38 



manner. Initially, in the absence of the bar, a certain distance exists 

 between the points where the wave begins to break and the breaker is 

 completed; this distance may be referred to as the breaker distance 

 and is denoted by s in figm-e 1. On flat beaches the breaker initially 

 is of the spilling type. As the bar is formed it changes into the plung- 

 ing type and the distance, s, increases. The magnitude of s is a func- 

 tion of the slope of the beach and of the depth of water at the point 

 of impending wave break. The functional dependence may be 

 expressed as 



sJH,=j{aol\„ i) (21) 



or 



V^i=/(^'^) 



which probably assumes a different form when no bar is presents 

 When bars are present, it is found that, to a rough approximation 

 s/Hi=5, 8, 9, and 11 when ^= 1/15, 1/30, 1/50, and 1/70, respectively. 

 It is the interpretation of the writer that the two changes mentioned 

 above have a bearing on the energy distribution in the bar environ- 

 ment. This may be explained by reference to the two curves in figure^ 

 4 which represent the maximum and minimum elevation of the water 

 surface with reference to the still water level during the passage of 

 waves. In a sense these curves represent the paths of travel of the 

 crests and troughs. In tracing the travel path of the crests, it is- 

 seen that the crest def<)rms rapidly and develops a central curl some-^ 

 where between the point where the wave starts to break and the 

 plunge point. The position of the fuUy developed curl is shown in 

 figure 14. The curl, or rotating body of water, causes a greater- 

 internal dissipation of energy than otherwise would be possible. 

 After the appearance of the central curl when the water elevation is 

 maximum at the bar crest, a strong current moves parallel to the 

 seaward slope of the bar over the crest and is directed shoreward. 

 The current in passing over the crest develops a curl and falls into 

 the trough of the bar. At the instant of the fall of the curl its linear 

 momentum is imparted to the waters ahead and the rotary momen- 

 tum is consumed locally. The impact of the ciu-l reforms the wave. 

 In following the path of the wave troughs seaward it is seen that the 

 current attains a maximum at the crest of the bar. The trough path 

 at this point is a concave surface just above the seaside face of the bar, 

 as shown in figure 14; here the hydraulic gradient is quite steep. This 

 together with the fact that depths are small indicate the existence of a 

 strong seaward current. The net result is that in all probability the 

 main role of the bar is to reduce the energy of the incoming waves 

 causing them to impart a lesser amount of energy to the reforming: 



