New Orleans, La. – The U.S. Army Corps of Engineers released the final Environmental Assessment (EA) and Finding of No Significant Impact (FONSI) for the proposed Neptune Pass flow reduction structure on April 16, 2025.
The final environmental assessment, “Mississippi River, Baton Rouge to the Gulf of Mexico, Louisiana, Neptune Pass Rock Closure, Plaquemines Parish, Louisiana EA# 589 and Finding of No Significant Impact (FONSI)”, evaluated the potential impacts associated with constructing flow control structures in both Neptune Pass and Quarantine Bay.
The EA and FONSI evaluated alternatives to eliminate a navigational hazard in the Mississippi River at Neptune Pass, a natural crevasse which existed prior to 1985 but has increased in size and flow during recent annual high river events.
Based upon hydraulic and geotechnical analysis completed in December 2024, USACE determined that a phased construction approach of the inlet and outlet structures was warranted to further assess the real time effects on navigation during periods of high-river flow and to be able to plan efficient and cost effective follow up actions, as needed.
The proposed Neptune Pass Inlet Structure is a stone sill that would reduce the cross-sectional area at the outlet of the Mississippi River and the inlet of Neptune Pass. The revised design of the structure would include a modified, less restrictive stone inlet structure at the at the entrance of Neptune Pass that is similar to the proposed structure as described in draft EA #589 released in August 2024. The stone structure will reduce the opening at the inlet to Neptune Pass from its current cross-sectional area of 32,000 square feet to 10,300 square feet. The structure will be constructed with approximately 330,200 tons of stone.
The Corps will continue to monitor navigation conditions during and after Phase 1 construction of the inlet sill. Concurrently, USACE is performing analysis and design of additional features of the project as described in EA #589. If warranted, those additional features would consist of raising the Phase 1 stone structure to further reduce the cross-sectional area of the entrance of Neptune Pass and construction of Sediment Retention Enhancement Devices (SREDs) in the receiving outlet at Quarantine Bay.
The environmental assessment determined that, while unavoidable impacts would occur due to project actions within Neptune Pass and Quarantine Bay, the proposed action would not constitute a major federal action significantly affecting the human environment. Construction of the proposed action would result in the elimination of the present navigational threat within the river. Without the proposed construction of the flow control feature, conditions would continue to deteriorate resulting in an increased threat to navigation.
Construction for the project is anticipated to start in early Summer of 2025.
An electronic copy of the final report and its appendices are located on the New Orleans District web site at: https://www.mvn.usace.army.mil/Portals/56/docs/environmental/RPEDS/EA%20589%20FONSI%20Document%20Appendices%20-%20FINAL.pdf
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