Field and Numerical Studies of Near-Bed Aggregate Dynamics
201
contained less than 1 % of the initial mass (Table 2). Again, more than 99% of the
initial mass ended up in the aggregate sink term. Within the processes of aggregate interaction, turbulent aggregation became more important than differential settling for the more vigorous hydrodynamic regime 2.
10
Aggregate disaggregation
In a first attempt to describe the near-bed particle dynamics more completely,
an aggregate disaggregation term was included into the diffusion-advection
equation. Similarly to the aggregate production terms [Eqs. (9-11)]' aggregate
disaggregation [Eq. (l2) J is a function of the hydrodynamic regime and mainly
related to the ratio of the aggregate diameter and the Kolmogorov length. The
amount of mass transfer between the various aggregate size classes is calculated
as the sum of the mass of aggregate production minus the mass of aggregate disaggregation [Eq. (13) J. If this sum is positive, the difference is transferred to the
next largest size class. If this sum is negative, i.e. aggregate disintegration dominates the aggregate interaction, the difference is transferred to the next smallest
size class. Consequently, the vertical distribution of aggregate interactions will
be dominated by aggregate production or aggregate disintegration as a function
of the hydrodynamic regime.
The results of the numerical simulations suggest that disaggregation influenced the vertical concentration profiles and the size class distribution of the
three aggregate classes close to the seafloor. Compared to the simulation without
disaggregation, higher concentrations of each size class were observed when aggregate disintegration was included (Fig. Sa-f). For the calmer hydrodynamic
regime 1 this influence was prominent only for the smallest size class, where aggregate disaggregation results in ten times higher concentrations than with turbulent aggregation and differential settling alone (Fig. Sa). In experiment 8 in
the calmer hydrodynamic regime 1, without disaggregation, the smallest aggregate size class was unimportant, accounting for less than 0.00002 % (Fig. 6b, Table 2), whereas with aggregate disintegration this aggregate class contains five
times more of the initial mass (0.001 %; Fig. 6c, Table 2). Only minor differences
between the experiments with and without disaggregation were observed in the
two larger aggregate size classes of settling velocities of 50 and 80 m daTI (Fig.
Sb, c). Even though the disaggregation term resulted in increased concentrations
of small aggregates compared to the simulation without aggregate disintegration, the majority of the initial mass (>99.7%) was found in the aggregate sink
class.
For the more vigorous hydrodynamic regime, the effect of particle disaggregation and thus transfer of mass from a larger to a smaller size class is important
for all size classes (Fig. Sd-f). The most striking effects of particle disaggregation turn out in the relative composition of the total suspended mass (Fig. 6). In
contrast to the simulation without disaggregation, the smallest aggregate class
contributed about 75 times more to the total mass when disaggregation is in-
201
contained less than 1 % of the initial mass (Table 2). Again, more than 99% of the
initial mass ended up in the aggregate sink term. Within the processes of aggregate interaction, turbulent aggregation became more important than differential settling for the more vigorous hydrodynamic regime 2.
10
Aggregate disaggregation
In a first attempt to describe the near-bed particle dynamics more completely,
an aggregate disaggregation term was included into the diffusion-advection
equation. Similarly to the aggregate production terms [Eqs. (9-11)]' aggregate
disaggregation [Eq. (l2) J is a function of the hydrodynamic regime and mainly
related to the ratio of the aggregate diameter and the Kolmogorov length. The
amount of mass transfer between the various aggregate size classes is calculated
as the sum of the mass of aggregate production minus the mass of aggregate disaggregation [Eq. (13) J. If this sum is positive, the difference is transferred to the
next largest size class. If this sum is negative, i.e. aggregate disintegration dominates the aggregate interaction, the difference is transferred to the next smallest
size class. Consequently, the vertical distribution of aggregate interactions will
be dominated by aggregate production or aggregate disintegration as a function
of the hydrodynamic regime.
The results of the numerical simulations suggest that disaggregation influenced the vertical concentration profiles and the size class distribution of the
three aggregate classes close to the seafloor. Compared to the simulation without
disaggregation, higher concentrations of each size class were observed when aggregate disintegration was included (Fig. Sa-f). For the calmer hydrodynamic
regime 1 this influence was prominent only for the smallest size class, where aggregate disaggregation results in ten times higher concentrations than with turbulent aggregation and differential settling alone (Fig. Sa). In experiment 8 in
the calmer hydrodynamic regime 1, without disaggregation, the smallest aggregate size class was unimportant, accounting for less than 0.00002 % (Fig. 6b, Table 2), whereas with aggregate disintegration this aggregate class contains five
times more of the initial mass (0.001 %; Fig. 6c, Table 2). Only minor differences
between the experiments with and without disaggregation were observed in the
two larger aggregate size classes of settling velocities of 50 and 80 m daTI (Fig.
Sb, c). Even though the disaggregation term resulted in increased concentrations
of small aggregates compared to the simulation without aggregate disintegration, the majority of the initial mass (>99.7%) was found in the aggregate sink
class.
For the more vigorous hydrodynamic regime, the effect of particle disaggregation and thus transfer of mass from a larger to a smaller size class is important
for all size classes (Fig. Sd-f). The most striking effects of particle disaggregation turn out in the relative composition of the total suspended mass (Fig. 6). In
contrast to the simulation without disaggregation, the smallest aggregate class
contributed about 75 times more to the total mass when disaggregation is in-