1()() BREAKERS AND SURF 



landing operations, for to attempt to bring a boat in through heavy 



breakers may well be fatal. Surf is also one of the chief factors that 

 must always be taken into account in the construction of breakwaters, 

 of sea walls, of piers, or of other installations on shore lines that are 

 exposed to it. The planning of these structures to withstand the force 

 of the breakers taxes the resources of engineering to the utmost; and 

 even so. severe damage may occur. 



The energy of a wave is of a twofold nature: (a) "dynamic," or 

 "kinetic." whichever term may be preferred, resulting from the com- 

 bined momentum of the innumerable water particles of which it is 

 composed, and (b) "static," or "potential," due to the elevation of the 

 center of gravity of the wave crest above sea level — to its "head," in 

 other words. Half its energy is dynamic, the other half static, with 

 its total energy proportional to its length and to the square of its 

 height. 32 The total energy, for example, of a wave 500 feet long. and 

 10 feet high is 400,000 foot-pounds per linear foot of its crest ; and a 

 wave '200 feet long and 6 feet high would carry an energy of 57,600 

 foot-pounds per foot at its crest. 



The total energy of a wave is slightly lessened when it comes into 

 water shoal enough to alter its form. On the other hand, a consid- 

 erably greater proportion of its total energy then lies above still water 

 level and moves forward with the wave form; it is largely for this 

 reason that the destructive power of breakers is so great. (For fur- 

 ther discussion, see Gaillard, 1004, pp. 45 and 135-136, pi. 5.) 



Many measurements have been made with dynamometers, of one 

 sort or another, in the breaker zone along different coasts; these may 

 he typified by the following readings, taken at Skerry vore Rocks, and 

 at Tyree Island, off the west coast of Scotland, in 1845 (table 27). 

 (For a general discussion of the pressures exerted b}' breakers, see 

 Gaillard. 1904, pp. 124-134 and 145-211.) During the two previous 

 years, the readings averaged 2,086 pounds per square foot in winter, 

 Oil pounds per square foot in summer, a difference that obviously re- 

 flects the seasonal difference in the sizes of waves there. The observed 

 values summarized in table 28, for Florida and for Lake Superior, are 

 a further example of the relationship between the dimensions of 

 breakers and the pressures they have been found to exert. Further- 

 more, djmamometers of the types used in the foregoing experiments 

 measure only the dynamic pressures of the breakers, not the static, i. e., 

 they record only a part of their energy. 



32 According to the equation F.— 's M'TJi-. where E is the total enerpy in foot-pounds 

 per unit width of the crest in one wave length, W is the weight of 1 cubic foot of sea water, 

 L is the wave length in deep water, and // is the wave height. For an extensive table, 

 giving the force exerted by deepwater waves of different sizes and shapes, see Gaillard, 

 1904, p. 41 and O'Brien 1942, p. 14. 



