Date of Award

8-9-2014

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

First Advisor

Alexander E. Yankovsky

Abstract

Estuaries are important systems that link fresh, inland waters to oceanic salt water, where they act to deliver large amounts of nutrients, sediments and pollutants into the ocean. Traditionally, the study of estuarine systems has been marked by difficulty owing to the complex hydrodynamics influenced by strong bathymetric changes, changes in tidal range, intricate geomorphology, among other factors; thus beginning to unravel the complexities of estuarine hydrodynamics will help to illuminate the nature of estuaries as well as to provide a foundation for their further study. In this study I focus on the transition zone from tidal to fluvial regime, which is defined as an area where tidal and river discharges are comparable. Recently, the transition zone has been the focus of attention as an important region within an estuary.

Tides are subject to frictional dissipation as they propagate inland through estuaries and river channels. Previous studies suggest that there is an enhanced tidal dissipation in the transition zone from a tidal to fluvial regime when the tidal flux and river discharge become comparable. The aim of this study is to understand the kinematics and dynamics within the transition zone. In particular, I hypothesize that there is an enhanced tidal dissipation in the transition zone due to (i) additive effects of tidal and river currents subject to the quadratic bottom friction, and (ii) to the presence of variable topography and enhanced bathymetric gradients in the transition zone. I analyzed time series of velocity profiles and bottom pressure that resolve the along-channel depth-averaged momentum balance in the transition zone of the Santee River, SC, USA. The following momentum balance terms are estimated: inertia (local acceleration), along-channel advective acceleration, pressure gradient, and bottom friction terms. Instruments were deployed in a 1-km long river reach characterized by a decreasing depth in the upstream direction from over 4 m to less than 2 m. Tides in the study area are predominantly semi-diurnal, flood-dominant. The leading terms in the depth-averaged momentum balance are found to be inertia, pressure gradient, and bottom friction. The pressure gradient and inertia dominate the momentum balance during the flood and subsequent current reversal from flood to ebb. However, during the ebb the pressure gradient is nearly balanced by bottom friction. A dissipative term is defined as a residual of inertia, advection, and pressure gradient force terms. I found that the dissipative term is comparable with the bottom friction term under steady river discharge. However, the bottom friction term underestimates the dissipative term when the river discharge exhibits abrupt variations. This yields a record-mean with a linear regression slope of 0.54. I hypothesize that the lateral eddy viscosity also contributes to tidal dissipation, especially when the pressure gradient force increases. Although tides are flood-dominant, most of the dissipation occurs during the ebb due to a superposition of comparable fluvial and tidal currents.

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