Thesis (M.Sc.) - Cairo University - Faculty of Engineering - Department of Civil Engineering

Turbidity currents are sediment{u2013}laden gravity currents that exchange sediment with bed by erosion and deposition, as flow travels downstream. They constitute a major mechanism for the transport of sediment into sea and lakes floor. A 3D multiphase numerical model that simulates the dynamics and flow structure of turbidity currents is utilized. The governing equations are solved by Computational Fluid Dynamics code (FLUENT). Analysis of the spatial and temporal variations in turbidity current dynamics is made for confined decelerating depositional surge current and unconfined continuous current. Results revealed that in general turbidity currents consist of two distinct layers, an upper dilute layer and a lower dense layer near bed, and also have distinctive head and body. In case of surge like, confined turbidity current, fixed release of sediment mixture generates a turbidity current with initial constant flow front velocity (slumping phase) and then decelerates regardless of the initial flow sediment diameter. When comparing different initial grain size proportions indicates that grain size plays a dominant role in propagation and sedimentation of particle laden gravity currents. In case of continuous unconfined turbidity current model results indicated that all turbidity currents are supercritical through the inclined channel then changes into subcritical at the end of basin regardless of initial sediment concentration. Investigating the impact of various flow controlling parameters (bed slope, bed roughness, initial suspended sediment concentration, inflow velocity and suspended sediment diameter) in the hydrodynamic of turbidity currents and the extension fan deposited, showed that these parameters affect the flow dynamics with different degree. The model is up scaled using various scale factors, the results of this study indicated that the numerical model is capable of predicting velocity values for large scale prototype, where as values for near bed suspended sediment concentration will be overestimated.

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