submarine valley located south of San Pedro 5 and leads to a maximum diver- 

 gence between orthogonals c and do Aie to the focusing of wave energy by 

 San Pedro Shelf j the wave crests between orthogonals g and k converge 

 on the eastern portion of the breakwater 5, the greatest convergence occurr- 

 ing between i and j_o On both sides of the breakwater the refraction factors 

 indicate divergence o 



Figures U and 5 show the detailed refraction diagrams for southern 

 hemisphere swell advancing toward San Pedro i*ay from 180° and 170° respec- 

 tivelyo Although the main features of these figures are similar to those 

 of Figure 3s several significant differences exist o These variations are 

 shown best in Figure 6 which gives the refraction factors along the 

 breakwater for all three diagrams » In addition^ the refraction factors 

 are plotted for 20 second waves from 160° as computed from the diagram 

 presented by O'Brien (1947)o 



Figure 6 shows that an eastward shift in the direction of wave 

 approach causes the zone of convergence to shift westward along the break- 

 water » The westward shift of the general pattern of wave refraction for 

 160° beyond bhat of 170° serves to support thiSo In the region of greatest 

 divergence along the western portion of the breakwater greater intensity 

 of wave energy is experienced from 160° than from the other three directions 

 because the shadowing effect of the islands is lacking for 160° o In this 

 connection the combined effects of the islands and San Pedro Shelf in 

 shifting the entire refraction pattern may be compared to the action of a 

 fulcrum^ about which the wave aoproach and wave advance are pivoted „ 



The region of convergence indicated for directions of wave approach 

 from 160° to 170° coincides closely with the location of the damaged por- 

 tion of the Long Beach Breakwater in 1930 » Applying a refraction factor 

 of 3o50 for this zone of convergence and utilizing the available obsarva- 

 tions^ of wave heights which were made during this critical period at 

 locations seaward of the outer end of the breakwaters a deep water wave 

 height of between 3 and 4, feet is obtained » 



For southern hemisphere swell of 20 second period the refraction 

 factor would be soirewhat modified for slightly different period swell j 

 shorter wave periods would be accompanied by less convergence o 



Part II - laves from a Tropical Storm o 



The large waves at San Pedro Bay on September 24. and 25^ 1939? were 

 produced by unusually strong winds in the area of the North Pacific to the 

 south of Long Beach » Unlike southern hemisphere swell which originates 

 beyond the present weather reporting area^ these waves could be estimated 

 in advance by using available synoptic weather data„ In this study the 

 wave characteristics at San Pedro Bay have been computed for September 24 

 and 25 by use of the forecasting technique given by Francis (1944) and a 

 set of wave forecasting graphs prepared by Arthur (1948) from the revised 

 theory given by Sverdrup and Munk (1947) » 



Investigation of synoptic weather situation » - l/l7ien the tropical 

 storm appeared on the weather maps it was progressing northward along the 



5 

 -'■All footnotes are given at the end of the article. 



