Date of Award

2012

Document Type

Campus Access Dissertation

Department

Civil and Environmental Engineering

Sub-Department

Civil Engineering

First Advisor

Jasim Imran

Abstract

Submarine channels have been documented as ubiquitous features of continental slopes and fan systems.These channels with intricate meandering patterns and extensive levees are recognized as products of density-driven flows known as turbidity currents. The flow field and the morphodynamics of these channels, however, remain poorly understood due to dormancy of these systems during the present geologic time. Both laboratory and numerical modeling allow the study of submarine flow and morphodynamics in a controlled condition. The present study has two main focuses; (i) experimental study of the morphodynamics and stratigraphy of sinuous submarine channels, and (ii) numerical investigation of the flow field in sinuous submarine channels.

Experiments were conducted by releasing turbidity currents in two relatively large pre-formed sinuous channels with multiple bends, trapezoidal cross section, and different initial axial slopes emplaced in a large tank. A total of 29 and 21 runs were made for a low and a high gradient channel, respectively. The dominant mechanism of overbank flow in both cases was curvature-induced stripping. Numerical simulations were conducted using a 3-D model considering different centerline slope, cross sections, confinement, curvature, and inlet discharges to study secondary flow in sinuous submarine channels.

The following main observations are made from the experimental study:

(i) the deposition rate was significantly higher inside the channel compared to the over-bank area, especially in the upstream region; (ii) asymmetric channel cross sections developed due to higher deposition rate on the outer bank of the channel bends; (iii) an apron or sediment wedge with overlain bedforms appeared near the channel entrance that prograded and aggraded with successive runs; (iv) over-bank flow due to flow stripping at bend apexes resulted in lobe-shaped deposits; (v) channel narrowing and higher gradient increased the flow velocity during the latter runs; (vi) sediment deposited on the outer overbank area had similar grain size distribution as the channel thalweg; (vii) an initial steep gradient of the channel promoted higher deposition rate on the overbank area; (viii) a generalized polynomial model can satisfactorily describe the levee shape near a submarine channel bend.

The following main observations are made from the numerical simulations:

(i) the orientation of secondary flow in a submarine channel is influenced by both channel gradient and inlet densimetric Froude number; (ii) lateral convection resulting from the interaction between in-channel and overbank flows leads to the reversal of secondary flow in an unconfined channel at a lower discharge and lower channel slope than in a confined channel with the same dimensions; (iii) a sinuous channel with variable curvature provides a more favorable condition for the reversal of secondary flow compared to a constant curvature channel; (iv) reverse secondary flow occurs at densimetric Froude number above 1.

Share

COinS