120 



Water 



1.^:3^2^^ 



Figure 106. Wave refraction at San Nicolas Island. Swell from northwest is refracted around the island, meeting at a 

 spit on the east side of the island and producing crossed wave crests beyond the end of the spit. Photograph by U. S. 

 Geological Survey, courtesy of J. B. Schoellhamer. 



elongate ellipse. Breaking (Fig 109) occurs 

 where the waves drive into water too shallow 

 to fill the circular orbit, usually about four- 

 thirds the breaker height (Munk, 1949a). 

 Breakers can occur at somewhat greater 

 depths (to twice the breaker height) when the 

 onshore wind velocity is great or where the 

 bottom has a sharp break in slope. At that 

 point the wave rises to as much as 20 per cent 

 higher than before breaking, and the crest 

 curls over and collapses on the hollow orbit. 

 When the wave fails to become steep enough 

 to break, the crest merely spills down the 

 steepened front of the wave. After either 

 breaking or spilling the wave changes to one 

 of translation, in which the water drives to- 



ward the shore, more nearly keeping pace 

 with the crest. 



Breakers and Associated Currents 



The orbits of water particles in waves are 

 larger near the surface than at depth, giving 

 rise to a shoreward mass transport of water 

 in the direction of wave propagation. After 

 the waves break, their onshore momentum 

 greatly increases the transport so that more 

 water is brought into the breaker zone than 

 can escape between wave crests. This wave 

 pressure can result in water piling up as 

 much as a meter above the level outside the 

 breaker zone. Most of this water escapes in 



