Numerical Investigation of Marsh Terracing As A Coastal Restoration Technique

Marsh terracing is a new coastal restoration technique implemented within the Northern Gulf of Mexico, particularly the Louisiana Coast. Its application is intended to combat the devastating land loss rates occurring as a result of sea level rise, land subsidence, and anthropogenic alterations to the hydrologic system. The technique involves dredging in-situ subtidal marsh soils and placing the excavated material into subaerial berms, or terraces, adjacent to the dredged 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, which is hypothesized to promote decreased shoreline erosion and increased marsh emergence. This study aimed to (1) quantify the ability of marsh terracing to reduce shoreline erosion, (2) determine the potential depositional effects and subsequent marsh emergence, 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 creation 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 development and calibration, six generalized terrace configurations were examined using post-processing tools and developed metrices. This gave insight to various terrace performance aspects, including [1] the magnitude of wave energy attenuation experienced on the leeward coast, [2] the storm-induced coastline erosion rates and hypothetical amount of coastline saved during a localized wind event, [3] the estimation of depositional area and sedimentation patterns within a configuration in relation to the terrace area constructed, and [4] the optimization of project benefits to project costs. Site specific conclusions were drawn for the terrace configurations within Vermilion Bay, Louisiana. 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 further provide a strong foundation for future marsh terracing modeling efforts.