Project Focus
Dynamics of subaqueous sediment gravity flows

Background and Motivation
Sediment gravity flows occurs in a vast variety of environmental,geological, and industrial situations caused by both natural and man-made causes. Such flows may occur in both subareal and subaqueous environment in different forms, such as mud flows, debris flows, and turbidity currents. The main objective of this project was to understand the flow structure and dynamics of subaqueous fine-grained cohesive sediment gravity flows.  

Method
The research was carried out through laboratory experiments that involves releasing dense fine-grained cohseive sediment suspension (often referred to as 'fluid mud') under ambient water in a rectangular flume. Two types of sediments gravity flows were investigated: flows generated from releasing fixed-volume of fluid mud release (dam break, Fig. 1a) and from constant-supply of fluid mud under a gate (Fig. 1b).

Fig. 1a. Subaquesous fluid mud gravity flows generated by releasing a fixed volume of fluid mud. Fig. 1b. Subaquesous fluid mud gravity flows generated by releasing a constant -supply of fluid mud.

Fig. 2. 3D flow strucutre at the frontal zone of sediment gravity flows. Lobe-cleft patterns at the leading edge of the current (left figure), nose and billows above and behind the head of the current (right-top figure), and decay/breakdown of billows (right-bottom) behind the nose can be seen.

 


Key Findings/Contributions

• First, I studied the three-dimensional structures at the leading edge of the gravity flows in the experiments by recording using a high-definition video camera from the top and side of the tank with different oblique angles. The currents exhibited three dimensional flow patterns that consist of protruding regions (lobes) that are separated by sharp cusps (cleft) at the leading edge of the current and billows due to gravitational instability that form above and behind the head of the current (Fig. 2). The results of the study was appeared in a portfolio paper (Chowdhury et al., 2009).
• Then, I studied propagation characteristics of  fixed volume released fluid mud gravity flows theoretically and experimentally (Fig. 1a). All the phases (slumping, inertia- buoyancy, viscous-buoyancy) of the flows were investigated and modeled by the theoretical models. In paricular, the investigation showed that Fluid mud gravity flows have a non-Newtonian rheology and their transition from one phase to another and propagation characteristics in the viscous phase differ vastlyfrom their Newtonian counterparts.  The results of the study was appeared in Chowdhury and Testik (2010).
• Then, I studied propagation characteristics of  constant-flux released fluid mud gravity flows (Fig. 1b).The intial phase for those flows was a wall jet phase, followed by the inertia- buoyancy  and the viscous-buoyancy phase. I developed a box model for viscous propagation of the flows. Details of the study can be found in Chowdhury and Testik (2012).
• I have also contributed in a co-authored article (Yilmaz et al., 2014) in which we proposed frictional resistance parameterizations for fluid mud bottom gravity flows.