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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 39 (1989), Pages 403-411

Massive Sediment by Passing of a Wide Tidal Inlet; Cat Island Pass, Louisiana

Bruce E. Jaffe (1), Asbury H. Sallenger, Jeffrey H. List (2)

ABSTRACT

The Isles Dernieres is a rapidly eroding transgressive barrier arc located in central Louisiana about 90 mi (150 km) west of the Mississippi River. Much of the islands' Gulf shoreline has retreated over 0.6 mi (1 km) during the past 100 years. Today, the island area is less than 25 percent of what it was 100 years ago. To study the erosion of the arc, we used surface-modeling software to represent historical surveys of the region.

Comparisons of three combined bathymetric/topographic surveys taken of the Isles Dernieres barrier-island arc during the past 100 years show many expected features, such as shoreface retreat, inlet development and migration, and downdrift spit growth. An unexpected feature was a large volume of sediment that began bypassing a 6 mi (9 km) wide inlet system to the east of the Isles Dernieres sometime between 1934 and 1986.

Sediment bypassed the inlet at 13 to 26 ft (4 to 8 m) depths, depositing a 2.5 mi (4 km) by 11 mi (18 km) shore-parallel lobe of sediment along the path of transport. The volume of sediment in the lobe was 1.5 billion cubic feet (44 million cubic meters), more sediment than was deposited by longshore transport near the islands during the same period. Sediment supplied by the bypassing nourished the eastern Isles Dernieres, resulting in a decrease in both shoreface and shoreline erosion. The average shoreline erosion rate in the area supplied by the bypassing decreased from 36 ft/yr (11 m/yr) between 1890 and 1934 to 16 ft/yr (5 m/yr) between 1934 and 1986. If the bypassing continues, the rate of erosion of the eastern Isles Dernieres will be mitigated and the life of the arc prolonged. This decrease in the erosion rate will only lessen the rate of destruction of the islands; the trend of area loss in the Isles Dernieres is not likely to be reversed.


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