118 



Water 



their movements may easily be studied by 

 visual observation from the air (Fig. 104). 



In addition to being caused by two or 

 more different storm centers, crossing sets of 

 waves are also produced by refraction 

 around both sides of islands and banks. In 

 fact, the Polynesians used crossing wave sets 

 on the lee side of islands as a navigational 

 aid, incorporating the information on their 

 famous stick maps (Emery, Tracey, and 

 Ladd, 1954, p. 45, PI. 1). Still other, but 

 smaller, waves are reflections from shores of 

 the mainland and islands. The many islands 

 off southern California give rise to compli- 

 cated patterns of crossing sea and swell 

 which can be observed visually (Pierson, 

 1951) or recorded by wave meters. 



Commonly, the records show very com- 

 plex patterns which have been produced by 



interference between many different sets of 

 waves coming from various directions and 

 with different periods (Shepard and LaFond, 

 1940; Sverdrup, Johnson, and Fleming, 1942, 

 p. 530). Some records, however, show rela- 

 tively simple envelopes of alternating se- 

 quences of small and large resultant waves 

 produced by only two main sets of interfer- 

 ing waves (Fig. 105). Usually each record 

 is dominated by one set of high waves which 

 may be of either long or short period. 



The velocity of waves depends on wave 

 length and depth of water. 



V= ^{gL/l'n) tanh (IttD/L), 



where V = velocity, L = wave length, and 

 D = water depth. In water deeper than L/2 

 this equation is approximated by V = 

 \/gL/2'7T, showing that the velocity is con- 



^liHiii^^ 



117 



SWELL AND WAVE PATTERN 



15 OCTOBER 1957 (0900-1500) 



34 



LONG SWELL-FROM NW 

 NTERMEDIATE SWELL-W 

 WIND WAVE-FROM NNW 



34 



33^ 



32 



32 



121 



120 



119 



117 



Figure 104. Pattern of crossing wave trains consisting of swells from two distant offshore storm centers and of waves 

 from local winds. Production of crossed waves in the lee of islands is suggested but is not clearly shown because of 

 the wide spacing of lines of observation. Data from visual examination aboard Marlin seaplane of Fleet Air Wind 

 Fourteen, San Diego, at altitudes of 300 to 600 meters on October 15, 1957. 



