Corps of Engineers Builds Underwater Sill in Mississippi River to Slow Saltwater Intrusion During Low Water Conditions

Under normal conditions, the downstream flow of the river prevents significant upriver progression of the salt water. However, in times of extreme low volume water flow, unimpeded salt water can travel upriver and threaten municipal drinking water and industrial water supplies. As such, the greatest risk that could occur with the saltwater intrusion is unsafe salinity levels at the intakes of municipal drinking water intakes in Plaquemines Parish.

To help combat this saltwater intrusion, the U.S. Army Corps of Engineers (USACE), New Orleans District, began construction of an underwater sill across the bed of the Mississippi River channel July 11, 2023, to slow further upriver progression of salt water from the Gulf of Mexico. They completed construction of the sill nearly two weeks later on July 23.

“Once the river volume begins to fall in low-water season, we regularly monitor the progression of the saltwater wedge on the Mississippi River,” said David Ramirez, chief of the New Orleans District’s Lower Mississippi River Management Branch. “The projected location of wedge’s toe in combination with current National Weather Service forecasts help us determine when we reach our triggers for constructing the barrier sill.”

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The intrusion of salt water into the river is a naturally occurring periodic condition, because the bottom of the riverbed between Natchez, Mississippi, and the Gulf of Mexico is below sea level. Denser salt water moves upriver along the bottom of the river beneath the less dense fresh water flowing downstream. However, this is the first time in the history of the New Orleans District Corps of Engineers that they have had to construct an underwater sill two years in a row to prevent salt water from creeping farther up the Mississippi River during low-water conditions.

USACE constructed a similar underwater sill in October 2022 at river mile 64, near Myrtle Grove, Louisiana, in Plaquemines Parish to arrest the progression of saltwater intrusion during that year’s low-water season. That particular sill eroded away when the Mississippi River returned to levels above 300,000 cubic feet per second, pushing the saltwater wedge back down the river to the Gulf of Mexico.

Weeks Marine’s hydraulic cutter suction dredge J.S. Chatry at work on the Mississippi River constructing an underwater barrier to saltwater intrusion. The Corps awarded a contract to Weeks Marine, which used a dredge, tow boats and a series of pipes to construct the underwater barrier near river mile 64, which is near Myrtle Grove, Louisiana, in Plaquemines Parish. USACE PHOTO BY RYAN LABADENS

This year’s sill was constructed at approximately the same location as last year, near river mile 64 near Myrtle Grove. The initial phase of construction brought the sill to an elevation of -55 feet, meaning that the top of the sill was 55 feet below the surface of the river water. Even after the sill reached that height, USACE has continued to monitor progression of the saltwater wedge to determine if the sill would need to be built higher to keep salt water from intruding farther upriver. Because of the continued low-water, low-flow conditions, the existing underwater sill was raised from a depth of -55 feet to a depth of -30 feet. A 620-foot-wide navigation lane was left at a depth of -55 feet to ensure deep-draft shipping continues along the nation’s busiest inland waterway.

USACE initially constructed the underwater barrier sill to create an artificial basin to delay the ingress of salt water beyond river mile 64 above Head of Passes, but as a result of the river’s prolonged extreme low-flow rate, the underwater sill was overtopped Sept. 20, 2023.

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“As a result of continued falling conditions, this existing sill was overtopped and the toe of the saltwater wedge has reached River Mile 69, near the community of Jesuit Bend,” said Col. Cullen Jones, USACE New Orleans District commander. “Our modeling indicates that by augmenting the existing sill, we can support state and local preparedness and response efforts by delaying further upriver progression of the salt water by approximately 10 to 15 days. As new information becomes available, we will reevaluate the projected movement of the salt water and share this information with our partners and the public for their preparedness, readiness, and response.”

USACE constructed similar underwater sills in 1988, 1999, and 2012 to arrest the progression of saltwater intrusion during low-water conditions for those years. Each of those sills naturally eroded when the Mississippi River low-water conditions abated and the water flow returned to the levels required to push the saltwater wedge back into the Gulf of Mexico.

This article featured in the 2023-2024 edition of America’s Engineers: The People, Programs, and Projects of the U.S. Army Corps of Engineers

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