Nature-Based Solutions (Dec 2022)
A review of issues related to formation, deterioration and restoration of the Chenier Plain, Mississippi River Delta, LA - Combining nature based and engineered approaches
Abstract
Sand and mud reaching the Mississippi River Delta (MRD) via distributary channels created the delta lobes that, in aggregate, make up the current Deltaic Plain (DP). Sediments transported to the west in the coastal boundary current, in turn, have built the “downstream” Chenier Plain (CP). We review options for restoring the CP coast between the beach dune system and the first series of Cheniers that fronts the Gulf of Mexico (GOM) for ∼200 km to the west of the DP, and contributes 30% to total area of the MRD. The CP was built by marine rather than fluvial processes that have resulted in episodic westward and onshore transport of fine-grained sediments delivered to the inner shelf by the Atchafalaya River distributary. Long-term restoration of the MRD is currently proceeding under the Louisiana Coastal Protection and Restoration Authority (CPRA). Projects to stabilize the CP coast are being proposed and built to test whether coastal defense structures like breakwaters and groins designed to retard erosion on sandy coasts can be adapted for use on muddy shores where cohesive sediments predominate. An estimated 30 to 50 Mt y−1 of resuspended mud is moved past the CP coast by the coastal boundary current in a feature described as the “Atchafalaya Mud Stream.” Mid- to late-Holocene MRD lobes on the west side of the DP similarly contributed mud to the same part of the inner shelf. Then, as now, a portion of this near-shore suspended sediment flux was diverted into shore-welded mudflats that were colonized by marsh vegetation that offset local shoreline retreat. The CP is a regressive coastal feature, formed since sea level rise and marine transgression slowed, and it has experienced a 19% wetland loss since the early 1930s. Coastal retreat, up to more than 10 m y−1, prevails along more than half of the CP shore. Most structures built on the CP coast are constructed of lightweight materials to reduce settlement and lower deployment costs. If the generally positive results to date are validated, then designs can be optimized to increase transmissivity of high-density “fluid mud” (>10 g l−1) during low-frequency swell, while reducing energy transmission in the higher frequency band occupied by locally generated seas that seem to be most destructive to newly created mudflats and marshes. We show that restoration in the CP can be enhanced by taking advantage of the land building capabilities of mud stored in the nearshore and delivered to the coast during storms. The goal is to use engineered structures to increase the rate at which mud is deposited and retained at the shore, thereby harnessing natural processes to reinforce the coast. The use of dredged sediments for wetland creation augments coastal shoreline enhancement.
