Effect of tropical storm and frontal passage on marsh erosion in terraced coastal wetlands

Marsh terracing is a wetland restoration technique that has been widely implemented in coastal Texas and Louisiana over the last 30 years. Marsh terraces are segmented ridges of sediment constructed from in situ material within open water areas of coastal wetlands that are designed to mitigate marsh platform erosion through the reduction of wave fetch and resulting incident wave energy. The success of marsh terraces is, in part, a function of their capacity to reduce the erosive potential of wind driven waves relative to the mechanical strength of soils composing the adjacent marsh platform as well as the terraces themselves. We present the results of an extensive field data collection effort designed to assess the spatiotemporal relationship between wind conditions, wave parameters, water level, soil shear strength, and suspended sediment concentration in marsh environments modified with terraces. Data collected between November 2018 and September 2019 at two terraced study sites in coastal Louisiana includes time series of wind vectors, wave spectra, and turbidity as well as over 250 observations of terrace and marsh platform soil shear strength. The time series observations, which captured the passage of multiple frontal storms as well as Hurricane Barry, indicate that the primary driver of marsh erosion at the study sites is high-energy wind-driven wave events resulting from tropical storms and fronts. Whereas tropical storms winds were observed to have a greater magnitude and duration, frontal winds events were more frequent and exhibited more consistent wind fields. Results suggest that wave energy and sediment erosion resulting from tropical storm passage is roughly equivalent in magnitude to that resulting from frontal passage. Marsh terracing can be an effective means of mitigating wave erosion, as symmetry of marsh terraces allow wind driven waves to be attenuated effectively from multiple prevalent wind sources.