102 BREAKERS AND SURF 



tances, even up to the almost incredible weight of 2,600 tons, though 

 anchored or fastened in various ways with iron rods. Striking ex- 

 amples, often quoted, are : a concrete block of 20 tons lifted vertically 

 to a height of 12 feet and landed on top of a pier 4 feet, 10 inches above 

 high water mark at the entrance to the canal to Amsterdam Harbor; 

 stones weighing up to nearly 7,000 pounds thrown over a wall 20 feet 

 high at Cherbourg on the southern shore of the English Channel ; and 

 — most famous case of all — an enormous mass of large stones set in 

 cement, and bound together with iron rods, the whole weighing 1.350 

 tons, broken loose and moved bodily at Wick Breakwater, Scotland. 

 (For more extensive discussions of the subject, see Gaillard, 1904, pp. 

 125-134 and 137-144, and Johnson, 1919.) 



Engineers concerned with the design of breakwaters, and so forth, 

 find it necessary to reckon with presures up to 2,280 pounds per square 

 foot in the Baltic ; 3,450 pounds per square foot in the North Sea ; and 

 4,120 pounds per square foot in the Bay of Biscay (Krummel, 1911, p. 

 118). 



THE CAUSES OF SURF 



The underlying cause for the development of breakers and surf is 

 the alteration that takes place in the shapes of waves as these move 

 in shoreward over a shoaling bottom, after they have reached the 

 point where the depth is less than one-half their own initial lengths. 



This alteration, as summarized on p. 56, consists in a decrease in 

 their lengths, often combined with an increase in their heights, by 

 which their crests are progressively steepened until they break. And 

 waves running toward the shore so commonly advance into water 

 shallow enough to transform them into breakers, that one is apt to 

 forget that the water may continue so deep, right up to the strand, 

 that the drag, so to say, of the waves on the bottom may not be sufficient 

 to steepen them to the breaking point before they actually arrive 

 at the barrier of the shore line. Thus, waves advancing against a 

 steep promontory, or cliff, that rises from water, say, twice as deep as 

 the wave heights, may simply surge up and down against the barrier, 

 breaking not at all or only in a confused manner, unless they strike 

 a part of the barrier where irregularities in its face cause them to do 

 so. It is often easy to observe this phenomenon when small waves are 

 running against a stone or steel pier with sheer walls, and we read 

 that it is sometimes taken advantage of in the construction of break- 

 waters. Surf breaks heavily, however, against sea walls, etc., if the 

 water is made relatively more shallow by the accumulation of sand or 

 gravel at their bases, as commonly happens after a time; in such 

 cases the incoming waves are altered into breakers by the sloping 

 bottom that they meet there. 



