‘River Power’: New Mississippi River Canal Demands Corps Action | environment

A gaping gap in the banks of the Mississippi River in the southernmost reaches of the waterway widened to an entire canal, pumping huge amounts of water through it and prompting the Army Corps of Engineers to move to shut it down. But state officials are questioning whether it could be a blessing from nature.

The Corps decided to close the entrance to the newly formed “Neptune Passage” with rocks on the east bank of the Mississippi, across from Boras. A Corps spokesperson confirmed Thursday that slowing downstream water flow is causing navigational problems in the river’s main shipping channel. At the same time, the state is wondering if allowing the trail to continue diverting water into Breton Sound could help rebuild lost wetlands there.

The sediment carried by the channel is already forming new landmasses in Quarantine Bay, which is part of Breton Sound, and is rapidly filling landlocked Denasse Bay, according to Alex Kolker, a coastal research scientist at the Maritime Consortium of the Universities of Louisiana.

In a research survey of the new 850-foot-wide waterway last week, Kolker and Dallon Weathers, a surveyor at Delta Geo-Marine, found that up to 118,000 cubic feet per second of water and sediment was flowing through the channel, which may have deepened to as much as 100 Feet near the junction of the river.

060422 map of the neptune corridor rift

The fault connecting the river to Quarantine Bay is one of several small distribution channels formed along the eastern bank of the Lower River at Plaquemines Parish, piercing low levees and boulders set in place to keep most of the river’s water flowing south through the southwest and south. passes in the Gulf. In 2016, the channel was only 150 feet wide, but it has widened significantly since then, possibly as a result of repeated years of high river flows and numerous hurricanes.

Diverting part of the river’s flow eastward through the expanded channel slowed the flow of water in the river’s main channel, causing sediment to seep in, posing a threat to seagoing ships moving north toward New Orleans, said Corps spokesman Ricky Boyet. .

“As of May 25, we measured just over 16% of the Mississippi River flow diverted at this location,” Boyett said. “Recently, we discovered shoals south of the site in an area where we had never seen shoals before.”

The Mississippi River Shallow Water Channel

This map shows where sediments caused erosion in the Mississippi River Channel south of the New Neptune Pass. The Army Corps of Engineers had to have contractors dredging the area to keep the river clear of shipping. (Army Corps of Engineers)

He said the Corps had to have a dredge to remove the shoals from a distance of 7/10 miles along the western side of the main navigation channel in mid-May.

“This diminishing means we’re starting to see impacts on the deep navigation channel and we’ll need to close the rift,” Boyett said.

Contractors will place a rock blanket over the riverbank mud bed at the mouth of the canal, to prevent further erosion, starting in July. Then a rock dam will be built in the corridor east of the river bank line.

“While the primary purpose of this structure will be to address impacts on the navigation channel, we are studying features that will allow some water, sediment, and small vessels to continue to pass through the closure,” he said.

The result could be that the closure would include a 10-foot-deep threshold to allow some freshwater, sediment and small boats to continue using the canal to enter the bay, said Brian Haas, executive director of the Louisiana Coastal Protection and Restoration Authority. .

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Narrow Passage of Neptune in 2016

This Google Earth image shows that the Neptune Passage fault was still narrow in November 2016 (Google Earth, Professor Alex Kolker, LUMCON)

“We encourage the Corps to address this as an opportunity (to restore coastlines by creating new land as sediment flows through the channel), and not just as a potential problem for the Navigation Channel,” Haas said.

Part of this discussion should include a review of the many other channels that allow river water to flow east into Breton Sound, to see if some could be closed to increase water flow in the area where the shoals occur, which could allow more water to flow, he said. Through the New Neptune Passage.

According to Kolker, the river’s flow at Belle Chasse was measured at 776,000 cubic feet per second by the US Geological Survey.

Mardi Gras Pass, a rift that formed 10 years ago near the Bohemia Passage, pulls 25,000 cubic feet to the east. Ostrica Lux, just north of the New Neptune Pass, picks up 13,400 cfs, while the channels are near the Fort. St. Philip captured another 104,100 cfs.

This means that up to 36% of the river’s flow down Belle Chasse never reaches south of the new pass, slowing water flow and causing erosion.

The Neptune Pass is expanded in 2022

This Sentinel 2 satellite image shows that Neptune Pass has expanded significantly by the end of April 2022. (Sentinel 2, European Space Agency, by Professor Alex Kolker, LUMCON)

“This, again, highlights the need to move toward more comprehensive river management, as we consider preserving the longevity and resilience of the navigation channel, but also consider flow control as a tool for ecosystem restoration,” Haas said, as an opportunity, not necessarily a problem. .

Haas said that while the new channel presents an opportunity for unexpected land growth on the eastern side of the river, it is too early to say whether it will lead to any changes in the state’s broad coast restoration strategy.

“It is certainly an interesting event, however, to be able to observe and understand the strength of the river,” he said. “There are some underwater bars forming there that look a lot like Wax Lake (on the western side of the main estuary of the Atchafalaya River), and in other areas where rivers have created new connections to the coast.”

new land

Scenes apart from how the land in Quarantine Bay grows from the sediment that flows through the Neptune Pass. (Google Maps, from Professor Alex Kolker, Lumkon)

Equally important, Kolker said, is to look at this rift as part of a broader scientific review of the future of land masses along the river, particularly the bird-footed delta at the mouth of the river.

“The immediate event is the failure of a structure along the bank and the river exploiting the weak point,” he said. “But was this because the structure was not built or maintained properly, or was this part of a larger delta behind it?

“Some people think that the estuary might move backwards as the bird-footed delta sinks, and that might be part of the larger change that’s happening in the river delta,” he said.

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