Figure 13. Growth and movement of breakpoint bar with elapsed time 

 and location of break point 



Average breaker locations are indicated in the figure for the profiles where 

 such information was available. 



237. Another mechanism for bar formation is long-period wave motion 

 generated, for example, by reflection from the beach (Bowen 1980). Standing 

 waves as a possible mechanism for bar formation have been investigated by 

 Carter, Liu, and Mei (1973), Lau and Travis (1973), Short (1975a, b) , and Mei 

 (1985). An oscillating velocity field induces a steady mean current in the 

 boundary layer close to the bed. If the oscillations are produced by purely 

 progressive waves , the mean drift in the boundary layer will always be in the 

 direction of the propagating waves. Partial reflection of the incident wave 

 may cause the direction of mass transport in the lower part of the boundary 

 layer to reverse if reflection is sufficiently large. (Theoretically the 

 reflection coefficient should exceed 0.414, according to Carter, Liu, and Mei 

 1973.) A complete standing wave induces mass transport toward the nodes in 

 the lower part of the boundary layer and toward the antinodes in the upper 

 part of the boundary layer. Depending on the height to which the grains are 



80 



