DIFFRACTION (of water waves). The phenomenon by which energy is transmitted 

 laterally along a wave crest. When a part of a train of waves is inter- 

 rupted by a barrier, such as a breakwater, the effect of diffraction is 

 manifested by propagation of waves into the sheltered region within the 

 barrier's geometric shadow. 



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

 flooding. 



DIURNAL. Having a period or cycle of approximately one TIDAL DAY. 



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

 (See Figure A- 10.) 



DIVERGENCE. (1) In refraction phenomena, the increasing of distance between 

 orthogonals in the direction of wave travel. Denotes an area of 

 decreasing wave height and energy concentration. (2) In wind-setup 

 phenomena, the decrease in setup observed under that which would occur in 

 an equivalent rectangular basin of uniform depth, caused by changes in 

 planform or depth. Also the increase in basin width or depth causing such 

 decrease in setup. 



DOLPHIN. A cluster of piles. 



DOWNCOAST. In United States usage, the coastal direction generally trending 

 toward the south. 



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) Floating material deposited on a beach 

 (driftwood). (4) A deposit of a continental ice sheet; e.g., a drumlin. 



DRIFT CURRENT. A broad, shallow, slow-moving ocean or lake current. 



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

 (See Figure A-7.) (2) BED FORMS smaller than bars but larger than ripples 

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

 them. 



DURATION. In wave forecasting, the length of time the wind blows in nearly 

 the same direction over the FETCH (generating area). 



DURATION, MINIMUM. The time necessary for steady-state wave conditions to 

 develop for a given wind velocity over a given fetch length. 



EAGER. See BORE. 



EBB CURRENT. The tidal current away from shore or down a tidal stream; 

 usually associated with the decrease in height of the tide. 



EBB TIDE. The period of tide between high water and the succeeding low water; 

 a falling tide. (See Figure A-10.) 



A-11 



