With LMR Stubbornly Low, Corps Begins Construction Of Sill To Block Salt Water Intrusion

With the Lower Mississippi River languishing at around 3 feet at the Carrollton Gage and with salt water creeping up from the Gulf of Mexico, the New Orleans Engineer District has begun construction of an underwater sill of river sand across the ship channel.

Salt water is always in the river near its mouth, moving upriver in the shape of a wedge. But with the river’s extreme low stage and slow flow rate—the Mississippi River at Belle Chasse has been dipping well below 100,000 cubic feet per second (cfs.) of late—the wedge is traveling alarmingly far upriver, threatening the freshwater supply of Plaquemines Parish, in particular.

To combat the intrusion of salt water, the Corps and its dredging contractor, Weeks Marine, is constructing the sill at Mile 63.8 (above Head of Passes), just upriver from Myrtle Grove, La. During the construction process, carried out by Weeks Marine’s cutter suction dredge JS Chatry, the Corps will monitor salinity levels in the river to determine how high the sill will need to be built.

The U.S. Coast Guard has restricted navigation to one-way traffic in the vicinity of the sill, with the channel reduced to 500 feet wide and favoring the left descending bank.

The JS Chatry will work around the clock, seven days a week, to build the sill, with an initial height set at a depth of negative 55 feet, or 55 feet below the surface. The Corps will adjust the height of the sill up in 5-foot increments and will likely not need to go higher than negative 45 feet, which would reduce the available draft in the river by five feet.

“Draft restrictions may be issued as construction of the sill progresses,” the New Orleans District said in announcing the start of construction.

Already, Plaquemines Parish has installed a pair of reverse osmosis machines to supply water to its east bank communities and to the Boothville-Venice area on the west bank. Currently, the parish pipes freshwater from Belle Chasse south to Port Sulphur.

Low Water Extending To Red River

Low water isn’t just a Mississippi River issue in Louisiana. To the west, the Red River is low, particularly below Lock & Dam 1 (Lindy C. Boggs Lock & Dam) in Marksville. Lock 1 is currently closed to navigation for dewatering and maintenance, and there’s a growing concern that, when the lock reopens on November 12, the channel will be too shallow for navigation.

“Our main concern is for the CLECO power plant at Boyce, La.,” said Richard Brontoli, executive director of the Red River Valley Association, in a letter to the state’s congressional delegation. “Their fuel is delivered by barge and, after a two-month closure, they must replenish it for continued operation.”

Besides the power plant, Brontoli mentioned supplies bound for a fuel distribution terminal in Alexandria, La., and steel coils bound for the Shreveport-Bossier, La., region.

“If there is no navigation, CLECO will be in a serious situation and all industries will question the reliability of using navigation for transporting their products,” Brontoli said.

Brontoli is calling for the Vicksburg Engineer District to receive funding for emergency dredging by October 28 to allow time to execute a contract and dredge the channel below Lock 1. Below Lock 1, towing vessels can opt for either navigating down the Atchafalaya River or accessing the Mississippi River through Old River Lock.

The Red River Valley is in a drought, just like much of the United States, but the Mississippi River has a direct impact on water conditions below Lock 1 on the Red.

“The Mississippi River has the largest impact on the height of the Acme [gage on the Black River] and Lock 1 lower gage in low water periods,” Brontoli said.

Tennessee-Tombigbee Waterway Holding Pool

East of the Mississippi River, the Tennessee-Tombigbee Waterway is holding pool, thanks to its locks and dams, from the Jamie L. Whitten Lock & Dam in the north to Howell Heflin Lock & Dam in the south.

“While most waterways are subject to drought, we are much less impacted than the Mississippi River,” said Justin Murphree, the Mobile Engineer District’s project manager for the Tennessee-Tombigbee Waterway.

The Tenn-Tom offers a 9-foot channel, maintained by its 10 locks and dams, that extends from the Tennessee River in the north to the Port of Mobile in the south.

“While most boats from the Mississippi River will consider the Tenn-Tom a restriction in itself, we can verify that we are a reliable system for anything that can fit in a 600-foot by 110-foot lock chamber,” Murphree said.

“It’s a slack water waterway,” said Mitch Mays, administrator of the Tennessee-Tombigbee Waterways Development Authority and president of the Tennessee-Tombigbee Waterway Development Council. “The water might get low, but the channel is still open.”

Mays said the ports and terminals along the waterway have the ability to move cargoes from truck to barge, and there’s ample room for vessels to come down from the Mississippi and Tennessee rivers.

“The bottom line is the Tenn-Tom has capacity,” he said. “All the locks operate 24/7, and the channel is open.”