126 



Water 



Table 8 



Seiche Data in Los Angeles Harbor, December 22, 1955 



from the lighthouse to the beginning of a 

 narrow channel near Station A-2. Leypoldt 

 (1937) suggested that this harbor seiche, 

 which had been noted as long ago as 1853, 

 owes its origin to the natural period of oscil- 

 lation of the water between the shore and 

 the east side of the Santa Rosa-Cortes 

 Ridge, but the dimensions of this body of 

 water are such that its seiche should have a 

 period of only 14 minutes, far different from 

 the observed period in the habor. Very 

 possibly, however, such oscillating bodies of 

 water contribute to the irregularity of wave 

 and tide gauge records. 



Although the seiche in Los Angeles Har- 

 bor has a much smaller amplitude than the 

 one in the Bay of Fundy, the associated 

 horizontal movements of water can be great 

 enough to drive ships sideways, damaging 

 both ships and docks and thus requiring 

 remedial measures to reduce the movements 

 (Vanoni and Carr, 1951). Measurements 

 made in the spring of 1944 by Knapp (1952) 

 showed the presence of two wave periods in 

 the outer harbor, 15 seconds and 3 minutes. 

 The vertical motion of ships in that area 

 corresponded to the 15-second wave period, 

 but the longitudinal and transverse motions 

 had intermediate periods of between 100 

 and 150 seconds. Longitudinal movements 

 as great as 4 meters were noted. Subsequent 

 construction of harbor works has reportedly 

 decreased the ampUtude of ship surge in the 

 outer harbor. A brief examination of ship 

 surge at Berth 60 near the entrance to the 

 inner harbor was made on September 2, 

 1957, by measuring the fore and aft move- 

 ment of a 10,000-ton freighter. Movements 



as great as 35 cm occurred, but the average 

 period was only 23 seconds, closer to the 

 period of the swell (at that time 14 seconds) 

 than to that of the seiche. Since surge en- 

 countered during loading operations is re- 

 ported to be greater at Berth 60 and other 

 berths near the harbor entrance than at 

 berths farther within the harbor, it appears 

 hkely that at least this ship surge is due 

 more to swells entering the harbor than to 

 seiches. 



internal Waves 



Internal waves are periodic oscillations of 

 a density interface beneath the water sur- 

 face. They are easiest to recognize by 

 cyclical changes of temperature at some 

 fixed depth. Waves of this type have been 

 found in several environments in the region, 

 but they have not been studied extensively. 

 The first serious attempt to study internal 

 waves atop the mainland shelf of southern 

 California was made off' San Diego by Uf- 

 ford (1947a, \9Alb, 1947c). In order to learn 

 the wave length, speed, and direction of ap- 

 proach, he measured temperatures with 

 bathythermographs at intervals as short as 

 2 minutes for several hours at each of three 

 or more stations, sometimes using as plat- 

 forms three ships anchored in depths of 36 

 meters. The results of seven such sets of 

 measurements were interpreted as showing 

 the presence of internal waves having periods 

 of 9-136 minutes, lengths of 97-343 meters, 

 speeds of 3-16 cm/sec, and average heights 

 of 1.5-5 meters. Uff'ord concluded that the 

 waves were progressive and moving chiefly 



