Field and Numerical Studies of Near-Bed Aggregate Dynamics
197
4c). The largest size class accumulated the highest concentration close to the seafloor, whereas above 0.25 m above the seafloor the mass distribution was dominated by small aggregates (>0.5% of the total initial concentration). The medium size
class with a settling velocity of 50 mday-l was most important between 0.03 and
0.25 m above the seafloor. Therefore, for the slower current velocities the simple
1-D-diffusion -advection model supports the idea of hydrodynamic sorting, similar to that proposed by Muschenheim (1987), even though his approach is based
on the combination of the empirical Rouse and the Karman-Prandtl equation.
8
Aggregate Interaction
All experiments including particle interaction were also run with two different
hydrodynamic regimes (Table 1, experiments 6-9). In Fig. 5a-f, concentration
distributions of the three simulated aggregate size classes are summarised. Due
to their wide range of concentrations, a log-log form was chosen for the presentation of the vertical concentration distribution up to 100 m above the seafloor.
Similar to the previous simulations the current velocity at 1 m above the seafloor
(ulOO) was set at 0.1 ms- l (experiments 6 and 7) and at 0.5 ms- l (experiments 8
and 9). The results of these experiments were compared with the standard diffusion-advection runs (experiments 4 and 5). Compared to the standard runs of
the simple diffusion-advection approach (dotted line), aggregate interaction
produced significantly different concentration profiles (solid and dashed lines)
after 22 h of simulation time. The advantage of the experiments with aggregate
interaction included over the simple diffusion advection approach is that in
these experiments the initial mass is freely exchanged between the aggregate
size classes, a situation similar to processes in the field and in contrast to the
simpler approach without aggregate interaction, where the final concentration
distribution of a size class for a defined hydrodynamic regime is only dependent
on the initial mass of this class. In experiments 4 and 5, without aggregate interaction, all three size classes account for one third of the initial mass. In all experiments with aggregate interaction significantly different mass transfer between
the three size classes and the aggregate sink was observed (Table 2). Thus, aggregate interaction most likely affects the vertical concentration distribution of the
different interacting aggregate size classes.
9
Turbulent Aggregation and Differential Settling
In experiments 6 and 7 aggregate formation from turbulent aggregation and differential settling was analysed. Either process moves aggregates from one size
class to the next largest size class. Asproposed by McCave (1984), under the calmer
hydrodynamic conditions differential settling dominates over turbulent aggregation over the total vertical extension of the model (model results not shown here).
197
4c). The largest size class accumulated the highest concentration close to the seafloor, whereas above 0.25 m above the seafloor the mass distribution was dominated by small aggregates (>0.5% of the total initial concentration). The medium size
class with a settling velocity of 50 mday-l was most important between 0.03 and
0.25 m above the seafloor. Therefore, for the slower current velocities the simple
1-D-diffusion -advection model supports the idea of hydrodynamic sorting, similar to that proposed by Muschenheim (1987), even though his approach is based
on the combination of the empirical Rouse and the Karman-Prandtl equation.
8
Aggregate Interaction
All experiments including particle interaction were also run with two different
hydrodynamic regimes (Table 1, experiments 6-9). In Fig. 5a-f, concentration
distributions of the three simulated aggregate size classes are summarised. Due
to their wide range of concentrations, a log-log form was chosen for the presentation of the vertical concentration distribution up to 100 m above the seafloor.
Similar to the previous simulations the current velocity at 1 m above the seafloor
(ulOO) was set at 0.1 ms- l (experiments 6 and 7) and at 0.5 ms- l (experiments 8
and 9). The results of these experiments were compared with the standard diffusion-advection runs (experiments 4 and 5). Compared to the standard runs of
the simple diffusion-advection approach (dotted line), aggregate interaction
produced significantly different concentration profiles (solid and dashed lines)
after 22 h of simulation time. The advantage of the experiments with aggregate
interaction included over the simple diffusion advection approach is that in
these experiments the initial mass is freely exchanged between the aggregate
size classes, a situation similar to processes in the field and in contrast to the
simpler approach without aggregate interaction, where the final concentration
distribution of a size class for a defined hydrodynamic regime is only dependent
on the initial mass of this class. In experiments 4 and 5, without aggregate interaction, all three size classes account for one third of the initial mass. In all experiments with aggregate interaction significantly different mass transfer between
the three size classes and the aggregate sink was observed (Table 2). Thus, aggregate interaction most likely affects the vertical concentration distribution of the
different interacting aggregate size classes.
9
Turbulent Aggregation and Differential Settling
In experiments 6 and 7 aggregate formation from turbulent aggregation and differential settling was analysed. Either process moves aggregates from one size
class to the next largest size class. Asproposed by McCave (1984), under the calmer
hydrodynamic conditions differential settling dominates over turbulent aggregation over the total vertical extension of the model (model results not shown here).