



E 150 



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: "(969 



t I 

 /! i 



\| i'96S i 

 •'96?. ! 



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LOWER ENVELOPE 



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 (967^ 



1969* 



jon reb Mor Apr May June July Aug Sept Oct n ov Dec 



Month 



Figure 11. Seasonal variation of the position 

 of the mean high tide line, Stinson 

 Beach spit, 1948-1970. (Johnson, 

 1971.) 



Data which give more detail on the day-to-day changes in a profile on the Stinson Beach 

 spit are those of Thompson (1970) which are plotted in Figure 12. The plots resulted from a 

 field study conducted from 4 January through 2 May 1969 on a profile near the Corps of 

 Engineer's Range 3. (See Figure 4.) This profile was measured daily; the waves incident 

 upon the profile were obtained from the Bolinas Harbor District wave gage. The daily 

 position of the mean low water line, mean tide line, and mean high water line are indicated 

 along with other elevations with respect to the 1929 sea level datum. The wide variation in 

 the position of those lines (with time) in the zone of wave action is strikingly evident. 

 5. Inlet Characteristics. At most lagoon inlets, two opposing forces control the location 

 and size of the opening. These forces are the ebb currents and the wave forces. The ebb 

 currents provide a flushing action of the entrance and move sand offshore; wave action 

 moves sand shoreward to fill the entrance. Often, the strength of the wave forces exceed 

 those of the ebb currents, and the inlet remains completely closed (except possibly in times 

 of flood, if appreciable stream flow enters a lagoon from a watershed). At bays or lagoons 

 where wave refraction varies the wave height along the barrier beach between the lagoon and 

 the ocean, the tidal inlet will be located where the forces of the ebb currents and the wave 

 forces are approximately in balance. This is the condition at Bolinas Lagoon where charts 

 and photographs show the entrance historically to be always adjacent to the Bolinas 

 headland where refraction effects (Fig. 3) cause the least wave height of any location along 

 the Stinson Beach spit. 



28 



