132 CHARACTER OF SURF 



THE STAGE OF THE TIDE AS IT AFFECTS THE SURF 



It is common knowledge that the surf is much more violent at high 

 tide than at low in regions where the coast is fronted by off-lying 

 ledges that are bare at low tide but submerged at high, and where the 

 rise and fall of the tide is great. The reason is obvious: the ledges 

 act as natural breakwaters at low tide under these conditions, but not 

 at high tide. The many ledges lying off the Cohasset shore in the 

 southern side of Boston Bay (with which we ourselves have a life- 

 time familiarity) act efficiently in this way, when partly awash and 

 partly bare, even though the wind may be blowing so strong onshore 

 that a heavy sea is running outside. But the 9- to 11-foot rise of tide 

 submerges them so deeply at high water that storm waves run right 

 in across them, to beat directly upon the shore line. Situations of 

 this sort are common along other rocky shore lines, including coral 

 reefs. 



The stage of the tide may affect the character of the waves in this 

 same way over offshore bars because many of these rise so near to the 

 surface of the water that a surf develops there at low tide, even though 

 it may not at high. In such cases it may be possible to land at low 

 tide on the beach behind the bar, if the latter is long enough and if 

 one can come in around it, though the surf on the shore may be too 

 heavy for this at high tide. The subject of bars is discussed further 

 on p. 133. 



An impression is widespread that the surf is also likely to be more 

 violent and the spray to fly higher at high tide than at low along steep 

 beaches in general, even if they are not protected by off-lying ledges 

 or by bars. This impression may partly be due to one's natural ten- 

 dency to class the breakers as higher if they form closer in to the beach, 

 as at high tide, than if this happens farther out as at low, even 

 though the actual heights of breakers may be the same in the one case 

 as in the other. But there is in fact a considerable difference in the 

 character of the breakers at high tide and at low (at least for regions 

 where the rise and fall of the tide is considerable) along beaches that 

 slope more steeply, as many do, above low-water mark than below 

 it, because the alteration in the shape of the wave is distributed over 

 a considerably greater distance (i. e., the rollers steepen more gradu- 

 ally as they near the breaker line) at low tide than at high. The in- 

 crease in steepuess, for example, of a wave that was 100 feet long in 

 deep water would, theoretically, be spread over a distance of about 

 1,200 feet at low tide if the bottom sloped up evenly to the tide line at 

 an angle of 100 to 1. If, however, the beach then steepened to 20 to 1, 

 a common condition, the result of a rise of 10 feet in the water level 

 would be to condense most of the alteration of the wave into the last 



