Investigation of Near-bed Transport Dynamics of Microplastics in Open-channel Flows

• Author / Creator

  Yu, Zijian

• Microplastics (MPs), defined as plastic particles smaller than 5 mm, are widespread in aquatic environments. However, their transport mechanisms in flow remain underexplored despite being critical for understanding their environmental transport and fate. This thesis addresses this gap by investigating near-bed MP transport processes, from initial transport stages (settling and incipient motion) to ongoing transport dynamics (continuous and intermittent movements).
  
  For MP settling, no generalized formula exists for the drag coefficient (Cd) that accounts for variations in MP physical properties (shape, size, and density). In this thesis, 1,343 MP settling data were collected from the literature. A new equation for Cd was developed using the dimensionless particle diameter (d⁎) and two shape descriptors, yielding an absolute error of 15.2%, significantly lower than that of existing equations (42.5-72.8%).
  The incipient motion threshold of MPs was next examined. MPs can either be exposed to flow, making them more prone to movement, or shielded by bed particles, requiring higher flow conditions for mobilization. For exposed MPs, the effects of bed roughness and MP properties on the critical shear stress (τc) were experimentally investigated. A new explicit equation for the critical movability number (Λc) was proposed, incorporating d* and a new dimensionless parameter related to MP physical properties, resulting in a smaller absolute error (12.3%) compared to a previous equation (55.6%). For hidden MPs, a power law relationship was identified between τc and the density and size of MPs, reducing error by 40% compared to a previous equation. To quantify sheltering effects, parameters for hiddenness (ΔZ), exposure (ΔH), and longitudinal exposure (ΔX) were introduced, leading to a semi-empirical method for predicting critical flow velocity (Uc), which reduced errors by 70% compared to previous methods for sediments.
  MP transport can be continuous or intermittent, depending on the flow conditions relative to the incipient motion threshold. Using particle tracking velocimetry, this thesis explored both movement types, focusing on streamwise transport. For continuous movements, the streamwise velocity of MPs (vx) follows a normal distribution. An empirical equation for vx was proposed, resulting in a low relative error of 5.2% compared to the experimental data. The continuous MP movements are super-diffusive, with particle inertia identified as the primary source of this anomalous diffusion. For MP intermittent movements, streamwise hop length (Lx) and hop duration (Ttr) follow exponential distributions, and vx exhibits a truncated Gaussian distribution. Approximately 60% of Lx corresponds to long hops, characterized by Lx Ttr. By applying a Lagrangian framework to describe vx and autocorrelation, it was shown that the preference for long hops is related to particle inertia, a fundamental characteristic of millimeter-sized MP transport.

• Subjects / Keywords

  • microplastics
  • settling
  • incipient motion
  • continuous movement
  • intermittent movement

• Graduation date

  Fall 2024

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-7k0q-hz70

• License

  This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.