Turbulence modulation in non-uniform and unsteady clay suspension flows

de Vet, Marijke

Abstract

Cohesive sediment is common within natural environments, such as rivers, estuaries, shallow seas and deep oceans. High-magnitude, low-frequency events, such as storms, floods, and post-wildfire erosion, which occur more often due to climate change, can enhance the cohesive sediment supply to rivers. The higher sediment transport rates can have major impacts on water quality and aquatic ecosystems, including fish habitats, and also on channel morphology. Further, subaqueous sediment gravity currents are among the volumetrically most important sediment transport processes on Earth.
In contrast to non-cohesive sediments, suspended cohesive sediment particles may form larger particles, or flocs, when the distance between the particles is sufficiently small. Networks of flocs in the flow, i.e., clay gels, enhance viscosity and yield stress, and thus are a key control on flow turbulence. Research into steady, uniform clay flows indicates a close interaction between turbulent and cohesive forces, controlling the dynamic structure of clay flows.
Subaqueous sediment gravity currents can be classified into different flow types, similar to a clay flow classification scheme of open-channel flows. On one side of the spectrum, turbidity currents are relatively dilute flows, in which the particles are supported by the upward component of fluid turbulence; correlated to turbulent flows with characteristics of a turbulent, wall-bounded shear flow for open-channel flows. On the other side of the spectrum, debris or mud flows have limited internal turbulence and cohesive sediment provides grain support through yield strength; correlated to an almost fully suppressed turbulence in quasi-laminar plug flows for open-channel flows. Transitional flows bridge the gap between turbidity currents, i.e. turbulent flows, and debris flows, i.e. quasi-laminar plug flow. These transitional flow types contain transient turbulent behaviour.
The transitional flow properties of clay-laden flow are based on steady, uniform open-channel flow. However, open-channel flows and gravity currents are naturally non-uniform, i.e. varying in space, or unsteady, i.e. varying in time. The formation of bonds between cohesive sediment particles is a time-dependent (thixotropic) process and, therefore, clay-laden flows need time to adjust to spatial or temporal variations in flow velocity. Despite clay being the most abundant sediment type on earth, the present knowledge on turbulent dynamics of non-uniform or unsteady clay-laden flows remains limited, whereas it directly influences the sediment transport capacity.
To research the influence of suspended cohesive clay on changing flow dynamics under non-uniform flow conditions, new experiments were conducted using decelerating and accelerating clay suspension open-channel flows in a recirculating flume. The flows transition between clay flow types, with different degrees of turbulence enhancement and attenuation as the flow adapts to the change in velocity. The experimental results show that decelerating clay suspension flows have a longer adaptation time than accelerating clay suspension flows, which is potentially correlated to the formation of clay bonds in decelerating flows requiring more time than the breakage of clay bonds in accelerating flows. This hysteresis is more pronounced for higher concentration decelerating flows that pass through a larger variety of flow phases of turbulence enhancement and attenuation.
To research the adaptation time of clay suspension flows to unsteady flow conditions, new experiments were conducted using unsteady clay suspension open-channel flows. The flow velocity was adjusted with increments of 0.1 m/s after which the adaptation time to reach the equilibrium conditions was quantified. The experimental results show that accelerating, strongly turbulence attenuated, clay flows require more time than weakly or non-turbulence attenuated clay flows due to the time required for turbulence to penetrate the plug flow in order to break the clay bonds. Relative to turbulent flows, the adaptation time of decelerating flows is reduced as the flow evolves through clay flow types as the increase in clay concentration allows for a higher frequency of inter-particle collision. Adaptation time is then increased again with stronger turbulence attenuated flows as the increasingly dominant cohesive forces reduce inter-particle collision and consequently the formation of clay bonds takes longer.
To research the evolvement of non-uniform clay-laden gravity currents, new experiments with constant-flux flows in a submerged flume were conducted. Based on velocity measurements three different flow types are identified with increasing turbulence attenuation: a) turbidity current, b) turbulent plug flow and c) transitional plug flow. The combination of balance between turbulent and cohesive forces and the formation of clay bonds determines the evolvement of a gravity current. For low clay concentrations, entrainment of ambient water and the additional turbulence developed at the upper interface penetrates into the gravity current preventing the formation of clay bonds. Consequently, the gravity current evolves towards a more turbulent flow condition. For high clay concentrations, entrainment and turbulence generation remains in the outer region allowing the formation of clay bonds in the inner region. Consequently, the gravity current evolved towards a less turbulent flow condition as a plug flow develops in the inner region towards the bed.
Turbulent flow dynamics have a direct influence on sediment erosion, transport and deposition patterns. Research into non-uniform and unsteady clay suspension flows indicates a complex interplay in cohesive and turbulent forces as the flow adapts to changes in velocity, which likely affects erosional and depositional processes in a variety of fluvial and submarine settings.

Citation

de Vet, M. Turbulence modulation in non-uniform and unsteady clay suspension flows. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4240676

• Thesis Type: Thesis
• Deposit Date: Mar 15, 2023
• Publicly Available Date: Mar 15, 2023
• Keywords: Geography ; Environment
• Public URL: https://hull-repository.worktribe.com/output/4240676
• Additional Information: Energy & Environment Institute, The University of Hull
• Award Date: 2022-09