HO BREAKERS AND SURF 



long as high should, theoretically, be almost twice as great when it 

 reached the point where the depth was one-tenth as great as its own 

 initial length (and where it might be expected to break) as they were 

 to begin with. And the longer the wave, relative to its initial height, 

 the greater is the increase that takes place, in this way, in the hori- 

 zontal velocities of its water particles. (This summary has been 

 drawn from Gaillard, 1904, pp. 41, 97, 135-136, pis. 1 and 5.) The 

 water particles at the crest of a wave may thus be moving forward 

 several times as fast, when it is about to break, as they were originally. 

 The question of the orbital velocities at the top of the crest at the 

 instant of breaking has not received as much attention as it deserves 

 from a practical standpoint. But anyone who has had experience in 

 surf knows that any object, such as a plank or a small boat floating 

 on the top of a roller, may be swept forward with astonishing rapidity 

 just as the top of the crest falls forward, if the breakers are of the 

 plunging type (p. 111). And this is one of the reasons why it is so 

 difficult to bring even a surfboat in through high breakers, for it is 

 likely to be carried forward over the crest unless it is well handled, to 

 be pitched down bow foremost into the trough ahead, where it will be 

 in imminent danger of broaching to, as its stern continues to be swept 

 forward in the air, or at the least of filling with the water that pours 

 down upon it from above. Surf running of this sort should never be 

 attempted in small boats, except as a last resort. 



THE CHARACTERISTICS OF BREAKERS 



The breaking of waves out in deep water appears to be caused chiefly 

 by the pressure of the wind, which forces the backs of their 

 crests ahead faster than their leeward sides are advancing, until they 

 come to overhang the troughs. As a rule, it is only the very top of the 

 crest that falls over. Typical breakers on the beach, however, 

 are not caused directly by the wind (although the wind may aid 

 in their formation, if it is blowing strongly onshore), but by the 

 alteration of the wave forms that takes place over shoaling bottom, 

 as described above, by which the waves grow steeper, until they be- 

 come unstable. It is commonly stated that it is the friction with the 

 bottom that causes this alteration in shape by retarding the lower part 

 of the wave, while the upper part continues to advance. But experi- 

 mental studies of waves have failed to show that a frictional effect is 

 of importance. Neither is the developing profile of a breaker what 

 it should be if friction were the sole cause, for its front becomes hol- 

 lowed as the crest steepens, suggesting, rather, that it is a deficiency 

 of water on the front side that causes the crest to overhang the trough 

 in front of it, and consequently to fall forward. 



