  The settling of cohesive sediment is ubiquitous in aquatic environments. In
the settling process, the silt particles show behaviors that are different from
non-cohesive particles due to the influence of inter-particle cohesive force.
While it is a consensus that cohesive behaviors depend on the characteristics
of sediment particles (e.g., Bond number, particle size distribution), little
is known about the exact influence of these characteristics on the cohesive
behaviors. In the present work, three-dimensional settling process is
investigated numerically by using CFD--DEM (Computational Fluid
Dynamics--Discrete Element Method). The inter-particle collision force, the van
der Waals force, and the fluid--particle interaction forces are considered. The
numerical model is used to simulate the hindered settling process of silt based
on the experimental setup in the literature. The results obtained in the
simulations, including the structural densities of the beds, the characteristic
lines, and the particle terminal velocity, are in good agreement with the
experimental observations in the literature. To the authors' knowledge, this is
the first time that the influences of non-dimensional Bond number and particle
polydispersity on the structural densities of silt beds have been investigated
separately. The results demonstrate that the cohesive behavior of silt in the
settling process is attributed to both the cohesion among silt particles
themselves and the particle polydispersity. To guide to the macro-scale
modeling of cohesive silt sedimentation, the collision frequency functions
obtained in the numerical simulations are also presented based on the
micromechanics of particles. The results obtained by using CFD--DEM indicate
that the binary collision theory over-estimated the particle collision
frequency in the flocculation process at high solid volume fraction.
