PART VI: EVALUATION OF MIGRATION FROM 1853 TO 1980 



40. The migration of Isles Dernieres is controlled mainly by overwash, 

 longshore drift, tidal currents, and subsidence. Any change in one factor 

 will ultimately affect the relative importance of the other three. By placing 

 quantitative values on the processes affecting migration, the significance of 

 these changes was determined. From the data presented in previous sections, 

 the following conclusions are made: 



a. The Isles Dernieres are making the transition from a wave- 

 dominated regime to that of a mixed-energy regime. This shift 

 is evidenced by change in island morphology and the relative 

 change in the processes controlling migration. In a wave- 

 dominated regime the barriers are long, continuous (having few 

 tidal inlets), and have a predominance of washover deposition. 

 In 1853, the morphology of Isle Derniere was typical of 

 barriers found in a wave-dominated regime. The island was not 

 broken by tidal inlets along its entire length and wash-over 

 deposition was the dominant process. In a mixed-energy regime 

 the barriers are broken by several tidal inlets, wash-over 

 deposition is of decreasing importance, and ebb tidal deltas 

 are formed. In 1980, Isles Dernieres fit the description of a 

 barrier in a mixed-energy regime. By 1980 Isles Dernieres were 

 broken by three tidal inlets, washover deposition had decreased 

 significantly, and ebb tidal deltas had formed at all tidal 

 inlets indicating an increase in tidal influence on island 

 morphology. 



b. Migration of Isles Dernieres has been slowed due to a decrease 

 in washover deposition and longshore transport, and a 

 corresponding increase in the number of tidal inlets. 



c. Overwash and resulting washover deposition is the principal 

 cause of landward migration. Longshore transport controls the 

 lateral growth of Isles Dernieres. Sediment transported by 

 both astronomical and storm tides tends to be removed from the 

 system decreasing the sediment available for overwash and 

 longshore drift. By decreasing the migration of Isles 

 Dernieres, the effects of subsidence have begun to play a more 

 important role in the evolution of this island. 



d. Isles Dernieres are in a mature stage of delta barrier island 

 development. 



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