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The minimum height reached by a falling tide. The height may be due solely to the periodic tidal forces or it may have superimposed upon it the effects of meteorological conditions.

Related Terms

HIGH WATER STAND

The condition at high water when there is no sensible change in the height of the water. A similar condition at low water is called LOW WATER STAND

GULDER

Local name given to double low water occurring on the south coast of England

INSULAR SHELF

A zone around an island that extends from the low water line to a depth at which there is usually a marked increase of slope towards oceanic depths.

LAGGING OF TIDE

The periodic retardation in the time of occurrence of high and low water due to changes in the relative positions of the moon and the sun.

LOW WATER EQUINOCTIAL SPRINGS

Low water spring tides near the times of the equinoxes. Expressed in terms of the harmonic constituents, it is an elevation depressed below mean sea level by an amount equal to the sum of the amplitudes of certain constituents as given in the Tide and Current Glossary published by the National Ocean Survey.

LOWER LOW WATER DATUM

An approximation of mean lower low water that has been adopted as a standard reference for a limited area, and is retained for an indefinite period regardless of the fact that it may differ slightly from a better determination of mean lower low water from a subsequent series of observations. Used primarily for river and harbor engineering purposes. Columbia River lower low water datum is an example.

NEAP TIDES

Tides of decreased range occurring semimonthly as the result of the moon being in quadrature. The neap range of the tide is the average semidiurnal range occurring at the time of neap tides and is most conveniently computed from the harmonic constants. It is smaller than the mean range where the type of tide is either semidiurnal or mixed and is of no practical significance where the type of tide is diurnal. The average height of the high waters of the neap tides is called neap high water or high water neaps and the average height of the corresponding low waters is called neap low water or low water neaps.

BEACH

The zone of unconsolidated material that extends landward from the low water line to the place where there is a marked change in material or physiographic form, or to the line of permanent veg- etation (usually the effective limit of storm waves). A beach includes foreshore and backshore. The beach along the margin of the sea may be called SEABEACH. Also called STRAND, espe- cially when the beach is composed of sand. See also TIDELAND.

BARE ROCK

A rock that extends above the mean high water datum in tidal areas or above the low water datum in the Great Lakes

CHART SOUNDING DATUM

The tidal datum to which soundings and drying heights on a chart are referred. It is usually taken to correspond to a low water stage of the tide. Often shortened to CHART DATUM, especially when it is clear that reference is not being made to a geodetic datum.

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