Date of Award

Spring 5-10-2019

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Lauren Ross

Second Committee Member

Kimberly Huguenard

Third Committee Member

Aldo Sottolichio

Abstract

Secondary flows and mixing have been shown to influence sediment transport, the vertical salinity gradient, and the exchange flow in estuaries. Typically, these hydrodynamic properties are investigated with respect to time and depth and their variability across the estuary is neglected. However, recent studies have shown that secondary flows and mixing, along with mechanisms that influence them, can exhibit lateral variability. Additionally, the variability of these hydrodynamic properties has not been studied in macrotidal estuaries, where the increased tidal forcing could affect the strength and cross channel variability.

There are two primary objectives of this thesis. The first objective is to investigate the cross sectional and temporal variations in forcing mechanisms of secondary flows and analyze their impact on the observed flow structure. The second is to investigate the cross sectional and temporal variations in mixing and to link the observed patterns to variations in influencing mechanisms of mixing, such as density and vertical shear. To address these research objectives, in-situ collected data and numerical modeling techniques were utilized. Data include horizontal current velocities and TKE dissipation, which were complemented by salinity provided by the numerical model. These data were collected in the Gironde estuary located in southwestern France, a partially mixed macrotidal estuary.

The results indicate that the forcing mechanisms of secondary flows vary intratidally, with the baroclinic pressure gradient forcing a circulation pattern during flood tide and the combination of Coriolis and curvature driving an opposing circulation pattern during ebb tide. The magnitude of the forcing mechanisms vary across the estuary, and this variation is attributed to lateral variations in axial flows and density. Additionally, mixing, quantified through the vertical eddy viscosity, was found to vary across the estuary and exhibited an asymmetric pattern over flood and ebb tides. The lateral variation in mixing was attributed to the lateral asymmetry in peak axial flows, and the tidal asymmetry in mixing was attributed to temporal variations in TKE dissipation, shear, and axial flows. Observed magnitudes of mixing were found to be less than previous studies which was the result of a low dissipation to production ratio, instigated by elevated squared vertical shear.

These results imply that lateral variations in hydrodynamics of a macrotidal estuary with complex bathymetric and topographic features cannot be ignored. The sensitivity to bathymetry and topography suggests that changes in channel geometry could alter internal dynamics and have a wide spread effect on sediment transport and exchange flow.

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