56. Few studies appear to have been conducted on breaker vortices, 

 although the investigations discussed below indicate that vortices contribute 

 to bar formation, sediment transport, and wave height decay. Two studies are 

 discussed in the following paragraphs. The first study involved wave height 

 decay, and the second study concerned breakpoint bar formation. 



Review 



57. Sawaragi and Iwata (1974) conducted experiments in a wave tank on a 

 composite slope consisting of a 1/18 foreslope leading to a horizontal sec- 

 tion. They examined wave deformation shoreward of the break point and esti- 

 mated that 15 to 30 percent of the energy of the plunging waves was converted 

 to kinematic energy of the subsurface vortex. 



58. Miller (1976) investigated breaker vortices in a series of tests 

 with a tilting wave tank containing a sand- filled bottom. He observed that 

 vortices created by plunging waves might extend from the surface to the 

 bottom, whereas vortices generated by spilling waves were smaller and confined 

 to the region near the surface . The results indicated that bars were formed 

 in the presence of large vortices generated by plunging waves, but bars tended 

 to be eliminated when subjected to spilling waves. Miller commented that the 

 simplified set of wave tank results did not justify immediate extrapolation to 

 the field, but suggested that this was a promising area for further study. 

 Summary 



59. Sawaragi and Iwata (1974) and Miller (1976) discuss implications of 

 breaker vortices on wave dissipation and sediment movement. Since vortex area 

 cannot be measured easily in the field, the present study attempts to relate 

 vortex area to properties that can be measured easily, such as wave height, 

 period, and local bottom shape. 



Wave Height Decay 



60. A broken wave dissipates energy as it progresses shoreward. The 

 wave may remain turbulent to the beach or become stable and reform. A 

 reformed wave will also shoal and eventually break. Wave height decay is 

 needed to calculate radiation stresses, which drive currents, entrain sedi- 

 ment, and create setup in the surf zone. The broken wave height is also 

 needed to design structures to be located in the surf zone. Several models 

 have been developed to predict wave height decay, and some of these are 



35 



