168 



BREAKER DIRECTION AND HEIGHT 



cut, the only limit to their sidewise expansion, as they advance across 

 the basin, is such as is imposed by the sides of the latter. The rela- 

 tionship between the breadth of a basin, and the degree to which en- 

 tering waves are reduced in height as they spread sidewise within it 

 may be illustrated by the Duluth Harbor opening on Lake Superior 

 (fig. 52), where measured waves that were 9.9 feet high at the nar- 



D * " p.S* N „. '■= Du luth Ship Ca nal n 



\ ' 



" SCALE OF FEET 



; v 10 g.°ut?°uMS iff »y 



Figure 52. — Sketch map of Duluth Harbor. Soundings in feet. (After Gaillard.) 



row entrance were only 1.17 feet high at the station marked A on 

 figure 52, and 1.0 foot high at station B; while waves that were 11 

 feet high at the entrances were only 0.45 foot high at station C, 

 the distances in from the entrance to these stations being 1,200 feet, 

 2,600 feet, and 4,195 feet, respectively. But waves that were 9.9 feet 

 high at the entrance were still 2.5 feet high at the station marked D 

 on the narrow side of the harbor (a reduction of only 3.6 to 1 in a 

 distance of 3,857 feet) because there is so little room for the wave 

 crests to expand sidewise in that direction (Gaillard, 1904, p. 89). 



The rate of reduction in wave height that is to be expected in locali- 

 ties of this sort is calculable, according to Gaillard, by a formula that 

 takes account of the breadth of the entrance (for this limits the breadth 

 of the wave crests that pass through it), the heights of the waves as 

 they emerge from the entrance, the sidewise breadth of the harbor at 

 the place of observation, and the distance of the latter inward from the 

 entrance. 



SURF AROUND ISLANDS 



The factors that determine the differences in heights of the breakers 

 from place to place along coasts in general, act in the same way around 



