138 Shallow Water Wave Transformation through External Factors 



Despite the great diversity in maximum pressures, the area enclosed by the 

 time-pressure curve remained constant. The higher the maximum pressure, the 

 smaller its duration will be. Figure 66 gives a graphic presentation of a time- 



Fig. 65. Normal shape of an imbedded air pocket and its disintegration into several parts 



Oi 



02 



03 

 sec 



0-4 



05 



Fig. 66. Left: normal curve of shock pressure in Bagnold's experiments. Right: recordings 

 of pressure at the vertical breakwater at Dieppe. 



pressure curve, showing how rapidly the pressure increases and decreases in 

 shock-surf; besides, there are two recordings of shock-pressures of a surf 

 wave against the breakwater of Dieppe. We can see that the order of 

 magnitude of the shock-pressures of the waves is the same in the tank as 

 in nature. The maximum pressure obtained in the test was 5-6 kg/cm 2 , as 

 mentioned above. The linear dimensions in nature are increased in the 

 proportion 12:1, and one could expect here maximum pressures of 70 kg/cm 2 . 

 However, the maximum pressure found in Dieppe was only 6-9 kg/cm 2 and 

 the values, in all other respects, remain of the same order of magnitude as 

 the laboratory tests. Bagnold is of the opinion that this striking fact must 

 be ascribed to the considerable difference existing between sea water and 

 fresh water used in laboratory tests. The formation of foam which occurs 



