Pecan Island terraces built marsh, then struggled to keep it

Earthen terraces south of Pecan Island created marsh, but much of the early gain did not last. In its final report on the Pecan Island Terracing project, issued in March 2022, CPRA described a restoration effort that established vegetation yet could not sustain land building against storms and erosion.

The report’s aerial-photography analysis, page 12, tracked land in Area 1 from 46 acres in 2001 to 240 acres in 2004, then down to 110 acres in 2018. These figures describe that defined part of the project south of Highway 82 in Vermilion Parish. They are not land totals for Pecan Island as a whole.

This particular landscape had an agricultural history. CPRA’s account says marsh in Area 1 was converted to dry pasture in the late 1950s by building perimeter dikes and draining the interior. Later levee deterioration, trapped saltwater and loss of soil elevation helped turn it into shallow open water. The report’s explanation is distinct from the history of oilfield access canals elsewhere around Pecan Island.

The restoration design placed earthen terraces across shallow water to shorten the distance over which wind could build waves. It also sought to create marsh-edge habitat and allow vegetation to spread. Gaps between the terraces permitted water movement, according to the report’s project description.

The initial construction created 201 acres of terraces in Area 1. CPRA found that the project met its first goal of creating more than 100 acres of marsh, but not the further goal of increasing land by more than 300 acres. Losses followed Hurricanes Rita and Ike, and vegetation did not spread fast enough to overcome erosion.

In its conclusions, CPRA identified a design problem as well as the storm damage: terraces were likely spaced too widely. Cut banks on both sides of interior terraces indicated that wind-driven waves could erode them from within the grid. Without a consistent source of sediment, the marsh could not rebuild what it lost.

The report still found value in the remaining habitat. Submerged aquatic vegetation persisted in 2020, even after its early increase had faded. The authors recommended considering stronger outer protection and less open water between terraces. Their findings show why creating land at construction and keeping it decades later are different tests of a restoration project.