A multi-scale investigation of interfacial transport, pore fluid flow, and fine particle deposition in a sediment bed

We used X-ray difference microtomography (XDMT) and multi-scale lattice Boltzmann (LB) simulations to investigate the deposition of colloidal particles in a streambed from the micron scale to the bedform scale. Flume experiments were performed to deposit zirconia colloids from suspension into a sediment bed composed of glass beads and covered with dune-shaped bedforms. Following deposition, we extracted a series of cores over a single bedform, and analyzed the microstructure of colloid deposition patterns within these cores by XDMT. Colloids deposited primarily on the upstream sides of glass beads located at the upstream face of the bedform. Colloid deposits were also found in narrow pores and grain contacts, resulting from physical straining. We used the pore structure measured by XDMT as internal solid boundaries in pore-scale LB simulations of pore water flow in order to define a constitutive porosity-permeability relationship at the scale of the representative elementary volume (REV) for the porous medium. We then incorporated this information in a continuum-scale LB model to simulate hyporheic exchange flow at the bedform scale. The bedform-scale flow model was discretized at the REV scale, with the porosity-permeability relationship obtained from the pore-scale analysis used to represent the effects of micro-scale feedbacks between particle deposition, bed structure, and pore fluid flow. Colloids deposited rapidly in the subsurface, leading to a decrease in permeability and a modification of hyporheic flow paths near the bed surface. Colloid deposition reduced the mean stream-subsurface exchange flux, but increased the spatial variability in pore water flow, leading to higher exchange flux in some locations. Similarly, the mean hyporheic residence time increased after deposition, but the development of preferential flow paths led to more rapid exchange through some regions of the bedform. These results reveal how flow-boundary interactions, colloid influx to the streambed, and colloid filtration in pore spaces interact to produce structural heterogeneity in sediment beds.