Field and Numerical Studies of Near-Bed Aggregate Dynamics
199
Table 2 Final percent distribution of the initial mass into the distinct aggregate size classes
for the numerical experiments 4-9. Experiments 4 and 5 describe a diffusion-advection approach without aggregate interaction. Experiments 6 and 7 include aggregate production
due to turbulent aggregation and differential settling. Experiments 8 and 9 included aggregate production and desintegration. See Table la and b for detailed description of the experiments.
Experiment
Aggregate
Aggregate
Aggregate
Aggregate
size class 1,
size class 2,
size class 3,
sink,
diameter
diameter
diameter
diameter
0.000066 (m) 0.000128 (m) 0.000234 (m) 2: 0.000234 (m)
settling
settling
settling
velocity
velocity
velocity
300 mday-l
50 m day-l
80 m day-l
Hydro4
33.3
33.3
33.3
dynamic
6
0.0002
0.075
0.093
99.83
regime 1
8
0.001
0.094
0.158
99.75
Hydro5
33.3
33.3
33.3
dynamic
7
0.00002
0.041
0.166
99.79
regime 2
9
0.0015
0.152
0.878
98.97
run (Fig. 5b, c), respectively. Aggregate production and the following redistribution by diffusion and advection strongly influences the relative contribution of
each size class compared to the total mass at different heights above the seafloor.
In contrast to the standard run, where the smallest size class dominates the total
suspended matter above 0.25 m above the seafloor (Fig. 6a), particle interaction
practically removes the smallest size class (Fig. 6b). Here, the smallest aggregate
class accounted for less than 0.0002 % of the initial mass. Only aggregates with
settling velocities of 50 and 80 mday-l contributed to the total mass of suspended matter (Fig. 6b). For the hydrodynamic regime I, the medium size class (settling velocity of 50 mday-I) dominated the suspended matter above 0.08 m
above the seafloor, when aggregate disintegration was excluded. In this experiment aggregate formation transferred more than 99.8% into the aggregate sink
class (Table 2).
In the hydrodynamic regime 2 with the higher current velocity of UlOO of
0.5ms- I , the overall trend was similar (Fig. 5d-f). In this case, the higher eddy
diffusion coefficient Aziz) distributed aggregates further into the water column.
Thus, less mass of each size class accumulated in close proximity to the seafloor,
but, in contrast to the hydrodynamic regime I, the larger aggregate size class
dominated over the whole BBL (Fig. 6e). Similarly to the calmer hydrodynamic
regime, significant differences evolved between simulation with particle interaction (experiment 7) and the standard runs. Between these two experiments the
concentrations of suspended matter differed by various orders of magnitude
and with aggregate interaction included (experiment 7), the three size classes
199
Table 2 Final percent distribution of the initial mass into the distinct aggregate size classes
for the numerical experiments 4-9. Experiments 4 and 5 describe a diffusion-advection approach without aggregate interaction. Experiments 6 and 7 include aggregate production
due to turbulent aggregation and differential settling. Experiments 8 and 9 included aggregate production and desintegration. See Table la and b for detailed description of the experiments.
Experiment
Aggregate
Aggregate
Aggregate
Aggregate
size class 1,
size class 2,
size class 3,
sink,
diameter
diameter
diameter
diameter
0.000066 (m) 0.000128 (m) 0.000234 (m) 2: 0.000234 (m)
settling
settling
settling
velocity
velocity
velocity
300 mday-l
50 m day-l
80 m day-l
Hydro4
33.3
33.3
33.3
dynamic
6
0.0002
0.075
0.093
99.83
regime 1
8
0.001
0.094
0.158
99.75
Hydro5
33.3
33.3
33.3
dynamic
7
0.00002
0.041
0.166
99.79
regime 2
9
0.0015
0.152
0.878
98.97
run (Fig. 5b, c), respectively. Aggregate production and the following redistribution by diffusion and advection strongly influences the relative contribution of
each size class compared to the total mass at different heights above the seafloor.
In contrast to the standard run, where the smallest size class dominates the total
suspended matter above 0.25 m above the seafloor (Fig. 6a), particle interaction
practically removes the smallest size class (Fig. 6b). Here, the smallest aggregate
class accounted for less than 0.0002 % of the initial mass. Only aggregates with
settling velocities of 50 and 80 mday-l contributed to the total mass of suspended matter (Fig. 6b). For the hydrodynamic regime I, the medium size class (settling velocity of 50 mday-I) dominated the suspended matter above 0.08 m
above the seafloor, when aggregate disintegration was excluded. In this experiment aggregate formation transferred more than 99.8% into the aggregate sink
class (Table 2).
In the hydrodynamic regime 2 with the higher current velocity of UlOO of
0.5ms- I , the overall trend was similar (Fig. 5d-f). In this case, the higher eddy
diffusion coefficient Aziz) distributed aggregates further into the water column.
Thus, less mass of each size class accumulated in close proximity to the seafloor,
but, in contrast to the hydrodynamic regime I, the larger aggregate size class
dominated over the whole BBL (Fig. 6e). Similarly to the calmer hydrodynamic
regime, significant differences evolved between simulation with particle interaction (experiment 7) and the standard runs. Between these two experiments the
concentrations of suspended matter differed by various orders of magnitude
and with aggregate interaction included (experiment 7), the three size classes