Despite vocal opposition from the state and coastal advocates, an environmental analysis compiled by the U.S. Army Corps of Engineers seemed to support the closure of a newly-formed channel diverting part of the Mississippi River through Plaquemines Parish’s east bank.
An environmental assessment released Friday weighed the impact of closing most of Neptune Pass, the new river cut, against taking no action at all — and the Corps’ conclusion seemed to lean toward constricting the waterway.
“Without the proposed construction of the flow control feature, conditions would continue to deteriorate, resulting in an increased threat to navigation,” stated the report.
As proposed, the Corps would construct a stone control structure that would restrict the water flowing through Neptune Pass to a 100-foot wide, 10-foot deep opening, though the design won’t be finalized until after a 30-day comment period. A cost estimate will also come after the agency decides on a design.
That would be a major downgrade from its current size. Since 2019, the major channel has grown to about 850 feet wide and around 90 feet deep at some points, carrying more than an eighth of the Mississippi River’s flow.
As the river scours out Neptune Pass, land has begun to form where it empties out on the side into Quarantine Bay. An area that was once 10 feet deep is now just two feet deep, shallow enough to stand in. But on the river side, a slower Mississippi River has also allowed more sand and mud to drop out and build in the shipping channel, which forced the Army Corps to authorize emergency dredging earlier this year. That dredging cost about $530,000, according to Corps spokesman Ricky Boyett.
The Louisiana Coastal Protection and Restoration Authority and environmental groups advocating for Louisiana’s coast have argued that the Corps needs to recognize the value of the new delta forming at Neptune Pass’ outfall – a rare sight in a state losing about one football field of land every 100 minutes.
“We want to have the science behind the decision, to know what’s happening before there are changes made to the pass and try to manage for navigation and restoration,” said Amanda Moore, the National Wildlife Federation’s Gulf program director.
If it’s not possible to leave the channel open, critics of the Corps’ approach said the federal agency should consider other design options that would allow Quarantine Bay and other surrounding wetlands to maintain a stronger connection to the land-building power of the Mississippi River.
But maintaining navigation has long been the Corps’ first priority, and its environmental assessment found that the agency’s plan would eliminate the threat posed by Neptune Pass.
“In the absence of the proposed action, uncontrolled water and sediment flow would continue to be diverted from the Mississippi River, resulting in continued shoaling in the adjacent segment of the river,” stated the report.
The assessment also asserted that most of the land growth documented by satellite imagery was likely due to the redistribution of mud and sand that the river eroded from within the channel, rather than land built from new material carried in by the river.
But the state and advocates said that’s still an open question, and a new study is underway to try to answer it.
A new satellite images shows emerging mouths bars- teardrop shaped islands in #Louisiana. These is where Neptune Pass, a new channel carrying ~15% of the Mississippi River, meets Quarantine Bay. This may be one of the largest deltaic land building events in decades. pic.twitter.com/6lp96Mpynb
— Alex Kolker (@AlexSKolker) July 28, 2022
Alex Kolker, a geologist with the Louisiana Universities Marine Consortium, will lead the study alongside surveyor Dallon Weathers starting this week. They hope to finish the “mass balance study” by the end of the public comment period and submit the data to the Corps for consideration.
Kolker explained the mass balance study would use soundwaves, drone lasers and satellite imagery among other tools to compare how much sediment has eroded from the channel to the amount that has built up on the bay side. While it’s indisputable that some semi-permanent land has been built, those estimates will begin to paint a clearer picture of how it’s happening.
The 30-day public comment period opened on Sept. 16 and is expected to close on Oct. 16.
