Observations of wave attenuation, scour, and subsurface pore pressures acrossbthree marsh restoration sill structures on a sandy bed

Abstract

With rising sea levels and more frequent exposure to extreme storms, coastlines worldwide are vulnerable to increased erosion and loss of natural marsh lands. In an effort to lessen these impacts, there is a growing practice of adapting hard or “gray” coastline protection techniques to more nature-based features that promote habitat and ecosystem health. Living shoreline marsh restorations utilize natural and naturebased materials to protect marsh shores from erosion while also allowing intertidal flushing to promote the health and diversity of the marsh. Our study investigates three types of living shoreline sill designs exposed to average and storm-energy wave conditions at varying water levels. The sills were designed to mimic constructed sills in practice (rock, oyster shell, tree root wads), but more generally vary in structure porosity and material dissipation potential. Large-scale laboratory experiments were conducted in the large wave flume at the O.H. Hinsdale Wave Research Laboratory. Wave transmission and reflection are used to demonstrate wave attenuation capability of each sill structure. Scour of the sill, bedload sediment transport rates on the seaward and shoreward sides of the sill, and sediment pore-water vertical hydraulic gradients were used to demonstrate the potential for sediment transport and liquefaction. Results will contribute to understanding the effect of sill material porosity and mass on structure stability, and the effectiveness of using green living shoreline sill structures in the continued effort to establish design criteria for living shoreline implementation.