Phase-Resolved Wave Boundary Layer Dynamics in a Shallow Estuary

We present in situ observations of near-bed velocity profiles with high temporal and spatial resolution from a Nortek Vectrino Profiler deployed in South San Francisco Bay. Using Hilbert analysis, we ensemble-averaged near-bed velocity profiles by wave phase and calculated wave phase-dependent boundary layer thickness for varying wave and current conditions. We also applied mixing length relationships to derive a boundary layer thickness-based eddy-viscosity and compared this estimate to one obtained from the k- turbulence model. From the eddy viscosity estimates, we find that while turbulence responds instantaneously to shear, boundary layer thickness lags by a scaling estimate based on the turbulence response timescale. This analysis provides a method for wave phase decomposition of field-based velocity profile time series and shows that there is a finite-time response between turbulence dissipation and boundary layer thickness. Plain Language Summary We conducted field work in a muddy shoal of South San Francisco Bay to measure water velocity profiles within 2 cm of the bed. Motion in this region comes from waves and tidal currents; we present a new method for separating just the wave signal from a time series. We decompose the wave signal into bins determined by the instantaneous wave phase- and compare phase-binned profiles to predictions from theoretical models. For each of these phase-binned profiles, we calculate the size of the region in which the presence of the bed impacts flow characteristics. We investigate sources of discrepancy between observations and laminar theory, including turbulence and wave-roughness. Finally, we examine the relationship of these changing properties within the passage of a wave and describe the time it takes for the flow changes initiated by the wave to propagate upward. These results are important for understanding the contributions of waves to nearshore processes such as sediment transport and erosion.

Automated summary

In this study, near-bed velocity profiles were measured in South San Francisco Bay with high temporal and spatial resolution. Through Hilbert analysis, wave phase-dependent boundary layer thickness was calculated and compared to boundary layer thickness-based eddy viscosity derived from mixing length relationships. The analysis showed a finite-time response between turbulence dissipation and boundary layer thickness, with the latter lagging behind the former based on turbulence response timescale estimates.