DECAY OF WAVES - The change that 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 wave length increases, 



DEEP Vi[ATER - Water of depth such that surface waves are little 



affected by conditions on the ocean bottom. It is customary to 

 consider water deeper than one-half the surface wave length as 

 deep water. 



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

 by wind action, 



DELTA - An alluvial deposit, usually triangular, at the mouth of a 

 river. 



DEPTH - The vertical distance from the still water level (or datum 

 as specified) to the bottom. 



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

 breaks. Also BREAKER DEPTH. 



DEPTH CONTOUR - See CONTOUR. 



DEPTH, CONTROLLING - The least depth of water in the navigable parts 

 of a waterway, which limits the allowable draft of vessels, 



DEPTH FACTOR -See SHOALING COEFFICIENT. 



DERRICK STONE - Stone of a sufficient size as to req\rire handling in 

 individual pieces by mechanical means, generally 1 ton up, 



DIFFRACTION OF WATER WAVES - The phenomenon by which energy is trans- 

 mitted laterally along a wave crest. When a portion of a train 

 of xiraves is interrupted by a barrier such as a breakwater, the 

 effect of diffraction is manifested by propagation of waves into 

 the sheltered region iri.thin the barrier's geometric shadoxj, 



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

 flooding, 



DIURNAL - Daily, recurring once each day, (e.g. lunar day or solar 

 day) . 



DIURNAL TIDE - A tide with one high water and one low water in a tidal 

 day. (See Figure A-IO) 



DIVERGENCE - (l) In refraction phenomena, the spreading of orthogonals 

 in the direction of wave travel. This denotes an area of de- 

 creasing wave height and energy concentration; (2) In wind set- 

 up phenomena, the decrease in set-up observed under that which 



A-IO 



