Numerical investigation of marsh terracing design parameters to optimize performance as a coastal restoration technique

Marsh terracing is a newly implemented coastal restoration technique utilized within the Northern Gulf of Mexico, particularly the Louisiana Coast. Its application is intended to combat the devastating land loss rates currently occurring as a result of sea level rise, land subsidence, and anthropogenic alterations to the system. The technique involves dredging in-situ subtidal marsh soils and placing the excavated material into subaerial berms, or terraces, adjacent to the borrow pit. There is significant research addressing the ecological benefits of marsh terracing, such as increased marsh edge, enhanced bio productivity, and improved habitability for nekton and waterbird species. However, there is a lack of research investigating the proposed hydrodynamic benefits of marsh terracing, decreased shoreline erosion and marsh emergence. This study aimed to (1) quantify the ability of marsh terracing to reduce shoreline erosion, (2) create emergent marsh and (3) provide a set of metrics to assess project performance and determine the optimal terrace configuration for a specific site. The study site, Four Mile Canal Terracing and Sediment Trapping in Vermilion Bay, Louisiana, was analyzed through the cre-ation of a 2D numerical model using Delft 3D Flexible Mesh. Coupling of D-Flow and D-Waves allowed for the analysis of high-resolution flow and wave dynamics within terrace configurations. Following model creation and utilization, six hypothetical terrace configurations were examined using post-processing tools and developed metrices. Site specific conclusions were drawn for the terrace configurations within Vermilion Bay. However, the numerical modeling methods and performance metrices presented herein provide a methodology that can be used to determine the optimal configuration for any terrace project site and provide the most beneficial use of project funding.