Interference of Waves; Short-crested Waves; White Caps 



When waves of different heights and lengths are present simul- 

 taneously the appearance of the free surface becouies very compli- 

 cated. At some points tr.e waves are opposite in phase and there- 

 fore tend to eliminate each other, whereas at other points they 

 coincide in phase and reinforce each other. 



As a simple case, consider two trains of waves which have the 

 same height and nearly the same velocity of progress. Owing to 

 interference, groups of waves are formed with wave heights rough- 

 ly twice those in the component wave trains, and between the wave 

 groups are regions in which the waves nearly disappear (fig. 5A) . 

 Analysis shows that these groups advance with a velocity which is 

 nearly equal to one half of the average velocity of the two trains. 



As another example, consider the simultaneous presence of long, 

 low swell and short but high wind waves. The resultant pattern is 

 illustrated in Figure 5B from v;hich it is evident that the short, 

 high waves dominate to such an extent that the presence of the 

 swell is obscured. 



So far, the discussion has dealt only with long-crested waves, 

 that is, waves with very long straight crests and troughs. Waves 

 can, however, also have short, irregular crests and troughs. In 

 the presence of such short-crested waves the free surface shows a 

 series of alternating "highs" and "lows", as Indicated in Figure 6. 

 This figure illustrates the topography of the sea surface, "highs" 

 being shovm v;ith full-draTAOi lines and "lows" mth dashed lines. 



10 



