138 



CHARACTER OF SURF 



across a submerged roof, even if the pflssage seems to be safe as viewed 

 from the seaward, especially if a swell is running, because an occa- 

 sional wave may rise so high that it falls forward in an acre or two of 

 swirling foam and broken water, even at times when the common run 

 of waves cross without breaking. We have time and again seen this 

 happen. 



The number of lines of breakers varies as widely as does their dis- 

 tance out from the coast. If the first heavy fall of water is right on the 

 shore line, there may be only one line of true breakers, besides the zone 



Figure 37.— A typical surf, with two chief lines of breakers, off a rocky coast of 

 the Hawaiian Islands in moderate weather. (Photograph, courtesy of W. J. 

 Clench.) 



of foaming water that surges up and down the beach (fig. 34). If, 

 however, the first line of surf is far offshore, there may be 2 (figs. 24, 

 26, 37) or even as many as 5 or 6 lines of breakers, or even more (fig. 

 38. 39), the outermost of which is usually the most dangerous. 



The governing factors here are chiefly the degree to which succes- 

 sive waves differ in shape and in dimensions, one from the next, while 

 they are still out in deep water; and the contour of the bottom, whether 

 sloping gently or steeply, uniformly or in successive steps — the pres- 

 ence of an offshore bar represents an extreme case. The more widely 

 the successive waves differ one from the next in height and in length, 

 the more numerous will the lines of breakers be. because the higher and 

 longer waves will break in deeper water, hence farther out from the 

 land, than the lower and shorter ones. And there may be several lines 

 of breakers, even with very regular swells, for each of them may still 



