come much closer together in nature, considering the difference in scale 

 between model and prototype. 



EFFECT OF SECONDARY CURRENT ON SEDIMENT TRANSPORT 



The modern theory on mass transport in water waves indicates that 

 there exists a shoreward movement along the bottom and a movement at the 

 surface, which may move either towards the sea or towards the- land. The 

 direction of surface motion and the relative strength between the surface 

 and bottom movement depend on the parameter d/L, where d is the water 

 depth and L is the wave length. This implies that at the front of a 

 littoral barrier, there exists a secondary flow which moves either in the 

 clockwise direction, where the water particles move inward along the 

 bottom, upward at the front of the barrier, and then outward at the 

 surface as observed in this study, or in the counter-clockwise direction. 

 When the roller moves in the clockwise direction there will be a slow up- 

 ward movement of a large mass of water in the neighborhood of the barrier 

 which is helpful in keeping sediment in suspension, in addition to the 

 vertical fluctuating velocity of the turbulent flow. On the other hand, 

 the clockwise motion of the secondary flow also keeps some of the turbulence 

 generated at the cliff away from the beach, thereby reducing the strength 

 of the turbulence prevailing at the beach surface. The reverse is true 

 for the case of a counter-clockwise roller. The net effect of the 

 secondary flow on the sediment motion therefore depends upon the relative 

 strength of the turbulent flow and the secondary flow. No definite con- 

 clusion to this effect can be drawn at this moment. 



SUMMARY 



From the experimental results of this investigation and a qualitative 

 evaluation of the scale effect on the model study, the mechanics of sedi- 

 ment motion in the vicinity of a littoral barrier can be visualized as 

 follows: The upcoast littoral current, and in some cases the shoreward 

 motion along the bottom, bring the sediment toward the littoral barrier. 

 In the neighborhood of the barrier, the high local turbulence created by 

 and diffused from the barrier keeps the sediment in suspension. The 

 suspended sediment is finally carried downcoast by the current along the 

 barrier, due to the continuity of the flow. This description is based 

 upon the deduction that in nature the turbulence at the front of the 

 barrier is very much stronger than what can be reproduced in the labora- 

 tory scale. 



ACKNOWLEDGMENT 



This work represents research carried out for the Beach Erosion Board 

 under contract with the University of California. The study was con- 

 ducted under the guidance of Professor H. A. Einstein. For his most 

 valuable advice and criticism, the writer wishes to express his apprecia- 

 tion. Grateful acknowledgment is made to Shang-Wen Yuan, graduate student 

 at the University of California for his capable assistance in the experi- 

 mental phase of this study. 



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