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 hydro- 

 graphic 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; 

 Lowest Low Water - France, Spain, and Greece. 



A common datum used on topographic maps is based on Mean Sea Level. 



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. 



DIKE (DYKE) - A wall or mound built around a low-lying area to prevent 

 flooding. 



DOWNDRIFT - The direction of predominant movement of littoral materials. 



DRIFT (noun) - (1) Sometimes used as a short form for Littoral Drift. 



(2) The speed at which a current runs. (3) Also floating material 

 deposited on a beach (driftwood). (4) A deposit of a continental 

 ice sheet, as a drumlin. 



DUNES - (1) Ridges or mounds of loose, wind-blown material, usually 



sand. (2) Bed Forms smaller than bars but larger than ripples that 

 are out of phase with any water-surface gravity waves associated 

 with them. 



EROSION - The wearing away of land by the action of natural forces. On 

 a beach, the carrying away of beach material by wave action, tidal 

 currents, littoral currents, or by deflation. 



FETCH - The area in which SEAS are generated by a wind having a rather 

 constant direction and speed. Sometimes used synonymously with 

 Fetch Length. Also Generating Area. 



FOREDUNE - The front dune immediately behind the backshore. 



FORESHORE - The part of the shore lying between the crest of the seaward 

 berm (or upper limit of wave wash at high tide) and the ordinary low 

 water mark, that is ordinarily traversed by the uprush and backrush 

 of the waves as the tides rise and fall. 



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