130 CHARACTER OF SURF 



respect within the range that has been investigated, so this question 

 must be regarded as an open one, for the time being. 



It is well known that when an offshore wind develops, the surf does 

 not break until closer in to land (i. e., until in shallower water) than 

 when the wind is onshore, or when it is calm. No doubt this is be- 

 cause a head wind tends to delay the steepening of the crest, and the 

 development of an overhanging front. Thus, waves advancing over 

 a bottom slope of 1 in 100 at St. Augustine, Fla., have been described 

 as breaking where the depth was equal to their own heights in calm 

 weather, at a depth 1.25 times as great as their own heights when the 

 wind was strong onshore, but not until the depth was only 0.72 as 

 great as their height when the wind was strong offshore (Gaillard, 

 1904, p. 120). In another case, near Bournemouth, England, a swell, 

 causing a heavy surf, did not break until almost in on the beach after 

 an offshore wind had risen, although the height of the breakers was 

 about the same as it had been while the wind had been blowing on- 

 shore (Cornish, 1910, p. 88). The difference in this respect between 

 onshore and offshore winds is often great enough to be worth taking 

 account of in landing operations, even before the sea or swell shows 

 any appreciable diminution in its height offshore. 



The interaction, in short, is so complex between the factors involved 

 that the nearest approach to a definite rule that we dare offer for the 

 relationship between heights of breakers and depth of water, is as 

 follows: If the bottom slope is gentle (less, say, than 1 in 40), waves 

 may break where the depth is twice as great as their own heights if 

 the wind is strong onshore, or if a strong current is running against 

 them; they will be likely to break where the depth is about 1.3 times 

 their own heights if the weather is moderate to calm, with little or no 

 current ; but they may not do so until the depth is only about three- 

 fourths as great as their heights, if there is a strong offshore wind. 

 If the slope of the bottom is steep or if it changes slope abruptly, it 

 is safer to reckon on waves breaking where the depth is twice as great 

 as their own heights, especially if the wind is blowing strong onshore 

 or if there is a strong current against the waves, for while it is uncertain 

 if the slope of the bottom, per se, has any effect on the depth at which 

 the rollers break, it is always wise to be on the safe side when dealing 

 with surf. Waves may even break where the depth is as much as 

 three to six times their heights in extreme cases (p. 126). 



It must be remembered that it is not the height that a wave may have 

 had while out in deep water that is critical in this connection, but 

 the height to which it may rise during its advance into shoaling water, 

 which depends largely on its initial steepness, as explained on page 

 121. For example, a wave that was 2 feet high in deep water might 

 be expected to break when it reached the 2.8-foot line, if its initial 



