Posted on December 12, 2022
A 2,000 cubic foot per second freshwater diversion from the Mississippi River to the Maurepas Swamp has been proposed to compensate for environmental impacts related to the West Shore Lake Pontchartrain Levee.
Details regarding the diversion were included in a final supplemental environmental impact statement released December 2 by the U.S. Army Corps of Engineers New Orleans District. The documents are available at https://www.mvn.usace.army.mil/Missions/Environmental/NEPA-Compliance-Documents/Bipartisan-Budget-Act-2018-BBA-18/West-Shore-Lake-Pontchartrain/ for a 30-day public comment period ending January 3.
The Maurepas Diversion has been recommended as the “tentatively selected alternative.” However, the Mississippi Valley Division commanding general will have the final authority on which mitigation plan to implement.
According to the U.S. Army Corps of Engineers, the conveyance channel would be located immediately west of Garyville and would encompass Hope Canal. Embankment features would be located in St. John the Baptist and Ascension parishes with barriers located within Bayou Secret and Bourgeois Canal in St. James Parish.
The diversion would be operated yearly, and findings from the supplemental environmental impact statement suggest the mitigation plan could generate 1,240 habitat units of coastal zone swamp annually in an area immediately adjacent to the levee alignment. This equates to 104,746 acres of improvements to the Cypress-Tupelo swamp north of I-10.
The benefits are expected to offset the negative impacts that construction of the diversion would have on 95 acres of coastal zone swamp habitat. Meanwhile, construction of the West Shore Levee is expected to impact up to 10,892 acres of swamp in the coastal zone.
According to the U.S. Army Corps of Engineers, the Maurepas Diversion would also increase resiliency surrounding the levee project. In January 2022, an additional $453 million was announced for the West Shore Levee to cover inflation and the inclusion of new resiliency features.
Other resiliency alternatives mentioned in the supplemental environmental impact statement included a St. James project that would have involved converting agricultural land into swamp habitat and a Pine Island project where shallow open water would have been converted into swamp habitat.
If the Tentatively Selected Alternative is approved as the Recommended Plan, only the benefits occurring on public lands would be counted toward meeting mitigation requirements. Construction features would include a conveyance channel, embankment features, and submerged rock rip-rap weirs; some of these features would occur on what is now private property.
The report states that the construction corridor would extend 5.5 miles north from River Road just west of Garyville, ending roughly 1,000 feet north of I-10. Plans call for boxed culverts under River Road, the Canadian National Railroad and Airline Highway, as well as a bridge over the channel at the Kansas City Southern Railroad.
The updated estimated total construction costs for the West Shore Levee project are $1,169,208, including the Maurepas mitigation.