II. Scripps Institution of Oceanography ^arterly Progress Report 

 No. 10. October -December 1951 



SUBMARINE GEOLOGY 



Survey of Mission Bay Channel 



As a result of the numerous recent drownings caused by small boats 

 capsizing on the bar at the entrance to Mission Bay, a joint survey was 

 made of the channel on 14 December 1951 by Scripps Institution of Oceano- 

 graphy and the Corps of Engineers (see Figure 1). 



In May 1950, an 8 foot deep channel was dredged between the Middle 

 and North jetties connecting Mission Bay with the open ocean. Following 

 the opening of the new channel, surveys were made by the Beach Erosion 

 Board in June, September, and November 1950 and April 1951 •., 



The initial channel was dredged along the center line between the 

 two jetties. Study of the first three surveys shows a progressive deep- 

 ening of the channel. on the bay side, and shoaling on the seaward end 

 of the channel, where a bar formed. The location of the otiannel (raid- 

 way between the jetties) was little changed during this period. The 

 April 1951 swvey showed that the relatively straight channel of previ- 

 ous surveys had become somewhat sinuous. Also there was appreciable 

 shoaling along the seaward end of the Middle Jetty, and a bar extended 

 from the shoal area toward the end of the North Jetty. 



Comparison of the April 1951 survey with the survey of Deoeraber 

 1951 shows that the channel has increased in sinuosity, now having an 

 inverted "S" shape. On the bay end of the inlet the main channel runs 

 along the Middle Jetty while on the seaward end it is along the North 

 Jetty (see Figure 1). TOiere the channel runs next to the jetties it ■ 

 is narrow and deep, while the portion between jetties is broad and 

 shallow, having a silt depth of about 7^ feet below MLLW. The shoaling 

 along the seaward end of the Middle Jetty has continued, and a 10|-foot 

 deep bar extends across the inlet from the end of the Middle Jetty 

 to within 100 feet of the end of the North Jetty. The bar moved 500 

 feet seaward between the April and December surveys. It seems probable 

 that the capsizing of small boats in the inlet resulted from a combina- 

 tion of minus tides, strong ebb currents, large waves breaking over the 

 bar, and lack of local acquaintance with breaking entrances. 



Statistical Study of Currents in the Surf Zone 



The statistical study of the variability and prediction of long- 

 shore currents mentioned in previous progress reports has been completed. 

 It will receive a limited initial distribution as SIO Submarine Geology 

 Report No. 23. 



The study showed that the variability of the longshore component 

 as measured by its standard deviation is equal to or larger than the 



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