SECT. 5] 



WIND WAVKS 



679 



Theoretically it is also expected that streams should change the direction of 

 travel of waves that enter them obliquely- (Johnson, 1947). This may be a com- 

 mon event but the literature reports few examples of it, perhaps because it has 

 been confused with the refraction that waves quite obviously show when 

 entering shallow water ; perhaps also because laboratories studying ocean swell 

 with the aid of underwater instruments have not in the past paid much atten- 

 tion to the direction of wave travel. It has been noted, however, that tidal 

 streams over a wide continental shelf can cause the period of swell reaching 

 the coast to vary slightly with the state of the tide (Barber and Ursell, 1948). 



Fig. 6 shows refraction diagrams for waves passing a headland, the under- 

 water contours of which are indicated as broken lines. The diagram shows the 



DEPTH CONTOURS IN METRES 

 200 KDO 60 40 20 lO 



±o 



Fig. 7. Calculated wave paths for 14-sec period swell approaching the La Jolla coast froin 

 WNW. Although the coast appears well exposed the underwater canyons refract 

 waves away from B and tend to focus them at AA. (After Munk and Tray lor, 1947, 

 Fig. 8. Copyright 1947 by the University of Chicago.) 



wave "rays", that is, lines drawn everywhere normal to the wave crests. Energy 

 follows along these paths and the waves are higher where the lines crowd 

 together and are lower where the lines spread apart. The paths shown as full 

 lines refer to waves of period 7 sec, typical storm waves. These are only slightly 

 refracted and the bay behind the headland is well sheltered. The broken lines 

 refer to swell of period 14 sec. The waves of this lower frequency are more 

 refracted and are more likely to be noticed at sites that are not directly exposed 

 to the open sea. Diagrams like this are of use to engineers. They can be con- 

 structed from charts showing the underwater contours (Arthur, Munk and 

 Isaacs, 1952) because the velocity of the waves at all points in the region can 

 then be deduced from theory and the refracted "rays" can be built up piecemeal 



