Patterns of Shoreline Change 



Shoreline erosion derived from the OGMS data indicate that the highest erosion 

 rates within the Isles Dernieres occur along the central portion of the island arc (Figure 

 20). Here erosion rates in excess of I 5 m/yr are common. Downdrift, both east and west 

 of the central island arc, erosion rates decrease to approximately 5 m/yr. This erosion 

 pattern reflects the influence of barrier orientation to the dominant wave approach. 

 Throughout their evolution, the Isles Dernieres have faced directly into the dominant 

 southerly wave approach, creating a sediment transport diversion zone in the central 

 island arc. With sediment transported both east and west from this area, a spreading 

 effect results, dispersing sediments over a wider area than a more asymmetrical wave 

 approach, as at the Chandeleur Islands would. 



Coastal structures have not been built in the Isles Dernieres barrier system; 

 therefore, its sediment dispersal system is undisturbed. A plot of island area versus 

 hurricane landfall indicates that island area has been decreasing steadily. The area of 

 the Isle Dernieres diminished from 34.8 km"^ in 1887 to 10.2 km'^ in 1979 (Figure 21). 

 Island land loss is very rapid, indicating the possible destruction of the Isles Dernieres 

 within 50 years. High erosion rates must be related to rapid subsidence in the area and 

 the lack of a substantial coarse-grained sediment imput to help maintain these barriers in 

 this sinking coastal environment. 



ST. BERNARD BARRIER SYSTEM 



Barrier Development 



The Chandeleur Island system occupies the eastern margin of the St. Bernard delta, 

 abandoned approximately 1,800 years ago (Frazier 1967). This system represents an 

 advanced stage in the evolution of a transgressive barrier island arc system. The 

 Chandeleur Islands represent a merged system of earlier erosional headlands and flanking 

 barriers associated with major unidentified St. Bernard delta distributaries. Seismic and 

 vibracore data collected by the Louisiana Barrier Island Project indicate that shoreface 

 retreat can no longer penetrate through to the sand bodies associated with the St. 

 Bernard delta and supply coarse sediments to the island arc. These islands are presently 

 transgressing across fine-grained lagoonal facies of Chandeleur Islands (Figure 22). Since 



20- 



Isles Dernreres 

 Area 



Force 5 

 - Force 4 



UJ 



Forces -i 



o 



Force2 "^ 



UJ 



Force 1 < 



O 



Tropical (r 



Storm ^ 



1975 



Figure 21. Changes in the area of the Isles Dernieres in relation to the 

 effects of tropical cyclones. Note rapid land loss indicating the potential 

 destruction of the Isles Dernieres within 50 years. 



33 



