experienced high rates of erosion at its central-most point. Longshore drift 

 has been westward on the western part of the island and eastward on the east- 

 ern end of the island (Peyronnin 1962). Similar transport patterns have been 

 documented by Penland and Boyd (1981) on the Timbalier Islands--Caminada/ 

 Moreau Coast — Grand Isle complex, and the Chandeleur Islands. Figure 9 

 illustrates the orientation of Isles Dernieres and the directions of longshore 

 transport. The main source of sediment for spit growth is from the rapidly 

 eroding central portion of Isles Dernieres and exposed distributary sedi- 

 ments. Beach-front erosion rates of up to 26 m/year have been calculated for 

 this area. Much of the sediment potentially available for spit growth never 

 reaches the location of spit deposition. Rather, it is removed from the 

 shoreface by tidal currents at the inlets to a depth below effective wave 

 base. But, enough sediment is carried along a series of offshore bars to pro- 

 vide for some spit growth (Meyer-Arendt and Wicker 1982). The eastern spit is 

 highly recurved indicating that strong tidal exchange forces are present. 

 This spit has been kept relatively stable by Wine Island Pass, which is ap- 

 proximately 18 m deep and is directly adjacent to the spit (Figure 10). On 

 the western end of the island an elongated spit has accreted, but, recently, 

 it has started to retreat, presumably due to a decrease in sediment supply 

 (Figure 11). Although sediment is being supplied to the western end of Isles 

 Dernieres, it is not enough to overcome the effects of tidal current erosion 



ISLES DERNIERES 



Figure 9. Sketch of Isles Dernieres showing the islands' orientation 

 and directions of longshore transport 



23 



