Saltwater from the Gulf moving up the Mississippi River will cause the Army Corps of Engineers to add to the height of an underwater levee it built this summer to protect water supplies for Plaquemines, St. Bernard and New Orleans, the Corps announced Friday.
The earthen sill built in July in the river near Alliance will be elevated so its top will only be 30 feet below the water surface. But that still may not block saltwater from contaminating water supplies in upriver communities, officials warned, in part because a deep notch will be included in the elevation project to allow shipping to continue.
"Based off the current forecast, and if no action is taken, you potentially could see the saltwater wedge all the way up to the French Quarter," said Col. Cullen Jones, commander of the Corps' New Orleans District office at a news conference at its headquarters building. "But we have no intention of not taking any action."
The sill, now 45 feet high and built in an area where the river is 90 feet deep, will be extended outward beyond its present 1,500-foot length.
Lidar views of the saltwater sill built in the Mississippi River in July 2023. (Army Corps of Engineers)
But a 625-foot-long notch will be maintained in the sill to a depth of 55 feet below the water surface over the river's navigation channel to allow ocean-going vessels to continue to move up and down river, with special one-way traffic rules, Jones said.
Ocean-going vessels using the river are allowed to have a 50-foot-deep draft, deeper than the new elevated section of the sill if the notch was not there. The Corps did not give an estimate for the additional cost of elevating the sill. The July work cost $8.9 million.
The Corps hopes the increased height will slow the movement of saltwater upriver, but also is making plans to supply water in barges to areas that may still be affected by high saltwater levels.
This graphic explains where the saltwater sill is built and where it works. The toe of the saltwater, however, was at mile marker 55.8 on Wednesday, about eight miles south of the sill.
The Corps and the state Governor's Office of Homeland Security and Emergency Preparedness are helping arrange contracts to deliver reverse osmosis machinery to remove salt from water at local water treatment plants in Plaquemines Parish, and may also try to obtain similar equipment for water plants in St. Bernard and other locations upriver, if necessary.
Installation of reverse osmosis equipment should be completed at Plaquemines' East Pointe a la Hache water plant by early next week, said Plaquemines Parish President Keith Hinkley.
He said Jefferson Parish also is providing water to parts of Plaquemines through an 8-inch water line on the West Bank, and efforts are underway to supply water to east bank residents in the Braithwaite area with water from a St. Bernard treatment plant.
But he warned both those efforts would be affected by the saltwater moving north to St. Bernard and Jefferson parishes.
Colonel Cullen Jones is the commander and district engineer for New Orleans District U.S. Army Corps of Engineers. He met with the media to talk about the low river concerns in the Mississippi River on Friday, September 15, 2023. (Photo by Chris Granger | The Times-Picayune | NOLA.com)
Hinkley said the parish, with assistance of state agencies, has already distributed 1.5 million one-liter bottles of water to lower Plaquemines residents.
"In July, we constructed the underwater sill to impede the progression of saltwater and that system as been performing as designed," Jones said at a podium set up near the Carrollton Gauge in the river, which showed the water level on Friday in New Orleans was only 2.5 feet above sea level.
"However, based off of the extended conditions, and the fact we have no rain in the current forecast, we also see the potential for it to overtop (the sill) on or about the 22nd of September, and then eventually impacting the freshwater intakes at the Belle Chasse water treatment facility by the third of October."
The freshwater could reach the Belle Chasse plant's water intake by Oct. 3, and a St. Bernard Parish water intake by Oct. 8 without the elevation work. Officials didn't have an estimate of whether or when the top of the saltwater wedge will reach those locations with completion of the elevation work.
"As we move forward with augmenting the sill, I want to be clear that if the current conditions persist, it will not be able to prevent overtopping of the sill, but it can delay it," Jones said.
If the saltwater reaches plants in New Orleans, freshwater barged in could be added to the water supply to reduce its salt content, Jones said. Barges were provided by the Corps in 1988 and 2012 during similar low-river periods.
Jones compared this year's low river to 1988, the first time in the modern era when the Corps built a sill in Plaquemines Parish to block saltwater from moving upriver.
That year, the sill was built too late to keep saltwater from moving as far north as Kenner, which led to the Corps adopting rules now in place that triggered construction of the sill in July.
Present projections indicate that freshwater will be flowing downstream at rates as low as 130,000 cubic feet per second by October. That compares with a flow rate of 120,000 cfs in 1988.
Jones said the Corps has already considered and rejected reducing the flow of water from the Mississippi River into the Atchafalaya River at the Old River Control Structure as a way of increasing freshwater levels downstream.
Congress requires 30% of the Mississippi's water be transferred to the Atchafalaya at the Old River complex north of Baton Rouge.
Water levels in the Atchafalaya Basin already are also dangerously low, and additional reductions would just extend saltwater intrusion problems to that part of the state, he said.
