Wave Phase-Decomposed Near-Bed Currents and Turbulence on the Shoals of South San Francisco Bay

Sediment transport is a ubiquitous process that largely controls contaminant fate and transport, nutrient cycling, and water column light penetration in estuaries. In order to predict estuary-scale transport processes, it is critical to accurately parameterize erosion from the bed. However, the physics of cohesive sediment erosion is poorly understood, in part because it depends on complex interactions between waves, currents, stratification, and sediment bed properties.

To help elucidate some of the controlling physics within the bottom boundary layer, we conducted field work in South San Francisco Bay during the summer of 2018 and present detailed observations of near-bed flow over the shallow eastern shoals. A profiling acoustic Doppler velocimeter logged three components of the velocity at 64 Hz for 12 minutes every hour, with 1 mm vertical resolution over 1.5 cm above the bed. These highly-resolved data provide measurements of the mean velocity, turbulent Reynolds stress, turbulent sediment flux, and wave momentum flux in and above the wave-current boundary layer. We present vertical profiles of these statistics decomposed by wave phase for various wave and current conditions during the study period. Waves are a major contributor to instantaneous velocity profile deviations from the 12-minute mean profile and cause corresponding variations in turbulence Reynolds stress, wave momentum flux, and turbulent sediment flux. The results highlight the wave phase-dependent nature of momentum and sediment fluxes near the bed, and offer insight into the dynamics controlling cohesive sediment erosion in estuaries.