226 - Sediment Transport and Retention in Complex Biological Canopies: Applications to the Design of Natural Infrastructure

Mechanisms of sediment transport, specifically the contributions of shear stress and turbulence to mass transport, are challenging to predict in complex flow fields, such as those found within complex aquatic habitats. As biological canopies of vegetation, shellfish or corals are often features of nature-based infrastructure, better understanding of shear stress-turbulence relationships to sediment transport across varied canopy types can improve the design and performance of natural infrastructure. In this field study, near-bed turbidity and flow were measured simultaneously within complex biological canopies (mangrove forest and oyster reef) and in areas without canopy (bare sediment). Turbidity measurements were converted to suspended sediment concentration (SSC) by long-term calibration of measured turbidity and SSC. Preliminary results indicate that ranges of turbidity measured within biological canopies (10-25 FTU) were larger than those observed outside of canopies (15-20 FTU) and flows observed in canopies were more turbulent. Vertical turbulent energy (10-4-10-6 m2/s2 in canopy, 10-5-10-7 m2/s2 outside of canopy) and dissipation (10-5-10-7 m2/s3 in canopy, 10-6-10-9 m2/s3 outside of canopy) were over an order of magnitude greater within the biological canopies. However, during times of low velocity, turbidity within canopies was lower than over bare sediment. These results suggest that canopy-mediated turbulence may influence sediment transport, but perhaps only under certain hydrodynamic conditions. Improved models of sediment transport in canopies can be applied predict potential morphodynamic changes under varied designs of natural infrastructure to assess hazard mitigation potential.