Field and Numerical Studies of Near-Bed Aggregate Dynamics
191
Here the subscript ld indicates a linear increase in the BBL and a decrease above.
According to Gust (1989), the height-dependent distribution of the energy
dissipation rate (£) in the boundary layer is given by
3
u.
£(z)=-,
kz
(7)
closely related to the energy dissipation is the size distribution of the turbulent
eddies in the boundary. The Kolmogorov length (.\) describes the smallest possible eddy size and is calculated as
[
)
0.25
A (z)= ~: '
(8)
with v as kinematic viscosity. In the presented model .\(z) is used to parameterise aggregate disaggregation.
5
Aggregate Interaction
Generally, three mechanisms, Brownian motion, turbulent aggregation and differential settling, determine physicochemical aggregate production in the BBL (McCave 1984). All three processes are strongly dependent on size distribution, abundance of aggregates and in the case of turbulent aggregation on the hydrodynamic
regime. In the diffusion-advection equation [Eq. (1)] aggregate size classes are defined using the settling velocity. The aggregate size of each class is derived using
the relation of settling velocity and aggregate diameter Ws = 1.73 DO. 78 (Gibbs 1985;
see legend of Table 1). Assuming spherical shape and a density of2.65 kgdm- 3 , the
mass (Mi,j) of aggregates of each size class is determined. At each time step aggregate abundances [Nj(z)] are calculated by dividing Cj(z) by the aggregate mass M j .
Whether and how much aggregates interact strongly depends on aggregate encounter probabilities. The significance of the aggregate encounter processes
changes with distance to the sediment surface (McCave 1984). Depending on the
concentration distribution and the hydrodynamic regime, aggregate interaction
becomes a function of height above the seafloor (z) in the presented numerical approach. In the water column and the upper BBL, differential settling dominates aggregate interaction, a process which describes the scavenging of small suspended
aggregates by larger faster-sinking aggregates (Hill and Nowell 1990; Hill 1992).
The ensuing encounter rate (K ds ) is proportional to the square sum of diameters,
to the absolute difference of settling velocities of the involved aggregates and to aggregate abundance. It takes the form (e.g. Hill and Nowell 1995):
Kd (Z)=TC(d.+d.)2Iw ·-w ·IN.N.,
5
4
I
J
51
5J I J
(9)
191
Here the subscript ld indicates a linear increase in the BBL and a decrease above.
According to Gust (1989), the height-dependent distribution of the energy
dissipation rate (£) in the boundary layer is given by
3
u.
£(z)=-,
kz
(7)
closely related to the energy dissipation is the size distribution of the turbulent
eddies in the boundary. The Kolmogorov length (.\) describes the smallest possible eddy size and is calculated as
[
)
0.25
A (z)= ~: '
(8)
with v as kinematic viscosity. In the presented model .\(z) is used to parameterise aggregate disaggregation.
5
Aggregate Interaction
Generally, three mechanisms, Brownian motion, turbulent aggregation and differential settling, determine physicochemical aggregate production in the BBL (McCave 1984). All three processes are strongly dependent on size distribution, abundance of aggregates and in the case of turbulent aggregation on the hydrodynamic
regime. In the diffusion-advection equation [Eq. (1)] aggregate size classes are defined using the settling velocity. The aggregate size of each class is derived using
the relation of settling velocity and aggregate diameter Ws = 1.73 DO. 78 (Gibbs 1985;
see legend of Table 1). Assuming spherical shape and a density of2.65 kgdm- 3 , the
mass (Mi,j) of aggregates of each size class is determined. At each time step aggregate abundances [Nj(z)] are calculated by dividing Cj(z) by the aggregate mass M j .
Whether and how much aggregates interact strongly depends on aggregate encounter probabilities. The significance of the aggregate encounter processes
changes with distance to the sediment surface (McCave 1984). Depending on the
concentration distribution and the hydrodynamic regime, aggregate interaction
becomes a function of height above the seafloor (z) in the presented numerical approach. In the water column and the upper BBL, differential settling dominates aggregate interaction, a process which describes the scavenging of small suspended
aggregates by larger faster-sinking aggregates (Hill and Nowell 1990; Hill 1992).
The ensuing encounter rate (K ds ) is proportional to the square sum of diameters,
to the absolute difference of settling velocities of the involved aggregates and to aggregate abundance. It takes the form (e.g. Hill and Nowell 1995):
Kd (Z)=TC(d.+d.)2Iw ·-w ·IN.N.,
5
4
I
J
51
5J I J
(9)