Degree

Doctor of Philosophy (PhD)

Department

Civil Engineering

Document Type

Dissertation

Abstract

The physical processes that route sediments from nearshore to the continental margin provide vital information to the global assessment of the geochemically important matter and the life in the ocean. Therefore, understanding these processes at the fundamental level will help develop accurate models that can be integrated into operational ocean models. Wave- and current-supported turbidity currents (WCSTCs) are one of the mechanisms that deliver sediments to the continental margin. WCSTCs are slow-moving turbidity currents where near-bed turbulence driven by strong surface waves and/or currents, tide- and/or wind-driven, maintain the turbidity current in motion. This study investigates the along-shelf current-supported turbidity currents (ACSTCs) over an erodible bed, where only the along-shelf current drives the flow, and sediment suspension is sourced from the ephemeral fine sediment deposits. To mimic ACSTCs, direct numerical simulations of a flow in a steady, turbulent, sediment-laden channel with a mild spanwise slope were conducted over an erodible bed. The primary focus of this study is to determine the effect of various sediment settling velocity, erosion parameters, and associated sediment-induced density stratification on total suspended sediment concentration, velocity structure, and turbulent characteristics of the ACSTCs. Specifically, this study aims to analytically and numerically investigates the transition of alongshore current-supported turbidity currents to self-sustaining turbidity currents over erodible seabed composed of fine sediment. Thus, a simplified depth-integrated dynamic equation is developed for suspended sediment concentration. The stability of the developed equation is analyzed both in itself and through temporal linear stability analysis. The analyses find two criteria for the inception of the aforementioned transition. Both criteria indicate that transition is found to reflect the competition between erosion flux, enhanced by the cross-shelf motion of alongshore current-supported turbidity currents, and the deposition flux. In addition, drag coefficient associated with cross-shelf motion of ACSTCs is formulated as a function of the Reynolds number, sediment concentration, sediment settling velocity, and the bed slope.

Date

8-31-2020

Committee Chair

Ozdemir, Celalettin Emre

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