Waves 



123 



even though the number of bathers is smaller 

 than during the late summer when the water 

 is warmer. These rescues and the occasional 

 drownings are usually needless because rip 

 currents can easily be recognized and avoided 

 by noting the yellow-greenish color of the 

 water produced by suspended sand, the pres- 

 ence of a peripheral line of foam or debris, a 

 gap in the line of breakers, and a peculiar 

 peaked and splashy character of the water 

 surface. 



Generally, the rip currents are active in 

 spurts of several minutes duration. After a 

 sequence of especially large waves, the water 

 level shoreward of the breaker zone rises and 

 the resulting hydrauhc head is great enough 

 to force a passage through the crests of 

 advancing breakers, especially when the next 

 waves are smaller ones. When most of the 

 trapped water has drained away, the rip cur- 

 rent ceases; a few minutes later the cycle is 

 renewed when large waves build up the water 

 level again (Shepard and Inman, 1950). This 



intermittent rise and faU of water near the 

 beach produces a low wave of long pe- 

 riod (2 to 4 minutes) which has been termed 

 surf beat by Munk (194%), who recognized 

 it on tsunami recorders. The variations in 

 water level inshore of the breaker zone are 

 also shown by transgressions and regressions 

 of the swash zone (area alternately exposed 

 and covered by thin sheets of water) on 

 sandy beaches. Smaller transgressions and 

 regressions of the water-saturated part of the 

 beach result from advance of large waves far 

 up the beach and the breakdown of subse- 

 quent smaller waves by backwash from the 

 first one (Fig. Ill) (Emery and Gale, 195 1). 

 This has given rise to another misconception 

 of the sea, that every seventh wave is a large 

 one. 



Where the waves strike the beach at an 

 angle, the wave pressure develops longshore 

 currents inshore of the breaker zone. Other 

 things being equal, the fastest currents are 

 produced by waves having crests that make 



Figure 111. The advance of breakers and their run-up on the sandy beach at Point Dume as read from frames of 

 slow-speed motion pictures. Lined area is fully saturated sand; dotted area is partly drained or dry sand. Note that 

 only occasional waves run far up the beach and saturate the sand, which then slowly drains until covered by the next 

 large wave. From Emery and Gale (1951, Fig. 2). 



