average of 18.6 m/yr. Downdrift, the erosion decreases end switches to accretion at the 

 western end, averaging 17.4 m/yr. 



Changes in the area of the Timbalier Island group reflect the impact of the 

 navigation structures at Belle Pass. Between 1887 and 1935, 19 tropical cyclones, 7 of 

 which were at least force-2 on the Saffir-Simpson scale (Saffir 1977), made landfall in 

 the vicinity of these islands, resulting in only a slight decrease in total area (Figure 15). 

 Between 1935 and 1956, both island areas increased, reflecting the low frequency of 

 tropical cyclone landfall, with just one force-2 storm occurring. Following 1956, the 

 area of both islands started decreasing rapidly. Hurricane frequency had increased 

 slightly, with five tropical cyclones impacting the coast, two of at least force-2 

 strength. The reduction in the island area is most likely attributed to the construction of 

 jetties at Belle Pass and the seawall groin system westward along East Timbalier Island, 

 and not to tropical cyclone impact. All of these structures have interrupted sediment 

 transport from the source areas within the Bayou Lafourche headland. A major factor in 

 reduction of longshore sediment transport appears to be the 1968 extension of the Belle 

 Pass jetties, as reflected by the dramatic changes in island areas during this time. 

 Flanking barrier islands connected to an active sediment source are dynamic and tend to 

 build. This appears to be the case prior to 1950 at the Timbalier Islands, when periods of 

 frequent hurricane impact produced little reduction in island area. These flanking 

 barriers, as long as they are receiving sediment input from the erosional headland, exist 

 in a state of dynamic equilibrium even under the conditions of rapid subsidence. The 

 recent land loss observed at the Timbalier Islands is directly linked to the introduction of 

 coastal structures in the sediment dispersal system. 



Caminada Pass Spit-Grand Isle. East of the Bayou Lafourche headland lie the 

 downdrift barriers of Caminada Pass spit and Grand Isle. Along the Caminada Pass spit. 



The Timbalier Islands Area 



E 15 .__ 



/ 



■^^ 



Force 5 

 - Force 4 



UJ 



-J 



Force 3 o 

 en 



Force 2 uJ 



z 

 < 



Force 1 y 

 a. 



Tropical 5 



Storm ^ 



1900 1925 1950 1975 



1934 - Small jetty system - Belle Pass 



1950 to present 



Seawal I . groin , and 



breakwater construction- East Timbaher Island 

 1968- Jetty system and navigation channel exten- 

 sion - Belle Pass 



COASTAL STRUCTURES 



Figure 15. Changes in the area of the Timbalier Islands in relation to the 

 effects of tropical cyclones and coastal structures. Note the rapid decrease 

 in island area following construction of coastal structures updrift. 



29 



