Phase-Resolved Shear Stress and Sediment Flux Profiles in a Combined Wave-Current Bottom Boundary Layer

Phase-resolved profiles of shear stress and turbulent sediment flux were collected in the wave bottom boundary layer of South San Francisco Bay. Observations revealed a transitional boundary layer influenced by benthic roughness, matching the phase variability of classic laboratory experiments but deviating in vertical structure. Wave friction factor measurements were evaluated against various parameterizations, finding that the Nielsen (1992) and Rogers et al. (2016) formulations were most accurate. The Grant and Madsen (1979) combined wave-current shear stress model accurately predicted the total bed shear stress, though negative bias increased with wave strength. The phase relationship between free stream velocity and bed shear stress was, on average, close to that predicted by theory, though phase behavior was possibly contaminated by measurement noise during weaker wave conditions. Finally, turbulent sediment fluxes exhibited complex vertical structure, with divergent behavior within and above the benthic canopy. These results provide important validation for popular bottom boundary layer models, while highlighting the complexities of cohesive sediment erosion over rough beds and in combined wave-current forcing.