124 



Water 



the largest angles with the shoreline (Put- 

 nam, Munk, and Traylor, 1949). However, 

 as pointed out by Shepard and Inman (1950), 

 so much water may be piled up at local shore 

 areas of wave convergence owing to sub- 

 marine ridges that it can flow as a longshore 

 current against approaching breakers. The 

 most complete study of longshore currents 

 in the region was made by Shepard (1950a) 

 from the Mexican border to Newport. Cur- 

 rents were found to be dominantly south- 

 ward, in response to the regional pattern of 

 waves. This agrees with the net direction of 

 transport of sand into breakwaters and 

 against groins as reported by Johnson (1956) 

 and others. During summer, however, when 

 swell from the Southern Hemisphere is im- 

 portant, part of the area has northerly long- 

 shore currents. Only slight relationship of 

 current direction and speed to tide is evident. 

 Speeds range up to 1 meter/sec but average 

 about 0.25 meter/sec. 



Nearly all the foregoing discussion refers 

 only to shores having sandy beaches, but 

 this is the most common kind of shore of the 

 southern California mainland. Rocky shores 

 of the mainland and particularly of the 

 islands are so irregular that they break up 

 advancing wave crests into sections, most of 

 which crash directly against the rocks instead 

 of forming offshore breaker zones. As a 

 result, there is little or no trapping of water, 

 and rip currents form only where there is a 

 seaward deflection of general coastal currents 

 by projecting rocky points. Reflection of 

 individual waves by the rocks is common, 

 but the reflected waves are small and usually 

 lose their identity within a fraction of a mile 

 from the shore. 



Tsunamis 



At sea, earthquakes have commonly been 

 felt aboard ships, reportedly as a sudden jar 

 which frequently led to the belief that the 

 ship had run aground. During the Long 

 Beach earthquake in 1933 many of the shocks 

 were felt aboard naval vessels anchored in 

 the harbor (Bittinger, 1933). No tsunami 

 was produced by this quake; a tide gauge 

 record merely showed the earth movement 



(Fig. 112). Eight local off'shore earthquakes 

 between 1934 and 1946 also failed to produce 

 recognizable tsunamis on tide gauge records 

 (Clements and Emery, 1947), but the No- 

 vember 4, 1927, shock off" Point Conception 

 was followed by a 2-meter wave on the near- 

 by but uninhabited shore. Other small tsu- 

 namis, probably all from distant sources, 

 were noted along various shores or on tide 

 gauges of southern California in 1812, 1854, 

 1855, 1872, 1885, 1946, and 1957 (Bache, 

 1856; Davidson, 1872; Green, 1946; Heck, 

 1947; O'Brien and Kuchenreuther, 1958). 

 New types of recorders devised especially 

 for wave periods of 10 minutes to 3 hours 

 have shown the presence of other possible 

 tsunamis (mostly of about 20-minute period) 

 too small to be recorded by tide gauges 

 (Munk, 1953). Although only small tsunamis 

 have been reported along the coast of south- 

 ern California, the occurrence of infrequent 

 large ones off South America (Gutenberg 

 and Richter, 1949, pp. 94-97) suggests that 

 southern California may not be immune. 

 Certainly, one as great as those that struck 

 the Hawaiian Islands on April 1, 1946 (Shep- 

 ard, Macdonald, and Cox, 1950), and on 

 March 9, 1957, would cause enormous dam- 

 age and some loss of life in the low and 

 densely populated areas of Malibu Beach 

 and Newport Beach. 



Seiches 



In each of the larger bays of the Pacific 

 coast periodic surging of the water has been 

 noted. The surges are usually less than 15 

 cm high and have periods intermediate be- 

 tween those of wind waves and tides (Fig. 

 1 12). These are known as seiches (Sverdrup, 

 Johnson, and Fleming, 1942, pp. 538-542), 

 free oscillations of periods controlled by the 

 depth and length of the bays: T = 4L/ \/gD, 

 where T is the period, L is the length of an 

 open-mouthed bay, and D is the average 

 depth. Although it is generally believed that 

 any disturbance of water level can start them, 

 such as tides, strong gusty winds, barometric 

 pressure changes, and perhaps earthquakes, 

 tsunamis, and surf beats, no case history of a 

 seiche in southern California has been re- 



