Plan to fix Mississippi leak could also create land | Environment

The power of the Mississippi is evident far south of New Orleans, where the mighty waterway is forcing its way through an ever-widening gap in its banks – and provides the impetus for an innovative solution to the problem.

The gap, known as Neptune Pass, which holds nearly a fifth of the river’s water, is helping to form what one scientist calls “probably the largest new delta in North America.” But it has also become the focus of a debate between shipping interests and coastal conservation groups over the best use of the river.

The Army Corps of Engineers has been working to find a compromise, and a new plan being presented to the public aims to achieve such a compromise by incorporating elements already used in coastal restoration projects.

If the plan works as intended, it would greatly reduce the flow of the river entering the passage and limit the impact on shipping. However, it would still allow sediment to pass through. This could serve as a model as the state faces increasing land loss along Louisiana’s coast.

So far, the response has been mixed. While the Corps has shown a willingness to think more creatively to balance the competing interests, the current proposal does not include publicly available models and data that scientists say should be used to determine the solution’s potential effectiveness.

Alex Kolker, a coastal scientist at the Louisiana Universities Marine Consortium who has studied the strait extensively and says it is creating the largest new delta on the continent, emphasized this concern, pointing out that several factors must be taken into account.

“I think most of us in Louisiana would rather have more land than less, but at the same time, nobody wants a boat loaded with chemicals running aground,” Kolker said.

“So this might be a way to think about management and the need to get the science right. Right now I have more questions than answers.”

The Corps says it is working on a report on the modeling and will release it when it is ready. It was not clear if that would happen before the Aug. 31 public comment period for the plan’s draft environmental assessment.

For the Coalition to Restore Coastal Louisiana, which has been working on land loss since 1988, the new plan is cause for cautious optimism because it does not plan to close the passage completely and installs devices to support land development. James Karst, communications director for the nonprofit, called certain aspects of the plan “innovative.”

“We wanted that land survey tool to remain,” he said. “And it’s still in effect and would continue to be in effect under this plan.”

Where the river flows wilder

The reason why the seemingly simple task of closing a hole in the river bank is so controversial is because it was never as simple as it seems.

The debate surrounding this river is intertwined with many aspects of Louisiana’s past, present and future. As the river flowed freely and meandered thousands of years ago, it deposited sediment that eventually formed the land that later became southern Louisiana.

A catastrophic flood in 1927 and the need for a reliable shipping route prompted the Corps to embark on a massive levee-building project along the river’s banks that forever changed the geologic forces. This project, along with factors such as oil and gas development, triggered the devastating land loss that plagues coastal Louisiana today.

Since then, the state has found itself in a paradoxical situation: it needs the river’s sediments to replenish its disappearing wetlands, but the levees protect surrounding communities from flooding and ensure steady transport on one of the world’s most important transportation routes.

Neptune Pass is a little out of this situation. The levee system ends more than 15 miles upstream, and the Mississippi River flows more wildly down there, breaking its banks in several places.

The pass existed for years as a much smaller crevasse, but the floods of 2019 transformed it into a large-scale outburst, decreasing from about 6% of the river’s flow to about 16%, according to the Corps.

The Mississippi River was diverted so much that it slowed the flow in the main channel, allowing sediment to settle and build up. This forced the Corps to dredge in areas where it had not previously done anything to keep the channel deep enough for ships.

In addition, the current in the passage had become so strong that it posed a danger to navigation.

Previous proposals to close the passage have raised concerns among coastal conservationists. Just as the river previously created land, the passage has so far created an area of ​​several thousand hectares, Kolker says.

Some of the new land has come above the surface and its muddy streaks extend above the waterline, while other areas lie beneath and are slowly accumulating.

“Put on your loafers”

An earlier attempt at a compromise seemed to satisfy few. Coastal conservationists said it cut off too much from the flow of sediment and the chosen location of the sill meant that water could have eroded paths around it.

The Corps started over again, but in the meantime laid down two layers of rock to stabilize the bank and prevent further erosion, requiring nearly 150,000 tons of rock and costing $6.7 million.

The new plan calls for a rock sill at the entrance to the pass, with a notch in the middle to allow recreational boats, water and sediment to pass through. The notch would be 100 feet wide and 26 feet deep, rising at both ends and eventually reaching five feet above sea level at the shore of the pass. A total of 168,000 tons of stone would be used.

An interesting aspect of the new plan are the so-called Sediment Retention Enhancement Devices (SREDs), known in engineering jargon as Sediment Retention Enhancement Devices, which are made of mud, fabric and stone. They are designed to resemble terraces, form giant Vs and extend for 300 feet. They are to be built in Quarantine Bay, where Neptune Pass leads, and are designed to help slow water flow and deposit sediment for land development.

Total costs and construction period are not yet known.

The Corps’ goal is to restore flow in the passage to pre-storm surge levels in 2019 to eliminate the hazard to navigation, but the terraces add a unique layer to the project.

“This approach has been used to build land in coastal restoration projects,” Corps spokesman Ricky Boyett said of the terraces. “Basically, we’re applying it to a navigation project.”

To coastal conservationists, Neptune Pass is a costless river diversion, similar to the nearly $3 billion man-made Mid-Barataria Sediment Diversion the state has planned on the opposite side of the river. They point to Neptune Pass as proof that such diversions work.

“You can walk on land that you would have swum on five years ago,” Karst said. “So if you don’t believe the river can create land, go see for yourself and wear your slippers.”