GRAVITY WAVE - A wave whose velocity of propagation is controlled pri- 

 marily by gravity. Water waves more than 2 inches long are con- 

 sidered gravity waves. Waves longer than 1 inch and shorter than 

 2 inches are in an indeterminate zone between CAPILLARY and GRAVITY 

 WAVES. See RIPPLE. 



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 move- 

 ment in a harbor or bay. Usually the vertical motions are low, but 

 when oscillations are excited by a tsunami or storm surge, they may 

 be quite large. Variable winds, air oscillations, or surf beat also 

 may cause oscillations. 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.) 



A-15 



