and subsidence and build a subaerial deposit. Enough sdeiment is available 

 offshore so thtat increased wave energy could possibly build this material 

 above sea level in the future. 



35. By determining the total amount of sediment added or subtracted to 

 the eastern and western spits (using the planimeter), the importance of long- 

 shore drift to the migration of Isles Dernieres can be ascertained (Table 2A). 

 From comparison of the values found for the percentage of sediment deposited 

 by longshore drift at the spits, it is apparent that through time a decreasing 

 amount of sediment was able to bypass the tidal inlets and be deposited at the 

 spits. The annual rates of erosion and deposition for each time interval 

 (Table 2B) show that sediment erosion and spit deposition has decreased since 

 1887 (from 57 to 93 percent, respectively). These figures, like those calcu- 

 lated for overwash, demonstrate that the role longshore drift plays in island 

 migration has decreased significantly in the past 93 years (1887-1980) rela- 

 tive to the other processes. The decrease in longshore drift as an effective 

 factor in island migration is probably directly related to the increase in the 

 effect of tidal inlets. As the inlets increase in number, widen, and espe- 

 cially deepen the amount of sediment able to bypass the inlet channel will 



Table 2 

 Comparison of Total Erosion with Amount Deposited at Spits 



A. 



Time 

 Interval 



1887-1935 

 1935-1953 

 1953-1980 



Total Amount of 

 Eroded Sediment 



10 6 m 3 



82.1 



16.1 

 19.3 



Amount of Eroded 

 Sediment Deposited 



at Spits 10 m 3 



1.64 

 0.16 

 0.08 



Percent of Total 

 Deposited at Spits 



2 

 1 

 0.4 



B. 



1887-1935 

 1935-1953 

 1953-1980 



Amount of Sediment 

 Eroded Annually 



10 6 m 3 



1.71 

 0.89 

 0.71 



Annual Amount of Sediment 

 Deposited at Spits 



10 6 m 3 



0.03 



0.009 



0.003 



25 



