Hydrologic Impact of Constructed Oyster Reefs on Flow Attenuation and Sediment Transport at an Eroding Marsh

Wachapreague, VA is a coastal town located on the East coast of the Delmarva peninsula, and historically has faced severe coastal flooding. The main marsh that mitigates Wachapreague’s flooding has been eroding over the past decades, and there have been recent efforts to reduce these erosion rates through the use of oyster reef based living shorelines. Since 2020, the southern shoreline of Wachapreague marsh has been lined with constructed oyster reefs, creating a protective barrier against wave attack. However, other living shoreline projects have had varying degrees of success in reducing erosion. This project quantified the success of Wachapreague’s constructed oyster reefs at reducing erosion, attenuating waves, and inducing fluid drag. Constructed oyster reefs are currently 3-4 years old at Wachapreague marsh, on average, and display signs of healthy oyster population and shell growth rates. These reefs heavily attenuated waves during low and mid tides, but during high tide increased wave height by approximately 5%. Significant wave height was directly correlated with turbidity, which is indicative of suspended sediment concentrations and sediment transport. Despite a lack of desired sensor precision, Wachapreague’s shoreline was recorded losing nearly 1 cm of sediment between June and July, 2024, with slight offshore deposition occurring between July and September, 2024. Additionally, marsh platform loss rates varied spatially across the marsh, with oyster reef installation most significantly slowing erosion rates along the shoreline’s western edge. Model results reveal that oyster reefs heavily attenuate flow below reef height, but flow velocity is likely increased near the fluid surface. Additionally, oyster castles have a region of high shear stress within the model’s front chambers along the bed surface. These shear stresses are sufficient to erode sediments found within these intertidal lagoon systems, indicating that constructed oyster reefs may undergo spatially differential erosion, as opposed to natural reefs the face more evenly spaced erosional pressures. Based on these findings, constructed oyster reefs are capable of lowering incoming wave energy during low and mid tide, reducing rates of shoreline loss, lowering SSC, and fostering healthy oyster growth. Additionally, these constructed reefs attenuate both flow and shear stress below reef height; however, oyster castles may face long term issues with differential erosion and reef subsidence. Therefore, while constructed oyster reefs can reduce rates of marsh platform loss, a different oyster substrate geometry is recommended for long-term reef health and success.