Field Observation of Seasonal Wave and Current Dynamics around a Hybrid Marsh-Rock Sill Living Shoreline

The combination of low-crested breakwaters, or sills, and marshes forms a common and effective coastal nature-based solution. However, uncertainty about their performance has limited their widespread adoption when compared to conventional structure-based coastal protection. In this study, we conducted nearly one year of field measurements to evaluate the functionality of a hybrid structure and vegetation feature, commonly referred to as a marsh-sill, in a relatively sheltered bay in rural Virginia. The rock sill’s crest was ~1 m high, ~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 height 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.