UC Berkeley

The water column at Point Sal, on the coast of California, is stratified by temperature, and internal bores propagate through the region regularly. We collected velocity, temperature, and turbulence measurements in the bottom boundary layer at a 30-m deep site for 2 months during summer 2015. We estimated the turbulent shear production (P), turbulent dissipation rate (ε), and vertical diffusive transport (T), to investigate the near-bed local turbulent kinetic energy (TKE) budget. The local TKE budget showed an approximate balance (P ≈ε) during the observational period, and buoyancy generally did not affect the TKE balance. On a finer resolution timescale, we explored the balance between dissipation and models for production, and observed that internal waves did not affect the balance in TKE at this depth. Characterizing the turbulence budget near the bed can lead to understanding and prediction of resuspension and transport of sediments.

Estuary marshes need bay-sourced sediment to keep up with sea level rise, and managing such crucial habitats requires numerical models of sediment transport. Over the course of 20 months in the field, we examined three parameters commonly applied as external inputs in numerical models. Our instrumentation measured tidal currents, water level, turbulent stresses, wave conditions, suspended sediment concentration and (briefly) suspended floc size at up to 4 sites at 1 and 2 m mean lower-low water on the 11-km wide mudflat of San Pablo Bay, California. Through these measurements, we computed bottom roughness, bed erodibility, and settling velocity. Bed roughness decreased by an order of magnitude from 10-5 to 10-6 at the more storm-influenced site during the stormy season. We attributed this to strong winter storms which last longer and are oriented on-shore. The bed erodibility parameter increased with the spring-neap cycle during periods with low storm activity, and increased up to 6 fold concurrent with winter storms. Suspended particle size (and settling velocity) decreased with greater tidal currents. The reduced settling velocity associated with a smaller particle size contributed to the higher suspended sediment populations observed during spring tides. With many sediment models being developed and implemented around the world, we focus on ground-truthing numerical models through our search for bottom roughness, erodibility, and floc size from field data. This characterization of sediment parameters will enable numerical models to be more physically grounded and predictive in environments like San Pablo Bay.

Stratification in estuaries is important for tidal exchange, vertical mixing, and sediment transport. In estuarine shallows, a framework of strain-induced periodic stratification (SIPS) is traditionally applied for interpreting the onset of stratification. We explored the mechanisms of SIPS as well as frontal advection for inducing stratification in the estuarine shoals of San Pablo Bay, California. Our instrumentation measured salinity, temperature, water level, and (at a subset of the sites) velocity at 5 sites across the bay over a period of 20 months. At a single site during a handful of the periods, we also measured vertical gradients in salinity. These measurements revealed that both SIPS and frontal advection were responsible for inducing stratification. When SIPS was responsible for creating stratification, it was more likely that mixing would erase it. When frontal advection was responsible, stratification was erased by advection and by mixing. A theoretical investigation of the lateral movement of stratification depth showed that mixing was more important than advection for allowing stratification along estuarine shoals. As the bed slope decreased, the role of mixing became even more dominant in controlling stratification. This spatial view of stratification revealed the possibilities for stratification and destratification along broad estuarine shoals.

Estuary margins are the connection between land and sea. At three different margin regions of San Francisco Bay, we explore flow, turbulence, and sediment transport. Measurements of velocity, Reynolds stress, salinity, temperature, and suspended sediment at these sites over 6 hour periods suggest that some sites are depositing upstream during high water, while other are eroding. Salinity stratification developed at some sites during mid-ebb, and the smaller tidal creeks showed a basin-scale seiche. While this seiche motion had a strong role in driving near bed velocities, its role in suspended sediment flux was minimal. From high-frequency measurements of vertical velocity and suspended sediment concentration, we computed settling velocity, however our investigation suggests that sediment advection remains too important at these sites to rely on this measurement strategy. This study suggests numerous paths for future work, which will further characterize the connection at estuary margins.

The objective of this dissertation is to explore parameters and processes pertaining to sediment transport in estuarine and coastal environments. Chapter 1 offers a brief introduction to the concepts of turbulence, sediment transport, and salinity stratification. Chapter 2 investigates the turbulence budget in the bottom boundary layer of the California inner shelf, in a region influenced by internal waves, as published in Ocean Dynamics (Allen et al., 2018). Chapter 3 presents field investigations on a seasonal scale of three parameters necessary for numerical sediment modeling. In chapter 4, we explore the balance between two stratification mechanisms in broad estuarine shallows, and discuss a framework for considering the destratification mechanism. Chapter 5 presents our results from a field study in San Francisco estuary margins, focusing on observed seiching as well as our ability to investigate particle settling. Finally, chapter 6 contains a summary of the findings from this work.