The optimization of marsh terracing as a wetland restoration technique: Mitigating cohesive sediment erosion from wind driven waves

Abstract:

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 linear segmented ridges of sediment constructed from in situ material within open water areas of coastal wetlands in order 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, sediment shear strength, and suspended sediment concentration in marsh environments modified with terraces. Results collected at two study sites in coastal Louisiana between November 2018 and August 2019 indicate that the periods of greatest wave erosion and sediment transport potential, based upon soil shear strength, occurred during the passage of strong cold fronts in the fall, winter, and spring, which had a frequency of approximately 4–7 days. For reducing erosion of the adjacent marsh platform, the optimal terrace spacing was found to be (235°/55° & 270°/90°). Optimal marsh terrace spacing was found to be approximately 200 meters which inhibits wind-driven wave growth such that resultant shear stresses are less than the strength of cohesive sediments at the study sites. Thus, where terraces are implemented with marsh erosion as a primary objective, aligning the features to prevailing wind directions and increasing spacing by at least a factor of two is the optimal design.