Impacts of rock sill design on sediment dynamics and marsh stability in living shorelines.

Rock sills are commonly placed at the seaward edge of constructed marshes in living shoreline projects to reduce erosion, stabilize shorelines, and improve coastal resilience. However, it is not clear how the design of these sills, whether continuous or segmented, affects sediment dynamics and marsh stability. In this study, we investigate two marsh sill living shorelines - one featuring tidal gaps and the other without - in comparison with an adjacent natural marsh in Ocean City, Maryland, USA. Our analysis focuses on changes in surface elevation, sediment and vegetation characteristics, and offshore suspended sediment concentration (SSC) through field survey. Building on previous research, we use the Delft3D-SWAN model to simulate sediment dynamics incorporating tidal gaps in the sills. Our drone surveys of surface elevation show a significant decrease within 1 m of the marsh edge and a noticeable increase beyond 2 m, suggesting that lateral marsh edge erosion supplies sediment for vertical accretion. Compared with the continuous sill living shoreline, we observe higher SSC in front of the natural marsh, along with higher sedimentation rates on the marsh platform. This indicates that while rock sills effectively reduce shoreline erosion, they may also limit the sediment supply needed to build vertical elevation. Segmented sill living shorelines with tidal gaps exhibit greater dynamism. Numerical modeling shows that erosion is more pronounced in marshes with segmented sills than in those with continuous sills. However, during high-energy events, the tidal gaps allow significantly more sediment to be transported to the marsh. Our study improves understanding of sediment dynamics in marsh sill living shorelines and offers insights for the practical design of tidal gaps.