Laboratory experiments on the coupling of oysters and breakwaters as a nature-based solution for coastal protection

Oyster populations within the Chesapeake Bay have been drastically reduced over the past few decades mainly due to unregulated human activities and diseases. Recently, regulations and restoration efforts have focused on restoring oyster populations while also considering their ability to provide ecosystem services, such as coastal protection, water quality improvement and many others. To promote oysters growth and the settlement of new populations, a recent technique used along the east cost of the US is the use of oyster castles. Oysters castles are concrete blocks, about 30 cm x 30 cm x 20 cm, which can interlock together in different shapes and combinations. They have proven effective in recruiting and retaining oysters and in promoting both vertical growth and horizontal expansion of oyster habitat. In this research, we aim to quantify the hydrodynamic differences that occur around these oyster castles during their early stage (i.e. castles without oysters), and with fully developed mollusks covering the surface of the castles. The experiments were conducted in a recirculating Odell-Kovasznay type channel at the Ecohydraulics and Ecomorphodynamics Laboratory (EEL) at the University of Illinois, with a 2 m long straight test section, 0.15 m wide and 0.6 m deep. We analyzed 4 different scenarios (with oysters and without oysters), varying the configuration of the oyster castle, the flow velocity, and the water level (emergent and submerged conditions). Oyster castles (both with and without oysters) were 3D printed at 1: 7 scale to fit the canal. Particle Image Velocimetry (PIV) was used for 2D flow characterization. Preliminary results showed noticeable differences in flow acceleration and flow separation atop the castles when covered with oysters, as well as an increase in generation and distribution of turbulent kinetic energy atop and around the oyster-covered castles. Magnitudes and spatial distribution of dissipation rates were also affected by the presence of oysters in both submerged and near-emergent conditions. Further research, in both unidirectional and oscillatory flow conditions, will quantify dissipation characteristics to provide relevant guidelines on the design and use of oysters-populated breakwaters as a viable nature-based solution for coastal protection.