39 



waves than would be the case if the bar were absent. It is beheved 

 that the longer breaker distance signifies a greater dissipation of 

 energy over the sand ripples as a result of increased turbulence. In 

 such cases the rotary motions of the central curl and plunging curl 

 are locally damped. Maximum currents over the bar are associated 

 with loss of energy in the boundary layers over the sand. 



The currents naturally affect the movement of sand in the bar 

 environment. When the water depth over the crest is greatest the 

 strong currents directed toward the shore move the layers of sand 

 from that part of the seaside slope of the bar surface and the crest 

 area into the trough of the bar. Subsequently, when the plunging 

 curl is falling in the bar trough, the rotating water picks up sand from 

 the trough and suspends it in the main body of water. At the instant 

 that the depth of water over the crest is minimum, the seaward ciurent 

 transports the suspended sand particles and at the same time induces 

 a movement of sand over the bar surface toward the seaside foot of 

 the bar. In fact, the cyclic movement of sand over the bar is quite 

 similar to that over a sand ripple, except that the advance of the main 

 body of the bar is imperceptible. In the case of bars, the sand trans- 

 ported to the seaside foot of the bar by sand ripples is collected at the 

 shore side of the bar by the recovering and reforming waves and is 

 transported to the shore. 



The formation of the main body of the bar is the final step in the 

 sorting of sand. The coarser sand reaches the shore through the 

 cyclic creeping movement on the surface of the sand ripples and the 

 bar; the finer particles return to the sea through the turbulent action 

 of the eddies of the sand ripples and the bar. This interpretation is 

 based on incomplete data and a detailed investigation on this particular 

 subject as an independent problem is necessary. 



Section VII. ACKNOWLEDGMENT 



The author acknowledges gratefully the numerous suggestions 

 made by Dr. M. A. Mason, Chief, Engineering and Research 

 Branch of the Beach Erosion Board, during the conduct of the tests. 

 The importance and significance of bar structures were brought to the 

 author's attention by Prof. W. W. Williams of Cambridge University, 

 and his general discussions of different problems relating to the 

 subject have been very valuable. The work of Messrs. W. H. Vesper 

 and D. G. Dumm, who operated the models and assisted in the com- 

 putations, was invaluable. 



