19. WIND WAVES 



N. F. Barber and M. J. Tucker 



1. Kinematics of Waves 



Sometimes at sea one finds the whole sea surface moving in long parallel 

 undulations at intervals of perhaps 200 metres. This simplicity is in striking 

 contrast to the confusion present in the storm regions where waves are 

 generated. 



The organized motion of a simple regular train of waves in deep water is 

 pictured in Fig. 1, which represents a vertical section through the water in a 



DIRECTION OF TRAVEL 



Fig. 1. In a progressive wave in deep water the water particles move on circular orbits 

 (very nearly) and rise in succession to become the crest of the advancing wave. The 

 surface particles crowd together near the crests and are pulled apart in the troughs. 



plane perpendicular to the long crest-lines of the waves. All the water particles 

 move on circular orbits in this vertical plane. All particles take the same period 

 of time to complete one cycle of their motion but they do not all reach the top 

 of their orbits at the same instant ; there is a progressive delay so that water 

 particles on the surface rise to the top of their orbits in succession and momen- 

 tarily form the crest of one of the advancing waves. On the crests the water is 

 moving forward in the direction of wave advance. Subsequently it moves 

 downward as the wave passes, moves backward while near the bottom of its 

 orbit in the wave trough and then rises and begins to move forward as the next 

 wave overtakes it. 



Below the surface, the motion of the particles grows less with increasing 

 depth, and at a depth D it is only a fraction, exp { — 'IttDJL), of that at the 

 surface, L being the distance between crests, i.e. a "wavelength". The factor 

 is very closely | for a submersion of one-ninth of a wavelength, \ for two-ninths 

 of a wavelength and so on. 



The profile of the wave shown in Fig. 1 has crests somewhat narrower and 

 troughs somewhat wider than a true sinusoid. The departure from sinusoidal 

 form is more marked with steeper waves, that is waves whose height is a greater 



[MS received June, 1960] 664 



