678 BARBER AND TUCKER [CHAP. 19 



charts shows that the swell is usually due to severe storms in the North Atlantic 

 some 1000 to 1500 miles away. Since swell travelling with its group velocity 

 will make some 400 to 600 miles a day depending on its frequency, there is 

 evidently time enough to recognize such storms on the charts and to give the 

 population of Barbados a warning one day in advance (Donn and McGuinness, 

 1959). Heavy swell can also hinder commercial shipping on coasts where cargo 

 must be transferred to boats to get it ashore, but swell prediction in seas other 

 than the North Atlantic is unfortunately not so certain because the detailed 

 wind and pressure patterns are less well known. 



6. Waves Approaching the Shore 



Theory and experiment both suggest that, in the open ocean, wave groups 

 tend to advance along great-circle paths. Near to land, however, they tend to be 

 refracted both by their entry into shallow water and by the stronger tidal 



50 40 



L^ 



■ I ; / / / / 





Fig. 6. Illustrating the refraction of waves around a promontory. Rays are shown for 

 waves of period 7 sec (full lines) and of period 14 sec (broken lines), both coming from 

 the left of the diagram. The underwater topography refracts the long waves much 

 more. The water depth in metres is showii by the broken contours. 



streams that exist there. An opposing stream reduces the speed of advance of 

 wave groups and crowds their energy into a smaller area of water. The waves 

 themselves are, therefore, higher after entering the stream (Unna, 1942); it is 

 well known that waves are higher on an opposing tide than on a following 

 one. In an extreme case, however, the stream may be so strong as to arrest the 

 wave groups. A very confused sea then develops in which all the wave energy 

 is lost to turbulence, but the up-stream waters will be calm because no waves 

 can arrive there. 



