CURRENT^ TIDAL -• A current, caused by the tide-producing forces of 

 the moon and the sun^ which is a part of the same general move- 

 ment of the sea manifested in the vertical rise and fall of the 

 tides c Also CURRENT^ PERIODIC „ See also CURRENT, FLOOD AND 

 CURRENT, EBBo 



CUSP - One of a series of naturally formed low mounds of beach material 

 separated by crescent-shaped troughs spaced at more or less regu- 

 lar intervals along the beach faceo Also BEACH CUSP„ (See Figure A-7) 



CUSPATE BAR - A crescent-shaped bar uniting with shore at each end„ 

 It may be formed by a single spit growing from shore turning back 

 to again meet the shore, or by ti'^ro spits growing from shore unit- 

 ing to form a bar of sharply cuspate form, 



CYCLOIDAL WAVE - A very steep^ symmetrical wave whose crest forms an 

 angle of 120° o The wave form is that of a cycloid„ A trochoidal 

 wave of maximum steepness. See also WAVE, TROCHOIDALo 



DAILY RETARDATION (OF TIDES) - The amount of time by which corresponding 

 tidal fhases grow later day by day (averages approximately 50 minutes), 



DATUM, CHART - See CHART DATUM » 



DATUM PLANE - The horizontal plane to which soundings, ground eleva- 

 tions, or water surface elevations are referred. Also REFERENCE 

 PLANE. The plane is called a TIDAL DATUI4 when defined by a 

 certain phase of the tide. The following datums are ordinarily 

 used on hydrographic charts; 



MEAN LOW WATER - Atlantic Coast (UoS„)^ Argentina, Sweden, 



and Norway? 

 MEAI^ LOWER LOW WATER - Pacific Coast (U<,S,)| 

 MEAN LOW WATER SPRINGS - Great Britain, Germany, Italy^ 

 Brazil, and Chile* 



LOW WATER DATUM - Great Lakes (U„Sc and Canada) | 



LOWEST LOW WATER SPRINGS - Portugal; 



LOW WATER INDIAN SPRINGS - India and Japan (See INDIAN TIDE PUNE) 



LOWEST LOW WATER - France., Spain^ and Greece o 

 A common datum used on topographic maps is based upon MEAN SEA 

 LEVEL. See also BENCH MARK. 



DEBRIS LINE - A line near the limit of storm x^rave uprush marking the 

 seaward limit of debris deposits, 



DECAY DISTANCE - The distance through which waves travel after leaving 

 the generating areao 



A-9 



