current and it will react a complex way with wind-generated bottom currents 

 and wave-drift currents. Thus^ it is possible to conceive of a situation 

 in which the angle of wave approach, the direction and velocity of the 

 wind, and the direction and velocity of the tidal current at a given time 

 will interact to reinforce or -.impede one another. A wind moving opposite 

 in direction to a tidal current, for example, will produce an increase in 

 the tidal- current velocity near the bottom (Reid, 1957) smd, should the 

 wave fronts be traveling in the direction of flow of the tidal current, the 

 net current velocity coTold have a velocity up to about l6 percent (cf . 

 Collins, 196^+) greater than the simp±e algebraic summation of the wave- 

 induced c-urrent and the reinforcing tidal cirrrent. As noted by Collins 

 (196U, p. 1051), "the effects of even very small currents on the mass 

 transport £~of fine material_7' by waves could be very large." D. R. Tuck 

 (oral communication) has studied the interrelationships of wind velocity 

 and direction, angle of wave approach, and tidal- current velocity on the 

 average mean- grain- size values for the beach slope in question. In instances 

 where the wave-drift current was reinforced by the tidal current, water 

 velocities of 21-29 cm/sec were' obtained near the bed. Mean particle sizes 

 actually observed at the bed under these circustances, and for the slopes 

 in effect at the time, were within the range of particle sizes predicted by 

 theory -as being moved by the net current. 



_ Thus, there is every reason to believe that a combination such as 

 Sg, T, Hq/Lq, Up, a, and C, mentioned at the outset as being the most- 

 influential combination of six variables in the most -influential lag period, 

 is in fact a valid combination for this area of the beach at Virginia Beach. 



Table B5^ shows that the most-_influential combination (Xl ignored 

 for 6 Xs at a time) includes: T, Lq, Up, a, h, and p. Variables XIO and 

 Xll take the place of variables X5 and X12 of the analysis in which XI was 

 included. Water depth merely acts to mediate the various process elements, 

 while water density will affect fluid drag at the bed. As seen in all of 

 tables B52-B56, water density, Xll, is a dominant variable in the strong- 

 est- combinations. 



Of further and more general significance is the rather persistent 

 contribution of variables X6, TJ , X8, and Xll and X12 in the strongest 

 combinations of Xs taken six at a time (tables B52-B56) . Wind velocities 

 onshore (x6) and offshore (X7) have also been seen to be of importance in 

 net erosion on the lower foreshore. In that section of the study it was 

 postulated that when the offshore wind reaches a certain velocity it is 

 important in producing a weak surface current beyond the breaker zone that 

 is capable of transporting fine particles put into suspension in the break- 

 eris out into the shoaling-wave zone. The onshore wind, however, may pro- 

 duce a seaward return flow of water on the bottom that will transport 

 somewhat coarser particles out of the breaker zone and onto the slope in 

 the shoaling-wave zone. Winds parallel to shore will probably interact 

 significantly with tidal currents, which are generally parallel to the 

 shoreline in the area of investigation, and eugment or decrease the current 

 velocities in the lower layers. 



50 



