10 

 9- 



8- 

 7- 

 6 - 

 5 

 4 

 3 

 2 

 1 - 



Grand Isle Area 



Force 5 



Force 4 _j 



< 



- Force 3 v) 



Force 2 ^ 



< 



Force 1 y 



(r 



_ Tropical ^ 



Storm I 



1951 to 1966 ■: J • Periodic groin construction 



and beacti nouristiment 

 1958- Eastern jetty Barataria Pass 

 1964- Eastern letty extended ■ Barataria Pass 

 1972 - Western lelly - Caminada Pass 



COASTAL STRUCTURES 



Figure 17. Changes in the area of Grand Isle in relation to the effects of 

 tropical cyclones and coastal structures. Note the rapid increase in island 

 area following construction of coastal structures downdrift. 



THE EARLY LAFOURCHE COASTAL BARRIER SYSTEM 



Barrier Evolution 



The isles Dernieres is the transgressive coastal barrier system associated with the 

 Early Lafourche Delta (Morgan 1974) abandoned 600 to 800 years ago. This barrier island 

 arc represents an advanced stage in evolution, resulting from extensive submergence and 

 reworking of the Caillou erosional headland (Figure 3). The historical map series of the 

 Isles Dernieres illustrates the transition from an erosional headland stage to a detached 

 barrier island arc stage (Figure 18). In 1853, Pelto and Big Pelto Bay separated this 

 barrier system from the mainland marsh by a narrow tidal channel less than 500 m wide. 

 By 1978, these bays had increased in size threefold and merged into Lake Pelto, and the 

 Isles Dernieres were located 7 km offshore. During this time period, the Isles Dernieres 

 shoreline retreated more than I km landward, and the original island of 1953 segmented 

 into four small islets. 



The geological strike section running through the Isles Dernieres (west to east) 

 shown in Figure 19 indicates at least two distributaries and a flanking beach-ridge plain 

 were the principal sand sources for barrier island development. In the central portion of 

 the island arc a thin (I m) washover and aeolian sand unit is seen transgressing across the 

 backbarrier marsh. Downdrift, east and west of the island arc, sand thickness increases 

 at Wine Island and Racoon Point, respectively. In these spit complexes, the barrier sand 

 body reaches a thickness of 5 to 6 m. With subsidence of these sand bodies, the Isles 

 Dernieres are receiving a diminishing sediment supply. This situation is the underlying 

 cause for the landward retreat and segmentation of the Isles Dernieres. 



31 



