UNDERTOVJ - A currentj below water surface, flowing "cawardj also thi 



receding water belou the surface from v.aves oreaking cij a shcO-Ving 

 beach. Actually "undertow" is largely mythical. As '''^e backwash 

 of each x.iave flows down the beach, a current is formed viiich flows 

 seaward, however, it is a periodic phenomenon. The most common 

 . phenomena expressed as "undertow" are actually the rip currents in 

 the' surf o 



UNDERWATER GRADIENT - The slope of the sea bottom. See also SLOPE. 



UNDULATION - A continuously propagated motion to and fro, in any fluid 

 or elastic medium, with no permanent translation cf the particles 

 themselves. 



UPGOAST - In United States usage; the coastal direction generally trend- 

 ing towards the north. 



UPDRIFT - The direction opposite that of the predominant movement of littoral 

 materials o 



UPLIFT - The upward water pressure on the base of a structure or pavement,, 



UPRUSH - T^ rush of water up onto the beach following the breaking of a 

 wave. Also SWASH, RUN-UP. (See Figure A-2) . 



VALLEY, SEA - A submarine depression of broad valley form without the 

 steep side slopes which characterize a canyon. 



VALLEY, SUBl^'iARINE - A prolongation of a land valley into or across the 

 continental or insular shelf, which generally gives evidence of 

 having been formed by stream 'erosion. 



VARIABILITY OF WAVES - (l) The variation of heights and periods between 

 individual waves within a wave train. (Viave trains are not com- 

 posed of waves of equal height and period , but rather of heights 

 and periods which vary in a statistical manner). (2) The variatior 

 in direction of propagation of waves leaving the generating area. 

 (3) The variation in height along the crest. This is usually called 

 "variation along the wave". 



VELOCITY OF WAVES - The speed with which an individual wave advances, 



VERY SHALLOW WATER - See SHALLOW WATER. 



VISCOSITY - Internal friction due to molecular cohesion in fluids. The 

 internal properties of a fluid which offer resistancej to flow. 



WATER LINE - A juncture of land and sea. This line migrates, changing 

 with the tide or other fluctuation in the water level » Where waves 

 are present on the beach, this line is also known as the limit of 

 backrush. (Approximately the intersection of the land with the 

 still water level). 



A-37 



