Date of Award

8-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Civil Engineering

Advisor

Testik, Firat Y

Committee Member

Hayter, Earl J

Committee Member

Khan, Abdul A

Committee Member

Misra, Ashok

Abstract

In this theoretical and experimental investigation, the propagation dynamics of the fluid-mud underflows form in pipeline dredge disposal operations in the designated areas are studied. These areas such as wetlands, nearshore waters are typically covered with stiff, cylindrical, emergent aquatic vegetation. The main goals of this study are to propose mathematical formulations for the bottom shear force and drag force of emergent stiff aquatic vegetation acting on the gravity current, and to present novel propagation modeling approaches. To be able to predict these forces, a friction coefficient (for the bottom shear force), and a drag coefficient (for vegetation skin friction and pressure drag forces) are formulated. The friction coefficient is defined in terms of Reynolds number that is formulated for non-Newtonian gravity currents. The drag coefficient is defined in terms of cylinder Reynolds number, which is defined for the non-Newtonian flow around a pack of cylinders in an array. The power-law rheology model, which has been shown to model fluid mud rheology well, was incorporated in the theoretical analysis. To verify the theoretical derivations of friction and drag coefficients, a series of constant-flux release gravity current experiments is conducted in a rectangular laboratory flume. Based on the experimental data, a relation between the Fanning friction coefficient and the Reynolds number is proposed for gravity currents propagating over smooth surfaces. The proposed relationship includes a proportionality constant (henceforth, the shape factor) that considers the shape of the current. For the non-Newtonian fluid mud gravity currents, a relationship associated with the shape factor was developed through experiments. Different potential applications of the experimentally developed friction factor - Reynolds number relationship are discussed. In this regard, a new viscous propagation model was developed and evaluated through comparison with experimental data for fluid mud gravity currents. Moreover, the data of the experiments with vegetation models are interpreted to observe the effect of emergent aquatic vegetation on the propagation dynamics and the anatomy of the non-Newtonian fluid mud gravity currents. The experimental observations showed that the presence of the vegetation significantly affects the propagation dynamics, hence the anatomy, of the gravity currents. Vegetation-induced drag force dominates the resisting forces acting on the gravity current, forcing the current to transition into a drag-dominated propagation phase. During this propagation phase the profile of the gravity current exhibits a well-defined triangular shape. At the very early stages of the current-vegetation interaction, the slope angle of the upper interface of the current with the ambient fluid evolves towards an equilibrium value, which remains constant throughout the remaining of the current propagation through vegetation. The equilibrium value of the slope angle was parameterized in terms of fluid mud rheological characteristics and the vegetation density. The distance travelled within the vegetated area until the slope angle is converged is also formulated in terms of the flow, fluid and vegetation properties. The experimental observations on the anatomy of gravity currents, in particular instabilities formed at the gravity current head, during the drag-dominated propagation phase are also discussed. Using the experimental data with vegetation models, group drag coefficient for emergent cylinders is formulated in terms of the vegetation areal fraction, the flow behavior index of the fluid-mud and the Reynolds number for cylinders in arrays, which is also defined in this study, as a part of the theoretical analysis. Using the drag coefficient formulation, a closed form prediction model for propagation of gravity currents through emergent vegetation is proposed.

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