Shoreline erosion is driving loss of coastal habitats and their ecosystem services in estuaries worldwide. Although hardened structures such as bulkheads and riprap have been historically used to mitigate these losses and protect coastal infrastructure, interest and investment in more natural approaches to achieve shoreline stabilization goals and sustain valuable ecosystem service benefits is escalating. Efforts to implement such living shorelines often fail in higher energy environments, however, where erosion may be particularly rapid. In this study, we present results from an on-going field experiment testing the efficacy of a new living shoreline technique designed to dissipate wave and boat wake action using brush-filled breakwalls, jumpstart oyster reef growth as a second line of shoreline defense using restoration substrates, and enhance salt marsh recovery. We also experimentally evaluate the mechanisms controlling biofouling and shipworm infestation of breakwalls to further improve the durability of this novel ecologically engineered living shoreline design.
Results/Conclusions
Monitoring and analyses of hydrodynamic and sediment transport processes revealed that boat wakes are a persistent force acting to erode salt marshes and oyster reefs along the Intracoastal Waterway in northeast Florida. Specifically, field measurements show that boat wakes exceeding 0.3m in height are particularly effective at driving salt marsh and intertidal mudflat sediment erosion during mid- to low-tide periods when these wakes shoal on the shoreline. However, the breakwalls dissipate this wake energy by >40%, on average, leading to sediment deposition on lower elevation intertidal mudflats and significant seaward expansion of salt marsh vegetation at higher elevations relative to control shorelines with no breakwalls. Oyster are growing rapidly on restoration substrates, indicating that the effects of these natural reefs on dissipating wave action and functioning as a second line of shoreline defense will increase further over time. Finally, we discovered that the bottom 20cm of the breakwalls are most vulnerable to shipworm infestation and biofouling, suggesting that the durability of this living shoreline design may be further enhanced through the strategic use of materials resistant to bio-erosion and fouling at the sediment interface. Together, these findings highlight that living shorelines can be engineered to succeed in high-energy environments.