Date of Award

Spring 5-6-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Advisor

Kimberly Huguenard

Second Committee Member

Kelly L. Cole

Third Committee Member

Lauren Ross

Additional Committee Members

Michael M. Whitney

Daniel G. MacDonald

Abstract

River plumes form at the river-ocean interface when fresh, buoyant river water merges with salty, dense ocean water and can significantly modify coastal water properties and circulation. It is important to understand how plumes physically mix into the ocean to inform predictive modeling of river-borne tracers to coastal seas. In tidally energetic regions such as New England, river plumes can form and evolve with each new tide and are referred to as “tidally pulsed”. In this dissertation, we explore the numerous mechanisms which can contribute to mixing tidally pulsed plumes (i.e., frontal, stratified shear [interfacial], and bottom-generated tidal mixing) their spatiotemporal variability, controlling processes, and the relative importance of each to plume dilution by utilizing numerical modeling and field observation techniques. The contributions of frontal, interfacial, and bottom-generated tidal mixing are first investigated using an idealized numerical model broadly inspired by the Connecticut River plume. A mixing budget is applied, and river discharge and tidal amplitude are varied between experiments to isolate the influence of each forcing on the budget. Results indicate bottom-generated tidal mixing can dominate the mixing budget for large tide, small discharge events, when the product of the nondimensional Estuarine Richardson number and inverse Rossby number (𝑅𝑖􀮾𝑅􀯢 􀬿􀬵) exceeds 1. When the nondimensional parameter is below 1, interfacial mixing dominates. Frontal mixing was found to never exceed 10% of total mixing in the budget. This is the first study to identify the potential for bottom-generated tidal mixing to dominate mixing in surface-advected river plumes. Wind controls on stratified shear mixing in tidal plumes is investigated using a realistic model of the Merrimack River plume system. A salinity variance approach is applied, allowing for the quantification of stratifying and de-stratifying processes (straining, mixing, advection) throughout the tidal plume. Winds countering the right-turning tendency of the plume are found to be most effective at increasing plume mixing. During the wind events, ambient shelf stratification is advected offshore, which creates a saltier shelf condition beneath the plume and increases the vertical salinity gradient. Simultaneously, plume layer velocities are enhanced, increasing shear and straining. The larger salinity gradient between plume and ambient coupled with increased shear leads to enhanced stratified shear mixing in the near and mid-field plume. The wind mechanism was found to be effective at modulating mixing at short, tidal time scales. The evolution of stratified shear mixing throughout the interior Merrimack River plume is characterized using observational data. Three source-to-front transects were conducted over a ~6-hour tidal pulse during low wind conditions. Data collection on each transect included continuous sampling of current magnitude and direction supplemented by profiles of turbulent kinetic energy dissipation rates and conductivity, temperature, and depth (CTD). Analysis shows stratified shear mixing transforms spatially and temporally over a tide and is characterized by three distinct regimes: plume layer mixing, nearfield interfacial mixing, and tidal interfacial mixing. Plume layer mixing is confined within the plume and decreases offshore of the nearfield as the tide progresses. Nearfield interfacial mixing facilities exchange between the plume and underlying ambient shelf throughout the tidal pulse. Tidal interfacial mixing mixes plume with ambient waters offshore of the nearfield at the end of ebb tide when shelf currents reverse direction beneath the plume. These observations provide some of the most robust spatiotemporal plume mixing estimates to date. This dissertation highlights the highly variable nature of mixing in tidally pulsed river plumes and the oftenimportant influence of the ambient shelf condition on mixing. Winds and tides impact the collective plumeshelf system to varying degrees which subsequently modulates mixing in a spatiotemporally varying manner. Analyses of static locations or times likely omit essential processes contributing to mixing. This research provides important context for future coastal model development.

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