GEOMETRIC MEAN DIAIffiTER - The diameter equivalent of the arithmetic 



mean of the logarithmic frequency distribution » In the analysis 

 of beach sands it is taken as that grain diameter determined 

 graphically by the intersection of a straight line through 

 selected boundary sizes (generally points on the distribution 

 curve where l6 and 8U percent of the sample by weight is coarser) 

 and a vertical line through the median diameter of the sample, 



GEOMETRIC SHADOW - In wave diffraction theory, the area outlined by 



drawing straight lines paralleling the direction of wave approach 

 through the extremities of the protective structure. It differs 

 from the actual protected area to the extent that the diffraction 

 and refraction effects modify the wave pattern, 



GEOMGRPHOLOGY - That branch of both physiography and geology which deals 

 with the form of the earth, the general configuration of its sur- 

 face, and the changes that take place in the evolution of land 

 forms , 



GRADIENT (GRADE) - See SLOHS, With reference to winds or current, the 

 rate of increase or decrease in speed, usually in the vertical, 

 or the curve which represents this rate, 



GRAVEL - See SOIL CLASSIFICATION, 



GRAVITY VJAVE - A wave whose velocity of propagation is controlled 



primarily by gravity. Water waves of a length greater than 2 inches 

 are considered gravity waves, 



GROIN (BRIT, GROYNE) - A shore protective structure (built usually per- 

 pendicular to the shore line) to trap littoral drift or retard 

 erosion of the shore. It is narrow in width (measured parallel to 

 the shore line), and its length may vary from less than one hundred 

 to several hundred feet (extending from a point landward of the 

 shore line out into the water) , Groins may be classified as per- 

 meable or impermeablej impermeable groins having a solid or 

 nearly solid structure, permeable groins having openings through 

 them of sufficient size to permit passage of appreciable quantities 

 of littoral drift, 



GROUND SWELL - A long high ocean swell. Also this swell as it rises to 

 prominent height in shallow water. Not usually so high or dangerous 



as BLIND ROLLERS o 



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 at which a wave group travels. In deep 

 water, it is equal to one -half the velocity of the individual 

 waves within the group, 



A-lU 



