Marsh Island’s protected shoreline gained ground while exposed reaches eroded

On the northeast tip of Marsh Island, shoreline protected by rock gained ground over two decades while an exposed section continued to retreat. That contrast is one of the clearest findings in CPRA’s June 2020 report on the Marsh Island Hydrologic Restoration project in Iberia Parish.

The state monitoring report, page 18, measured an average shoreline advance of 1.22 meters a year along the protected section from 1999 through 2019. The unprotected section lost an average 4.25 meters a year over the same period. Those are movements of the shoreline, not acres of new marsh.

The work addressed a landscape altered by oil exploration and moving water. CPRA’s report describes oilfield canals cut in the 1950s and spoil banks that later deteriorated or subsided. Openings in those banks allowed rapid tidal exchange between the bays and interior marsh, contributing to scouring and vegetation loss. The report also identifies erosion and subsidence in the island’s history.

Construction finished in December 2001, according to the report’s project history. The $2.9 million project included roughly 4,000 feet of rock breakwater, seven canals plugged with rock, an earthen canal closure and an additional closure at an oil-exploration canal. Its approximately 7,310-acre project boundary covered the island’s northeast tip, rather than the entire refuge.

Water levels responded differently within that boundary. CPRA found that the structures reduced water-level variability in the northern project area compared with a reference area. But the report rated its shoreline-erosion goal only partly met: protection worked along the rock-armored stretches while severe erosion continued elsewhere.

The report also cautions against crediting all land gains to the canal closures. A separate project, East Marsh Island Marsh Creation, added land within the area. CPRA attributed the improved overall land-loss trend partly to that later work.

By the June 2020 assessment, the remaining barrier between Lake Sand and West Cote Blanche Bay was a concern. Breaks had formed in the landbridge, and the authors warned that losing that separation would weaken the project’s ability to moderate water levels. Their recommendation paired continued upkeep with additional shoreline protection—a reminder that the useful life of a coastal structure depends on what happens around it.