136 Shallow Water Wave Transformation through External Factors 



This force is able to displace horizontally and lift considerable water masses simultaneously. 

 With a velocity u, a water mass can climb a height u 2 /2g. for instance with 10 and 14 m/sec res- 

 pectively the height can be 5 to 10 m. Therefore, it is not surprising that breakers can cause enor- 

 mous destructions. KrOmmel (1911, vol. II, p. 118) has given a compilation of such surf break- 

 ers with a powerful visible effect. 



Table 18. Pressures of wave impacts for different sizes of waves 



(According to Gaillard) 



Bagnold (1939), of the College of Science and Technology in London, 

 threw a new light on this process of surf breaking. These experiments were 

 made in a wave tank approximately 1 1 m long, 1 h m high and i m wide. 

 The waves were generated at one end and caused to overturn at the other 

 end (often after several reflections) by travelling on to an adequately sloping 

 surface and then to break by hitting a vertical wall. Different devices on 

 this wall were fixed to determine the pressure produced by the surf breakers, 

 either by visual observations or by recording continuously. The purpose of 

 the experiments was mainly to find a numerical relation between the maximum 

 shock-pressures and the dimensions of the observed waves. In Fig. 64 are 

 shown three different possibilities when surf hits a vertical wall whose base 

 is always submerged. In the first case, the wave overturns at such a distance 

 from the wall that no breaker will hit it. In the second case (2a) the wave 

 overturns just in time for the tip of the breaker A to reach the wall before 

 the breaker founders (2b). In this process, a large air cushion B is enclosed 

 between the wall and the lower part of the surf front. As this front advances, 

 the air in this space is being compressed (2c) and then explodes vertically with 

 a scarcely audible noise with a formation of many squirts (2d). In this case the 

 position of the lower point E in the surf eddy, is always below the normal 

 surface at rest. In the third case, the wave overturns still later and, according 

 to Bagnold, this is the only case in which shock-pressures can occur. The 

 enclosed air cushion is much smaller in the horizontal direction than before, 

 and the point E is already in ascending motion, before it contacts with the 

 wall at A. Also, in the vertical direction, the dimension of the air cushion 



