Sedimentation and resistance tensor of a river floc from 3D X-ray microtomography

Natural sediment flocs are highly porous particulate aggregates composed of biogenic and minerogenic materials. They can be an important component of suspended sediment load in rivers, estuaries and the marine environment and modelling floc dynamics and behaviour is very important for many aquatic industries, maintenance of waterways and conservation and management of aquatic water bodies. X-ray computed microtomography has recently been applied to quantify the complex three-dimensional (3D) geometry of natural sediment flocs. Here, X-ray images of 3 selected natural millimetre-sized flocs sampled from the Thames River have been digitalised and converted into geometries used in Stokesian Dynamics calculations of the hydrodynamic properties of the flocs, where each floc is represented as a rigid ensemble of spherical beads moving in the creeping flow regime. Our approach is a substantial step from previous attempts in which synthetic fractal structures were simulated. In addition to describe the complex dynamics of floc settling, we compute: (i) the hydrodynamic radius of the flocs; (ii) the floc mobility and resistance tensors; and (iii) the relation between sedimentation velocity and fractal dimension. The simulations show that the coupling of gravitational forces with lateral velocities, which we analysed by examining the cross-components of the mobility matrix, produces a helical motion of the flocs as they settle. We argue that this lateral motion may lead to an enhancement of floc-floc aggregation by differential sedimentation due to an increase in the effective collisional area. Furthermore, the simulations demonstrate significant differences in the dynamics of settling between the three flocs despite a similar gross shape. Our work exemplifies how high-resolution X-ray techniques can be coupled with accurate particle-resolved simulations to understand the settling dynamics of real (as opposed to synthetic) flocs collected from estuarine, coastal or waste-water environments. • X-ray images complex-shaped river flocs are combined with Stokesian simulations to extract settling properties of river flocs. • Flocs studied have realistic geometries, rather than being synthetic fractal aggregates. • Flocs investigated perform a helical motion while settling; helical motion may increase probability of floc-floc encounters. • The full hydrodynamic resistance tensor of 3 selected flocs from the River Thames is investigated in detail. • The effect of fractal dimension on hydrodynamic resistance is analysed.