Corps Announces Plans For Neptune Pass
The New Orleans Engineer District has released its environmental assessment (EA) and finding of no significant impact (FONSI) for its plan of action to reduce flows from the Mississippi River through Neptune Pass, an outlet on the east bank of the river between Mile 23 and 24 (above Head of Passes), or about 70 miles below New Orleans.
The crevasse dates back decades, but its size and the amount of the Mississippi River it captures has increased dramatically of late, particularly since high water events between 2019 and 2021. The distributary now diverts an estimated 15 to 17 percent of the Mississippi River’s flow, according to a study by a team led by Alexander Kolker, an oceanographer associated with the Louisiana Universities Marine Consortium and the Coastal Climates Institute. In high water conditions, that would make Neptune Pass similar in flow rate to the 10th largest river in North America, according to the study.
As the pass has grown in size and depth, sediment from the river and material scoured from the pass itself has settled into nearby Quarantine Bay, leading many to argue the crevasse is building free land for Louisiana, a state plagued by subsidence and coastal erosion.
The Kolker study found that the Quarantine Bay delta was 56 to 79 percent larger than the material scoured from Neptune Pass, “indicating that it is a net land building system.”
“These findings provide key insights that are critical to the restoration and safe management of the Mississippi River and its delta, the largest system of its kind in North America,” the study stated.
“What’s more, as Neptune Pass has formed, flow out of Southwest Pass has decreased,” Kolker said on social media just before the release of the study. “This means that Neptune Pass is contributing to shifts in the mouth of the nation’s largest river.”
It’s that threat to navigation in the Mississippi River Ship Channel and the river’s deep-draft outlet, Southwest Pass, that prompted the Corps of Engineers to study Neptune Pass and take action to protect navigation to Louisiana’s deepwater ports between Plaquemines Parish and Baton Rouge.
“The current, uncontrolled diversion is resulting in significant shoaling and the immediate need for dredging to maintain authorized navigational depths,” the Corps of Engineers stated in Environmental Assessment 589, published April 17. “In the absence of the proposed action, continued scouring within Neptune Pass would occur, resulting in an increase of flow being diverted from the Mississippi River and subsequent, increased shoaling within the river.”
The Corps document also said river pilots aboard deep-draft vessels had reported “experiencing suction effects as they transit the adjacent segment of the Mississippi River.”
“Without the proposed construction of the flow control feature, conditions would continue to deteriorate, resulting in an increased threat to navigation,” the report stated.
With that in view, the Corps has announced plans to construct a “Neptune Pass Inlet Structure” of stone at the entrance of the pass. The structure, constructed of around 330,000 tons of stone, will reduce the mouth of Neptune Pass from its current cross-sectional area of 32,000 square feet to 10,300 square feet, essentially returning it to its pre-2019 dimensions. The Corps is also studying the creation of sediment retention enhancement devices (SREDs) at the outlet of Neptune Pass in Quarantine Bay.
Construction is expected to start early this summer.
