Modeling approach of coupling oyster restoration and submersed breakwaters to prolong gray infrastructures effectiveness

Sea level rise (SLR) and increasing storm frequency threaten coastal environments. Breakwaters, a common engineering solution to prevent erosion and allow deposition, are littered throughout coastal areas. This grey infrastructure provides little habitat value and biodiversity. As sea-level rises, breakwaters will become ineffective for wave attenuation and erosion control. As a natural alternative, oysters create three-dimensional, complex reef structures that attenuate wave energy and increase sedimentation rates, providing also a wide variety of ecosystem services. If coupled with breakwaters, oysters can maintain the efficiency of these grey structures over time as they are able to grow with SLR. Here, we studied the coupling of submersed breakwaters and oysters in a small cove of the Choptank River (MD, USA), through the numerical model Delft3D-Flexible Mesh. For the coarse part of the domain, bathymetry was obtained from the NOAA National Ocean Service (30-meter resolution Digital Elevation Model) while for the more detailed part, flights were carried out via Unmanned Aerial Vehicles to provided high resolution bathymetric data. We focused on the impact these nature-based reefs induce on sediment transport and wave damping in the face of SLR. The model was validated with water level data obtained by NOAA at the Cambridge (MD) station and by means of bathymetric surveys using GPS, conducted along transects in the cove at the Horn Point Laboratory - UMCES. We then simulated how breakwaters keep pace with SLR with and without growing oysters under different combinations of external forcing. Modeling results showed landward erosion at the foot of the breakwaters and seaward deposition, in agreement with field investigations, overestimated up to 30%. The oysters addition provided protection and effectiveness of breakwaters over time by damping up to 40% more of the incoming wave heights compared to the use of gray infrastructures alone, which would otherwise lose their effectiveness in reducing wave energy with SLR. This work underlines the importance of preserving the efficiency of gray structures on coastal protection. It shows how coupling oysters with these infrastructures can maintain their structural efficiency over time and provides management indications about recovery plans for coastal wetlands