Lake Mechant Landbridge Restoration

MSMM was tasked by the Coastal Protection & Restoration Authority (CPRA) of Louisiana to perform all engineering services associated with the restoration of a sheet pile plug that was damaged in Terrebonne Parish during Hurricane Isaac in 2012. Steel Sheet pile Plug #2 was severely damaged, with a large portion of the 280’ long structure leaning over in the center, and since the original collapse, the condition of the sheet pile has deteriorated and become a boating hazard.  The plug was initially installed as part of the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA) of Louisiana to serve as a barrier to saltwater intrusion in order to improve the marsh behind the structure for environmental restoration habitat and for the multiple lines of defense theory for hurricane protection. The sheet pile plug crosses an access canal between Lake Pagie and Lake Mechant. The original plug was constructed using PDA-27 Grade 42 steel sheet pile sections to an elevation of 4 feet NAVD88. The sheet pile was installed to a maximum depth of -23.0 feet NAVD88. The plug is 282 feet long with earthen wingwalls construction at both ends to an elevation of 5 feet NAVD88. The wingwalls were constructed from dredged material and armored with articulated concrete mats on geotextile fabric.

As part of the design services for the project, MSMM completed an initial survey of the dilapidated structure and determined that a large scour hole had formed in the water bottom near the middle of the structure. The MSMM engineering staff determined that the structure would have to be re-located. Additional survey was collected to determine the extent of the scour hole and it was decided that the new structure required re-location outside of the scour hole footprint, approximately 175 feet to the North of the existing structure.  Multiple structures were evaluated, and it was determined that a combi-wall structure fortified with large stone at the toe of the structure would be utilized to replace the structure. The new structure was designed assuming a large storm surge and was fortified to withstand extreme wind and flooding conditions. The project design was finalized in late 2019 and all permits were obtained by the Summer of 2020. The project is currently out to bid, and construction is supposed to commence in late 2020.