Seasonal regulation of wave and current dynamics by a marsh sill living shoreline

The combination of low-crested, shore-parallel rubble mound breakwaters, also known as rock sills, and planted marshes on their shoreside forms a common coastal nature-based solution, usually referred to as marsh sill. However, uncertainty about their performance in wave attenuation has limited their widespread adoption when compared to conventional structure-based coastal protection. In this study, we conducted a ten-month field measurement to evaluate the functionality of a marsh sill living shoreline in a relatively sheltered bay in rural Virginia. The rock sill's was ∼1 m high and ∼1 m wide at the crest, with a 1:1.5 offshore slope, and the marsh was ∼6 m wide in the direction normal to the shoreline. Wave and current sensors were deployed at multiple locations to measure hydrodynamic variables, particularly wave heights and currents. Observations indicated that on average, the rock sill and marsh vegetation attenuate waves by 54% and 7%, respectively. Wave transmission through the rock sill was compared to existing empirical formulas that relate wave transmission to relative freeboard, relative structure crest width, and surf similarity, and new equations are proposed to describe the year-long field observations. Currents in the marsh behind the sill are 55% weaker than currents offshore of the rock sill. The marsh sill was found to be effective in reducing waves and currents consistently throughout the deployment.