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Flood zone

The land bordering a stream which is subject to floods of about equal frequency; for example, a strip of the flood plain subject to flooding more often that once but not as frequently as twice in a century.

Flood, maximum probable

The largest flood for which there is any reasonable expectancy in this climatic era.

Flood-control storage

Storage of water in reservoirs to abate flood damage. (See Retarding reservoir.)

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.

Floods above a base

See Partial-duration flood series

Floodway

1. 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. 2. 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.

Flow-duration curve

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

Fluidized bed combustion (FBC)

Process for burning coal more efficiently, cleanly, and cheaply. A stream of hot air is used to suspend a mixture of powdered coal and limestone during combustion. About 90 to 98 percent of the sulfur dioxide produced during combustion is removed by reaction with limestone to produce solid calcium sulfate.

Fluorocarbons

Carbon-fluorine compounds that often contain other elements such as hydrogen, chlorine, or bromine. Common fluorocarbons include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). See chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons, ozone depleting substance.

focus

The initial point within the Earth that ruptures in an earthquake, directly below the epicenter.

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