On the parameters of common settling velocity models for porous sediment aggregates

The settling behavior of sediment aggregates is a critical factor influencing the transport of fine-grained sediment in riverine and marine environments. Due to the small size and fragile structure of cohesive sediment aggregates, direct measurement of their porosity and permeability is challenging. While porosity often is estimated using settling velocity relations, permeability is frequently overlooked. The current study examines the impact of considering non-negligible permeability on the properties of flocs. Aggregate properties are compared by calibrating experimental data to two settling models in a dilute regime: one assumes a fractal structure of aggregates and neglects permeability, while the other assumes constant porosity and permeability. The current results demonstrate that both models describe the experimental data of highly porous aggregates with similar accuracy. Aggregate dynamics are further investigated in more complex flow conditions using numerical simulations applying a volume penalization method to geometrically resolve flocs. The behavior of permeable and impermeable flocs is compared in dense suspension regimes and during dilute settling in density-stratified environments. The current findings reveal that permeability significantly influences settling dynamics in complex scenarios and should be considered when determining aggregate properties.