Physical and Program Options for the Inland Migration of Louisiana's Coastal Wetlands in Response to Relative Sea Level Rise

Abstract

Louisiana contains over a third of the coastal wetlands in the contiguous U.S., but has seen a drastic reduction in total wetland area in the last century. This loss is especially troubling for coastal Louisiana where wetlands play a vital role in protecting and supporting the state’s economy and culture. Under natural conditions, coastal wetlands will move upland with rising sea level or sinking land. However, engineered structures and shore armoring, such as levees, seawalls, and bulkheads, impede this process. Advanced planning for wetland migration is needed to keep communities and infrastructure out of harm’s way from encroaching open water and to mitigate future wetland loss. This project investigates the potential for wetland migration in Louisiana through 1) the mapping and analysis of coastal wetland migration and 2) an examination of policy alternatives relevant to wetland migration. Wetland loss and migration were analyzed in Lafourche, St. Mary, and Vermilion parishes using the Sea Level Affecting Marshes Model. Moderate and less optimistic values of subsidence rates were modeled with constant global sea level rise projections to identify the impacts of dike and levee protection on wetland loss and the upland migration of coastal wetlands. The percent of wetland loss offset by wetland migration was calculated for each parish in an effort to aid in management decisions. It was found that wetland migration into dry land areas did not occur in any of the three parishes unless dike and levee protection of undeveloped dry lands was removed. The intensity of subsidence and the distribution of dry land greatly impact the overall benefits of allowing coastal wetlands to migrate into dry lands. This observation was exemplified in Lafourche Parish, which has a limited distribution of dry land and was modeled using higher subsidence rates than those found in both St. Mary and Vermilion parishes. Not only was the net loss of wetlands greater when dike and levee protection was removed in Lafourche Parish, but the total amount of wetland gain by means of wetland migration was incapable of offsetting a significant amount wetland loss. The policy analysis was developed with consideration for the framework of Louisiana’s Comprehensive Master Plan for a Sustainable Coast. An exploratory model was created to assist coastal managers and stakeholders in policy decisions regarding the migration of wetlands along Louisiana’s coastline. The model was constructed using five main criteria and six different policy alternatives. Policy criteria included wetland migration, flood risk, equity, adaptability, and political feasibility, and the policy alternatives assessed were rolling easements, density restrictions, transferable development rights, conservation easements, defeasible estates and voluntary acquisition. Applying the policy model in scenarios where dike and levee protection is removed reveals that rolling easements would only be appropriate in Vermillion or St. Mary parishes where potential for wetland migration is high. In Lafourche parish, where potential for wetland migration is low, an emphasis on minimizing flood risk suggests that transferable development rights would be the best alternative to pursue. Applying the policy model in areas where dikes and levees are present also favors transferable development rights as the optimal policy alternative. Moreover, this final demonstration suggests that a wetland migration policy can serve to address the CPRA priority of minimizing “induced risk,” while also adapting to changes in flood protection planning.