WATER DEPTH 127 



Rock, Oregon; and at 48 to 60 feet near Yaquina and Coos Bays, 

 Oregon. Surf has also been reported over depths as great as 72 feet 

 along the Washington coast, and even at 90 feet there in the most 

 severe weather, 40 as it also has in the southern part of the North Sea, 

 where the seas break heavily over Borkum Ridge in depths of 10 to 15 

 fathoms (60 to 90 feet) during onshore gales. Surf has been recorded 

 at depths of 50 to 56 feet off the coasts of the Guianas ; at 56 to 66 feet 

 off Yucatan ; at 66 to 100 feet around Madeira ; at 66 to 77 feet off Al- 

 geria ; also at 80 feet off northern Spain, although a conflict between 

 waves and currents may perhaps have been responsible for some of 

 these extreme cases (p. 51) . 



The instances cited are enough to show that the statement, often 

 made, that a wave may be expected to break where the depth is equal 

 to its own height is not an adequate one ; nor does this even apply to 

 the common run of surf. The range of variation which may be ex- 

 pected with waves of different sizes is shown in table 32. The chief 

 reason for the very considerable variation that has actually been ob- 

 served in the ratio between height of surf and depth of water is that 

 waves tend to break in somewhat deeper water with a strong onshore 

 wind than they do in moderate weather, because the direct pressure 

 of the wind against the windward sides of their crests increases the 

 steepness of the latter, thus hastening their overfall. Thus waves, 

 advancing over a bottom slope of 1 in 100, off St. Augustine, Fla., have 

 been described as breaking where the depth was 1.25 times as great as 

 their own heights with a strong onshore wind, though they did not do 

 so in calm weather until they reached the point where the depth was 

 equal to their own heights at the moment. 



It also appears, from various observations, that waves of equal 

 lengths and heights break in considerably deeper water where the 

 slope of the bottom changes abruptly than when they run in over a 

 uniform slope. Thus, waves may perhaps break where the depth 

 averages about 1.7 to 1.8 as great as the breaker height, where a steep 

 bottom slope is followed by a more gentle one, if the weather is calm 

 and if there is no current. This probably is the explanation for the 

 broken water that is often to be seen along the offshore edges of shoals 

 and of reefs, over depths greater than those at which surf would other- 

 wise be expected with waves of the sizes running at the time. 



The interference that often develops between trains of waves coming 

 from different directions, also may increase their heights and the 

 steepness of the individual crests, not only causing these to break in 

 considerably deeper water than would happen otherwise, but rendering 

 the breakers so much more complex in pattern, as greatly to increase 



40 The information as to the depth of which surf breaks along the northwest coast of the 

 United States is abstracted from a summary by Gaillard, 1904, pp. 115-117. 



