EM 1110-2-1100 (Change 1)
31 Jul 03
D
DAM
Structure built in rivers or estuaries, basically to separate water at both sides and/or to retain water at one side.
DATUM
Any permanent line, plane or surface used as a reference datum to which elevations are referred.
DATUM, CHART
See CHART DATUM.
DATUM, PLANE
The horizontal plane to which soundings, ground elevations, or water surface elevations are referred. Also
REFERENCE PLANE. The plane is called a TIDAL DATUM when defined by a certain phase of the tide. The following
datums are ordinarily used on hydrographic charts:
MEAN LOW WATER--Atlantic coast (U. S.), Argentina, Sweden, and Norway.
MEAN LOWER LOW WATER--Pacific coast (U. S.).
MEAN LOW WATER SPRINGS--United Kingdom, Germany, Italy, Brazil, and Chile.
LOW WATER DATUM--Great Lakes (U. S. and Canada).
LOWEST LOW WATER SPRINGS--Portugal.
LOW WATER INDIAN SPRINGS--India and Japan (See INDIAN TIDE PLANE).
LOWEST LOW WATER--France, Spain, and Greece.
A common datum used on United States topographic maps is MEAN SEA LEVEL. See also BENCH MARK.
DAVIDSON CURRENT
Deep-ocean boundary current off the west coast of the U.S. which brings warmer, saltier, low oxygen, high
phosphate equatorial type water from low to high latitudes.
DEBRIS LINE
A line near the limit of storm wave uprush marking the landward limit of debris deposits.
DECAY AREA
Area of relative CALM through which waves travel after emerging from the generating area.
DECAY DISTANCE
The distance waves travel after leaving the generating area (FETCH).
DECAY OF WAVES
The change waves undergo after they leave a generating area (FETCH) and pass through a calm, or region of lighter
winds. In the process of decay, the significant wave height decreases and the significant wavelength increases.
DEEP WATER
Water so deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the
surface wavelength is considered deep water. Compare SHALLOW WATER.
Appendix A Glossary of Coastal Terminology
A-21
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