Particle Dynamics

BBLs are often characterized by enhanced concentrations of suspended particles as compared to the water column above. Such nepheloid layers are generated by resuspension of particles from the sediment surface and subsequent upward transport. The potential of a turbulent flow to entrain sediment particles of size D is often described in a Shields diagram where empirical thresholds of sediment motion are provided as a function of a nondimensional shear stress d ā€” pĀ«2/((pf ā€” p)gD) and a particle Reynolds number Re* ā€” a*D/v,inwhichppis the particle density and g the gravitational constant. The d can be interpreted as the ratio between the lift force provided by the turbulent shear stress defined in eqn [3] and the gravitational force acting on the particle. While in suspension, the fate of the particle is determined by the balance between upward transport by turbulent diffusion and Stokes settling.

The quantitative characterization of resuspension and particle transport, however, is often complicated by cohesive properties of the particles. Cohesive particles require greater shear stresses to become resus-pended; moreover, they tend to form aggregates when in suspension, which alters their settling velocities.

The resuspension-settling cycles increase the contact area between particle surfaces and water and thus enhance the fluxes from and to the particles. In addition, suspended particles contribute to water density and locally enhanced resuspension, generated, e.g., by high near-bottom current velocities in the littoral zone or at the depth of the thermocline

(cf. Figure 1), may lead to the formation of turbidity currents.

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