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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

face

The external surface of a structure, such as the surface of a dam.

federal snow sampler

A snow sampler consisting of five or more sections of sampling tubes, one which has a steel cutter on the end. The combined snowpack measuring depth is 150 inches. This instrument was formerly the Mount Rose Type Snow Sampling Set.

fetch

1. The effective distance which waves have traveled in open water, from their point of origin to the point where they break.
2. The distance of the water or the homogenous type surface over which the wind blows without appreciable change in direction.

field (moisture) capacity

The amount of water held in soil against the pull of gravity.

field (moisture) deficiency

The quantity of water, which would be required to restore the soil moisture to field moisture capacity.

fill dam

Any dam constructed of excavated natural materials or of industrial wastes.

firn (snow)

Old snow on top of glaciers, granular and compact and not yet converted into ice. It is a transitional stage between snow and ice. Also called neve.

firn line

The highest level to which the fresh snow on a glacier's surface retreats during the melting season. The line separating the accumulation area from the ablation area.

flash flood

A flood which follows within a few (usually less than six) hours of heavy or excessive rainfall, dam or levee failure, or the sudden release of water impounded by an ice jam.

flash flood guidance (FFG)

An internal NWS product which indicates the amount of rainfall in a given amount of time (usually less than 6 hours) which most likely will produce flash flooding in a given area.

flash flood statement (FFS)

A statement issued by the NWS which provides follow-up information on flash flood watches and warnings.

flash flood table

A table of pre-computed forecast crest stage values for small streams for a variety of antecedent moisture conditions and rain amounts. Soil moisture conditions are often represented by flash flood guidance values. In lieu of crest stages, categorical representations of flooding (minor, moderate, major) may be used on the tables.

flash flood warning (FFW)

A warning issued by the NWS to warn of flash flooding that is imminent or occurring.

flash flood watch (FFA)

A statement issued by the NWS which alerts communities to the possibility of flash flooding in specified areas.

flashboards

A length of timber, concrete, or steel placed on the crest of a spillway to raise the retention water level but which may be quickly removed in the event of a flood by a tripping device, or by deliberately designed failure of the flashboard or its supports.

floc

A cluster of frazil particles

floe

An accumulation of frazil flocs (also known as a "pan") or a single piece of broken ice.

flood

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

flood control storage

Storage of water in a reservoir to abate flood damage.

flood crest

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

flood frequency curve

1. A graph showing the number of times per year on the average, plotted as abscissa, that floods of magnitude, indicated by the ordinate, are equaled or exceeded.
2. A similar graph but with recurrence intervals of floods plotted as abscissa.

flood loss reduction measures

The strategy for reducing flood losses. There are four basic strategies: prevention, property protection, emergency services, and structural projects. Each strategy incorporates different measures that are appropriate for different conditions. In many communities, a different person may be responsible for each strategy.

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 potential outlook

A statement issued by the NWS which alerts communities to the possibility of river flooding along specified reaches or near specified forecast points.

flood prevention

Measures taken in order to keep flood problems from getting worse. Planning, land acquisition, river channel maintenance, wetlands protection, and other regulations all help modify development on floodplains and watersheds to reduce their susceptibility to flood damage. Preventive measures are usually administered by the building, zoning, planning and/or code enforcement offices of the local government.

flood problems

Problems and damages which occur during a flood as a result of human development and actions. Flood problems result from

• Inappropriate development in the floodplain (e.g., building too low, too close to the channel, or blocking flood flows);
• Development in the watershed that increases flood flows and creates a larger floodplain; or
• A combination of the previous two.

flood profile

A graph of elevation of the water surface of a river in flood, plotted as ordinate, against distance, measured in the downstream direction, plotted as abscissa. A flood profile may be drawn to show elevation at a given time, crests during a particular flood, or to show stages of concordant flows.

flood routing

The process of determing progressively the timing, shape, and amplitude of a flood wave as it moves downstream to successive points along the river.

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.

flood statement

A statement issued by the NWS which provides follow-up information on river flooding.

flood warning

A warning issued by the NWS to warn of river flooding which is imminent or occurring. A flood warning is issued when a river first exceeds its flood stage, and it may be reissued if a new river forecast for a forecast point or reach is significantly higher than a previous forecast.

flood wave

A distinct rise in stage culminating in a crest and followed by recession to lower stages.

flooded ice

Ice which has been flooded by melt water or river water and is heavily loaded by water and wet snow.

floodproofing

The process of protecting a building from flood damage on site. Floodproofing can be divided into wet and dry floodproofing. In areas subject to slow-moving, shallow flooding, buildings can be elevated, or barriers can be constructed to block the water's approach to the building. These techniques have the advantage of being less disruptive to the neighborhood. It must be noted that during a flood, a floodproofed building may be isolated and without utilities and therefore unusable, even though it has not been damaged.

floodwall

A long, narrow concrete, or masonry embankment usually built to protect land from flooding. If built of earth the structure is usually referred to as a levee. Floodwalls and levees confine streamflow within a specified area to prevent flooding. The term "dike" is used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.

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.

The channel of a river or stream and those parts of the flood plains adjoining the channel, which are reasonably required to carry and discharge the floodwater or floodflow of any river or stream.

flood duration curve

A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.

flowing well

A well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also called an artesian well.

forebay

The water behind (upstream) of a dam.

forecast crest

The highest elevation of a river stage expected during a specific storm event.

forecast point

A location that represents an area (reach of a river), where a forecast is made available to the public.

flow duration curve

A cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded..

flowing well

A well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also called an artesian well.

foresight

A sighting on a point of unknown elevation from an instrument of known elevation. To determine the elevation of the point in question, the foresight is subtracted from the height of the instrument.

fountainhead

The upper end of a confined-aquifer conduit where it intersects the land surface.

fracture

Any break or rupture formed in an ice cover or floe due to deformation.

fracture zone

An area which has a great number of fractures.

fracturing

Deformation process whereby ice is permanently deformed and fracture occurs.

frazil ice

Fine spicules, plates, or discoids of ice suspended in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.

frazil slush

An agglomerate of loosely packed frazil which floats or accumulates under the ice cover.

free groundwater

Unconfined groundwater whose upper boundary is a free water table.

freeboard

The vertical distance between the normal maximum level of the water surface in a channel, reservoir, tank, canal, etc., and the top of the sides of a levee, dam, etc., which is provided so that waves and other movements of the liquid will not overtop the confining structure.

freezeup date

Date on which the water body was first observed to be completely frozen over.

freezup jam

Ice jam formed as frazil ice accumulates and thickens.

french drain

An underground passageway for water through the interstices among stones placed loosely in a trench.

frequency curve

A curve that expresses the relation between the frequency distribution plot, with the magnitude of the variables as abscissas and the number of occurrences of each magnitude in a given period as ordinates. The theoretical frequency curve is a derivative of the probability curve.

friction head

The decrease in total head caused by friction.