--> ABSTRACT: Erosion and Deterioration of Isles Dernieres Barrier Island Arc, Louisiana: 1842-1988, by Randolph A. McBride, Karen Westphal, Shea Penland, Bruce Jaffe, Abby Sallenger, and S. Jeffress Williams; #91029 (2010)

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Erosion and Deterioration of Isles Dernieres Barrier Island Arc, Louisiana: 1842-1988

Randolph A. McBride, Karen Westphal, Shea Penland, Bruce Jaffe, Abby Sallenger, S. Jeffress Williams

The Isles Dernieres barrier island arc is the most rapidly eroding coastline in the United States. Located on the Mississippi River delta plain in Terrebonne Parish, Louisiana, the Isles Dernieres consists of four smaller islands in a 32-km long chain. From west to east, these islands are known as Raccoon Island, Whiskey Island, Trinity Island, and East Island. The barrier island arc is separated from the mainland by Caillou Bay, Boca Caillou, and Lake Pelto lagoonal systems. The abandonment and transgression of the Bayou Petit Caillou delta (part of the larger Lafourche delta complex) over the last 600 years, along with sea level rise, repeated storm impacts, and rapid shoreface erosion, have led to the formation of the Isles Dernieres. Continued transgressive submergenc combined with a diminishing sediment supply are driving the extreme coastal erosion found in the Isles Dernieres.

Accurate shoreline change maps were constructed using historic coastal maps and recent aerial photography and compiled on an Intergraph computer mapping system. The shoreline of the central barrier island arc at Whiskey Island has retreated 2,154 m in 135 years (1853-1988) at a rate of 16 m/year. The erosion rates have ranged between 10.1 and 18.1 m/year depending on the frequency of storm impacts. In 1988, Hurricane Gilbert eroded the Isles Dernieres beach more than 40 m. As well as rapid beach erosion, the area of the Isles Dernieres is decreasing. In 1853, the barrier island area was 7,750 ha.; by 1988, it had decreased to 771 ha. (a rate of 52 ha./year). In terms of perimeter measurements, the perimeter of the Isle Dernieres was 130,081 m in 1853. However, as the Isles Dernieres b came fragmented through time, the perimeter increased dramatically to 212,466 m, 194,417 m, and 202,219 m in 1933, 1956, and 1978, respectively. As land loss continued, especially after the 1985 hurricanes, the perimeter of the Isles Dernieres decreased to 137,417 m by 1988. If the current rates of erosion and land loss persist, the subaerial superstructure of the Isles Dernieres will be destroyed, leaving only a subaqueous sand shoal by the year 2004.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.