AMPLITUDE, WAVE. (1) The magnitude of the displacement of a wave from a mean 

 value. An ocean wave has an amplitude equal to the vertical distance from 

 still-water level to wave crest. For a sinusoidal wave, the amplitude is 

 one-half the wave height. (2) The semirange of a constituent tide. 



ANTIDUNES. BED FORMS that occur in trains and are in phase with, and strongly 

 interact with, gravity water-surface waves. 



ANTINODE. See LOOP. 



ARMOR UNIT. A relatively large quarrystone or concrete shape that is selected 

 to fit specified geometric characteristics and density. It is usually of 

 nearly uniform size and usually large enough to require individual 

 placement. In normal cases it is used as primary wave protection and is 

 placed in thicknesses of at least two units. 



ARTIFICIAL NOURISHMENT. The process of replenishing a beach with material 

 (usually sand) obtained from another location. 



ATOLL. A ring-shaped coral reef, often carrying low sand islands, enclosing a 

 lagoon. 



ATTENUATION. (1) A lessening of the amplitude of a wave with distance from 

 the origin. (2) The decrease of water-particle motion with increasing 

 depth. Particle motion resulting from surface oscillatory waves 

 attenuates rapidly with depth, and practically disappears at a depth equal 

 to a surface wavelength. 



AWASH. Situated so that the top is intermittently washed by waves or tidal 

 action. Condition of being exposed or just bare at any stage of the tide 

 between high water and chart datum. 



BACKBEACH. See BACKSHORE. 



BACKRUSH. The seaward return of the water following the uprush of the 

 waves. For any given tide stage the point of farthest return seaward of 

 the backrush is known as the LIMIT of BACKRUSH or LIMIT BACKWASH. (See 

 Figure A-2.) 



BACKSHORE. That zone of the shore or beach lying between the foreshore and 

 the coastline comprising the BERM or BERMS and acted upon by waves only 

 during severe storms, especially when combined with exceptionally high 

 water. Also BACKBEACH. (See Figure A-1.) 



BACKWASH. (1) See BACKRUSH. (2) Water or waves thrown back by an 

 obstruction such as a ship, breakwater, or cliff. 



BANK. (1) The rising ground bordering a lake, river, or sea; or of a river or 

 channel, for which it is designated as right or left as the observer is 

 facing downstream. (2) An elevation of the sea floor or large area, 

 located on a continental (or island) shelf and over which the depth is 

 relatively shallow but sufficient for safe surface navigation; a group of 

 shoals. (3) In its secondary sense, used only with a qualifying word such 

 as "sandbank" or "gravelbank," a shallow area consisting of shifting forms 

 of silt, sand, mud, and gravel. 



A-2 



