Dynamics of a shallow, macrotidal, strongly stratified estuary [electronic resource]

Located at the coastal boundary between rivers and the ocean, estuaries are characterized by strong environmental gradients: gradients in salinity, temperature, turbidity, velocity, organisms, and other physical and bio-geochemical factors. Estuarine systems provide extensive biological and ecological functions ranking among the most productive habitats in the world. Additionally, estuaries contribute to extensive human uses ranging from recreation and food to wastewater disposal and land reclamation. These uses have contributed to estuarine degradation world wide. Understanding estuarine hydrodynamic processes can help predict the effects of human induced and natural changes to estuaries as well as help protect these ecologically important systems. This dissertation examines the dynamics in estuaries that lie outside of the commonly investigated partially and well-mixed estuarine regimes. I explore the dynamics of the Snohomish River Estuary, an energetic, macrotidal, shallow, strongly stratified estuary where the tidal range is similar to the total water depth and the advective length of the estuary is greater than the length of the salt-wedge intrusion. These features are critical in that they lead to violations of common assumptions employed in examining estuarine dynamics: the assumption of a small tidal amplitude relative to the mean water depth and the assumption of a background horizontal density gradient that is relatively constant in time and space. Many estuarine systems and tidal mudflats fit within a similar regime motivating the need to better understand these systems. In particular, I focus this dissertation on the analysis of both the intratidal (within a tidal cycle) and subtidal (residual/average over one or many tidal cycles) dynamics of the Snohomish River Estuary with the broader goal of connecting it to other estuaries within this strongly forced, strongly stratified regime, as well as importantly comparing it to the more commonly studied partially and well-mixed estuarine regimes. I investigate the intratidal variability of stratification, vertical mixing, and longitudinal dispersion as well as the local influence of a bathymetrically generated front on this intratidal variability. I then propose a method to interpret residual circulation in a system with such a large tidal amplitude to depth ratio such that I can consider the influence of intratidal dynamics on the residual flow. I observe some features common with partially and well-mixed estuaries such as the importance of tidal straining to intratidal dynamics and to the residual circulation as well as the importance of trapping mechanisms to frontogenesis and lateral dynamics. Likewise, I identify important areas where these estuarine regimes differ, such as the importance of interfacial mixing and the advection of stratification in highly stratified estuaries as well as describe the reasons for complex residual circulation profiles in estuaries with a large tidal amplitude relative to the mean water depth. Finally, I propose that a depth-normalized coordinate system may help reconcile differences between estuaries with vastly different tidal amplitude to depth ratios thus allowing their dynamics to be compared more readily to each other and to theory.

Advisor: Monismith Stephen Gene primary advisor. Thesis advisor
Fong Derek advisor. Thesis advisor
Fringer Oliver B. (Oliver Bartlett) advisor. Thesis advisor
Koseff Jeffrey Russell advisor. Thesis advisor
Stacey Mark (Mark T.) advisor. Thesis advisor