Corps To Build Sill To Block Salt Water Wedge

For the second time in less than a year, the New Orleans Engineer District is planning to construct a sill across the Mississippi River to block the intrusion of salt water from the Gulf of Mexico.

“We have coordinated closely with our state and local partners, as well as incorporated lessons learned from last year’s low-water season,” said Col. Cullen Jones, commander of the New Orleans District. “We are confident we will see the same level of success with this salt water barrier that we achieved with previous efforts.”

According to the Corps, salt water moving up the Mississippi River in lower water conditions is a natural occurrence because the riverbed between Natchez, Miss., and the Gulf is below sea level. Salt water, which is more dense than fresh water, moves upriver along the bottom of the river in the shape of a wedge. In normal stages, the salt water wedge is held in check by the flow of the Mississippi River. When the river gets extremely low, though, the wedge travels inland, threatening municipal and industrial water supplies.

To block the upriver intrusion of salt water, the Corps, through a dredging contractor, builds an underwater sill of river sand across the ship channel. This will be only the fifth time the Corps has ever built the salt water sill. Prior occasions include 1988, 1999, 2012 and 2022.

At the time the New Orleans District made the announcement July 5, the Mississippi River at the Carrollton Gage was hovering between 2 and 3 feet, with daily dips below 1.5 feet and rises up to 3.5 feet. That’s a bit lower than in October 2022 when the Corps last built the salt water sill.

Near Myrtle Grove

Similar to last year, the Corps will construct the salt water sill near Myrtle Grove, La., using sediment from a designated site. The sill will take about two weeks to complete, yet afford some protection against salt water intrusion before completion. In 2022, the Corps built the sill at Mile 64 to an initial depth of 55 feet. That sill eroded when river flows increased last fall.

At the close of June, the toe of the salt water wedge was at river mile 54.4 (above Head of Passes).

“We are working to have the contract in place this week, with construction to begin mid to late next week,” said Ricky Boyett, a Corps spokesman.

The salt water wedge has been affecting Plaquemines Parish, which straddles the last 70 miles of the Mississippi River, for more than two weeks. The parish government and its water supply contractor, Inframark, issued a drinking water advisory June 21 for residents living from the Empire Bridge to the city of Venice. At that time, the parish’s Boothville water system had been experiencing elevated levels of sodium and chloride, consistent with salt water intrusion from the Gulf. The parish’s drinking water below Empire exceeded the Environmental Protection Agency’s maximum contaminant level for chloride (250 milligrams per liter), with sodium levels also spiking.

Beginning June 21, the parish has been offering residents free water and ice from the Boothville-Venice and Buras fire stations.