weather Service.
Gage datum.
An arbitrary datum plane that is established for a particular gaging station to which water-surface elevations can be compared.
Gage height.
See Stage.
Gaging station.
A site on a stream, canal, lake, or reservoir where systematic observations of gage height or water discharge are obtained by a gage, recorder, or similar equipment.
Peak stage.
The maximum height of a water surface above an established datum. Same as peak gage height.
Rain, snow, hail, or sleet.
Real-time data.
Data collected by automated instrumentation and telemetered and analyzed quickly enough to influence a decision that affects the monitoring system.
Recurrence interval.
The average interval of time within which the magnitude of a given event, such as a flood, will be equaled or exceeded one time.
The height of a water surface above an established datum. Used interchangeably with gage height.
The margins of a stream channel. Banks are called right and left as viewed facing the direction of flow.
The discharge or flow that occurs in a natural channel. Although the term discharge can be applied to the flow of a canal, the word "streamflow" uniquely describes the discharge in a surface stream course.
Surface runoff.
That part of the runoff that travels over the soil surface to the nearest stream channel. It also is defined as that part of the runoff of a drainage basin that has not passed beneath the surface following precipitation.
Surface water.
Water on the surface of the Earth.
Water year.
The water year deals with the surface-water supply for a 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends and which includes 9 out of the 12 months. Thus, the year ending September 30, 1999, is called the "1999 water year."
NOTE: Some definitions were excerpted from other sources.


  1. Current and historic stream water levels are reported as stage above a gage datum or as water-surface elevation above sea level. which is the addition of the stage to the gage datum.
  2. Datum for each site can be found in the station description by clicking on the site of interest in the streamflow conditions table and looking under GAGE for datum of gage.
  3. Stream elevation could be useful during flooding for comparison to elevation of structures such as building pads or road surfaces.
  4. The elevation of structures and roads can be obtained from surveys and topographic maps.