Table 1 

 Comparison of Total Erosion with Amount Redeposited by Overwash 



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 by 



Overwash 10 m 



71.4 

 13.3 

 14.7 



Percent of Eroded 

 Sediment Redepos- 

 ited as Overwash 



87 

 83 

 76 



B. 



1887-1935 

 1935-1953 

 1953-1980 



Amount of Sediment 

 Eroded Annually 



10 6 m3 



1.71 

 0.89 

 0.71 



Amount of Eroded Sediment 

 Deposited by Overwash 



Annually 10 6 m 3 



1.49 

 0.74 

 0.54 



in Table 1A were expressed as annual rates of erosion and deposition for each 

 time interval. Table 1B shows that the annual rate of sediment eroded has de- 

 creased by 57 percent since 1887, and the annual rate of overwash deposition 

 has decreased by a slightly larger amount of 67 percent in the same period. 

 These figures reveal that there has been a change in overwash deposition, but 

 its relationship (percent) to the total amount eroded has stayed approximately 

 the same. This small change in overwash deposition results in a decrease in 

 the relative importance of overwash with time, because the rates of other pro- 

 cesses involved in island migration have changed signif icantly--relative to 

 the change in overwash. The decrease in overwash importance is also reflected 

 in the overall decrease in the rate of landward migration through time. In 

 the central Isles Dernieres area, the rate of migration has decreased from 

 27 m/year between 1887 and 1906 to 17 m/year in the last 27 years. Several 

 factors probably contribute to this decrease in landward migration: 



a. There has been a large increase in the number of tidal inlets, 

 i.e., from no tidal inlets in 1853 to three in 1980. Tidal in- 

 lets decrease the surge on the barrier front from overwash by 

 allowing water to channel between (instead of over) the island, 

 hence inhibiting landward migration. 



20 



