Date of Award

Spring 5-12-2018

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Kimberly Huguenard

Second Committee Member

Lauren Ross

Third Committee Member

Shaleen Jain

Abstract

Long-term material transport in estuaries is largely controlled by subtidal flow. Subtidal flows are driven by conditions specific to their geographic location, namely river discharge, tidal forcing, and wind. These conditions are further modified by mixing, curvature of the estuary, Coriolis, and friction. While many practices of measuring these various forcing mechanisms exist, the current literature fails to provide a standard for measuring turbulent mixing in well- mixed estuaries. As a changing climate affects these various forcing and modifying mechanisms via sea level rise and increased precipitation, the corresponding material transport scheme is expected to change accordingly. Subtidal flow and mixing dynamics in the Damariscotta River, a critical hub in Maine’s oyster-aquaculture industry, are investigated to explore how a changing climate may affect local dynamics. Multiple field surveys are performed to adequately characterize all three ‘reaches’ of the Damariscotta River, each characterized by unique bathymetric features, during varying river discharge and tidal conditions. In September 2016 and March - July 2017, a total of eight field surveys were performed during sequential spring and neap tides to cover both wet and dry seasons and a full range of tidal conditions. An acoustic Doppler current profiler measured current velocities and a shear probe microstructure profiler provided turbulent kinetic energy dissipation rates, density, and turbidity measurements at four locations across estuary. Results show that subtidal flow structure changes significantly between reaches, exhibiting a vertically sheared pattern in the lower reach and a mix of vertically- and laterally-sheared patterns in the mid- and upper reaches. These patterns are further investigated through an analysis of the subtidal momentum balance, which allows for the inspection of each forcing mechanism’s individual contribution to the observed dynamics. Lateral and longitudinal advection and frictional effects were found to dominate in the estuary, all of which increased in magnitude up estuary. Based on the momentum balance results, predictions for the dynamic response to sea level rise and increased precipitation can be made. Mixing conditions are also found to vary considerably between reaches with largest mean turbulent kinetic energy dissipation rates observed in the upper reach. These patterns exhibit increased tidal asymmetry up-estuary, indicating the possibility of significant intermittency. Intermittency in turbulence has recently received significantly more attention in the past decade as oceanic and atmospheric researchers become aware of the problems it poses on accurately measuring turbulence. A sensitivity analysis to dataset-size is performed link various scales of intermittency to tidal and hydrographic characteristics and identify how many profiles of turbulent kinetic energy are necessary to precisely represent turbulent mixing in well-mixed estuaries. Internal intermittency is found linked to regions of complex geometry, transitions phases of the tide, and regions of strong lateral and longitudinal straining of velocity shears. Appropriate recommendations of sampling technique are made for use in other like estuaries.

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