there is no present-day sediment source to the Chandeleur Islands, they are dinninishing 

 in size. The sedinnent dispersal system is recycling barrier sands within the island 

 complex. Storm waves transport sediment seaward into a broad inner -shelf sand sheet 

 and landward into backbarrier washover fans. 



Patterns of Shoreline Change 



The pattern of shoreline changes in the Chandeleur Islands is different from that in 

 the Isles Dernieres, due to differences in shoreline orientation to the dominant wave 

 approach. The Chandeleur Islands are oriented oblique to the dominant wave approach, 

 whereas the Isles Dernieres face directly into the dominant wave approach. The southern 

 end of the Chandeleur Islands receives the brunt of the wave attack; wave-refraction 

 attenuation along the shallow inner shelf increases towards the north, and is reflected in 

 decreasing shoreline erosion rates (Figure 23). Along the southern part of the Chandeleur 

 Islands, erosion exceeds 15 m/yr and is characterized by periodic hurricane destruction 

 followed by partial island re-emergence and rebuilding. Northward along the islands, 

 erosion rates decrease from 15 m/yr to around 5 m/yr at the northern end. A plot of the 

 area of the Chandeleur Islands versus hurricane landfalls shows the importance of periods 

 of high hurricane frequency to total island area (Figure 24). Between 1869 and 1924, nine 

 tropical cyclones made landfall in the Chandeleur Islands region, of which only two were 

 above force-2 strength, resulting in a slight decrease in island area. Between 1925 and 

 1950, five tropical cyclones made landfall; however, only one was of hurricane force; and 

 the remainder were tropical storm strength. For this time interval, the Chandeleur 

 Islands only slightly decreased in area. Between 1950 and 1969, rapid decrease in the 

 total island area of the Chandeleur Islands was related to a period of frequent hurricane 

 landfall. Five major storms impacted the island, one of which was hurricane Camille, of 

 force-5 strength and had deep-water wave heights measuring in excess of 20 m. As a 

 result of these high-intensity storms, the iota I island area of the Chandeleur Islands 

 decreased from 29.7 km^ in 1950 to 21 km^ in 1967. Sediment dispersal in this system 

 reflects the hurricane impact on barrier islands with the finite internal sediment supply 

 that is continually being recycled. Hurricane responses are offshore sediment transport 

 and barrier breaching, leading to sediment losses to the offshore and tidal delta sinks. 

 Sediment dispersal patterns are determined by barrier orientation to the prevailing 

 regional wave climate. Barrier rebuilding in the Chandeleur Island reflects the presence 

 of a southerly updrift sediment source supplying progradational episodes farther north. 



Breton ■ Chandeleur 

 Island Area 



---,; 



Force 5 



Fofce4 ^^ 



< 

 Force 3 u 



ui 



- Force 2 lu 

 z 

 < 



Force 1 y 

 cc 

 cr 



Tropical zi 



Storm ^ 



Figure 24. Changes in the area of the Chandeleur Islands in relation to 

 the effects of tropical cyclones. 



35 



