Shallow Water Wave Transformation through External Factors 137 



is smaller than it was previously. Only in this case, the noise of the surf 

 is sharp. 



It was expected that, by experimenting in the wave tank, the process could 

 be controlled in such a manner that each experiment could be repeated in 



Fig. 64. Several possibilities in the behaviour of the breakers on a vertical barrier (Bagnold). 



the same way at any time, thus permitting an exact analysis of the phenomenon, 

 especially of the maximum pressure. Although the greatest precautions were 

 taken, this could not be attained. With external circumstances unchanged, 

 and one wave being quite similar to the other, the maximum shock-pressure 

 turned out to be different in each case, and it seems that the smallest, hardly 

 noticeable changes in the wave itself and in the outside factors cause varia- 

 tions in the shock. The development of small air cushions has also a con- 

 siderable influence and much depends upon whether one single air cushion 

 is formed or whether, as shown in Fig. 65, it is split up into several small 

 cushions. The greatest shock-pressure that could be observed for a normal 

 wave of 25-4 cm high, and a velocity of 1-80-2-40 m/sec was, according 

 to visual observations, 5-6 kg/cm 2 . The tests showed that the shock-pressure 

 became very small when the thickness of the enclosed air cushion exceeded 

 half its height and that its intensity increased when this thickness decreased. 



