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 topographic maps is based on MEAN SEA LEVEL. See 

 also BENCH MARK. 



DEBRIS LINE. A line near the limit of storm wave uprush marking the landward 

 limit of debris deposits. 



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 signi- 

 ficant 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. 



DEFLATION. The removal of loose material from a beach or other land surface 

 by wind action. 



DELTA. An alluvial deposit, roughly triangular or digitate in shape, formed 

 at a river mouth. 



DEPTH. The vertical distance from a specified tidal datum to the sea floor. 



DEPTH OF BEIEAKING. The still-water depth at the point where the wave 

 breaks. Also BREAKER DEPTH. (See Figure A-2.) 



DEPTH CONTOUR. See CONTOUR. 



DEPTH, CONTROLLING. See CONTROLLING DEPTH. 



DEPTH FACTOR. See SHOALING COEFFICIENT. 



DERRICK STONE. See STONE, DERRICK. 



DESIGN HURRICANE. See HYPOTHETICAL HURRICANE. 



A- 10 



