Study Reveals That Sand Piles Used to Restore Coast Also Contributed to Subsidence

A new study underscores the unique difficulties Louisiana faces in maintaining its fragile delta and keeping the sea at bay: Researchers found work to replenish an eroding shoreline by pumping onto it massive amounts of sand itself caused the land to sink.

The study looked at a large-scale restoration project on the Caminada Headland, a 14-mile long strip of beach and marsh considered vital in efforts to keep the Louisiana coast intact.

Over a two-year period, researchers found the headland – made up of soft river deposits and decaying organic matter – compacted from the weight of the new sand by as much as 1 foot.

The study was the first of its kind to measure such compaction, according to Harry Roberts, a Louisiana State University coastal scientist and lead researcher.

“Almost as soon as they put sediment there, it was noticed,” Roberts said about the compaction.

The Caminada Headland is one of the largest restoration projects undertaken in Louisiana.

The $200 million restoration project has involved barging and pumping sand from Ship Shoal, a vast bank of sand left over from an ancient barrier island located about 30 miles away in the Gulf of Mexico, onto the headland. The dune along the Caminada beach has been raised to about 7 feet above sea level, or by more than 3 feet from what was there before.

This long shoreline is considered so important because it protects the oil-and-gas hub of Port Fourchon and acts as a natural buffer holding off even worse erosion from taking place in the marshland north of the headland.

The study looked at what happened when 3.3 million cubic yards of sand from Ship Shoal was dumped on the western portions of Caminada.

Scientists have long conjectured subsidence can be caused when such work is done to add sediment, but it had not been measured before, Roberts said.”No one ever knew what the values were. Now we know what the basic framework is.”

He said the lessons learned at Caminada can be applied elsewhere on the coast. Louisiana is engaged in a multibillion-dollar enterprise to combat land loss throughout its coastal plain. Since the 1930s, the state has lost an estimated 1,900 square miles of land, an area the size of Delaware.

The study was done in conjunction with state scientists.

Chip Kline, the head of the Coastal Protection and Restoration Authority, a state agency overseeing coastal restoration, said engineers predicted the compaction from the weight of the sand would occur and accounted for that in their designs.

“The bottom line is that we predicted the subsidence and overbuilt to accommodate the settlement,” he said in a statement.

Roberts said the compaction should decrease over time as the land under the piled up of sand gets squeezed tight. He estimated that about 2 feet of subsidence could be expected over a 20-year period.