GROIN (British, GROYNE). A shore protection structure built (usually 

 perpendicular to the shoreline) to trap littoral drift or retard erosion 

 of the shore. 



GROIN SYSTEM. A series of groins acting together to protect a section of 

 beach. Commonly called a groin field. 



GROUND SWELL. A long high ocean swell; also, this swell as it rises to 

 prominent height in shallow water. 



GROUND WATER. Subsurface water occupying the zone of saturation. In a strict 

 sense, the term is applied only to water below the WATER TABLE. 



GROUP VELOCITY. The velocity of a wave group. In deep water, it is equal to 

 one-half the velocity of the individual waves within the group. 



GULF. A large embayment in a coast; the entrance is generally wider than the 

 length. 



GUT. (1) A narrow passage such as a strait or inlet. (2) A channel in 

 otherwise shallower water, generally formed by water in motion. 



HALF-TIDE LEVEL. MEAN TIDE LEVEL. 



HARBOR (British, HARBOUR). Any protected water area affording a place of 

 safety for vessels. See also PORT. 



HARBOR OSCILLATION (HARBOR SURGING). The nontidal vertical water movement in 

 a harbor or bay. Usually the vertical motions are low; but when oscilla- 

 tions are excited by a tsunami or storm surge, they may be quite large. 

 Variable winds, air oscillations, or surf beat also may cause oscilla- 

 tions. See SEICHE. 



HEADLAND (HEAD). A high, steep-faced promontory extending into the sea. 



HEAD OF RIP. The part of a rip current that has widened out seaward of the 

 breakers. See also CURRENT, RIP; CURRENT, FEEDER; and NECK (RIP). 



HEIGHT OF WAVE. See WAVE HEIGHT. 



HIGH TIDE, HIGH WATER (HW) . The maximum elevation reached by each rising 

 tide. See TIDE. (See Figure A- 10.) 



HIGH WATER. See HIGH TIDE. 



HIGH WATER LINE. In strictness, the intersection of the plane of mean high 

 water with the shore. The shoreline delineated on the nautical charts of 

 the National Ocean Service is an approximation of the high water line. 

 For specific occurrences, the highest elevation on the shore reached 

 during a storm or rising tide, including meteorological effects. 



HIGH WATER OF ORDINARY SPRING TIDES (HWOST). A tidal datum appearing in some 

 British publications, based on high water of ordinary spring tides. 



A- 15 



