Field and Numerical Studies of Near-Bed Aggregate Dynamics
195
solid line]. For all three size classes the constant Ac(z) produced a rather homogeneous distribution of concentrations to about 1 m above the seafloor, but significantly lower than the initial conditions. Smaller aggregates (settling velocity of 30
m day-I) diffused further into the water column, whereas the larger and heavier
aggregates (settling velocities of 50 and 80 m day-I) stayed closer to the seafloor.
In contrast, both height-dependent, Azc(z) and AZd(z) parameterisations (Table
1, experiments 2-5) generated logarithmic concentration profiles of all aggregate
size classes and both hydrodynamic regimes (u lOO of 0.1 and 0.5 ms- I , Fig. 3a-f).
In all four experiments (Table 1, experiments 2-5) the concentration distributions of the observed aggregate size classes were the same for the two approaches
of the parameterisations of eddy diffusion coefficients AzcCz) and AZd(z) (Fig. 3bf). However, preliminary results of simulations with strong discontinuities of the
diffusion coefficient with rapidly decreasing values above the upper limit of the
boundary displayed accumulations of small aggregates at the limit of the boundary layer. Mass accumulation especially of small aggregates at pycnoclines, is a
well-known phenomenon (McCave 1986). In the model, the sharp gradient of the
diffusion coefficient at the upper limit of the BBL could influence the aggregate
behaviour like a strong density gradient in the field. Therefore, we decided to use
a modified diffusion coefficient with decreasing values above the height of the
boundary layer [Eq. (6)] in all experiments including aggregate interaction.
The magnitude of the diffusion coefficient strongly influences the vertical
transport distance of all aggregate classes. Comparable to the Rouse parameter
(u*k/w s )' in the present model the interaction of the diffusion coefficient and the
settling velocity Ws determines the concentration profile of each aggregate size
class. Accordingly, at a u lOO of 0.5 ms- I , more material of the largest aggregate
class (Fig. 3c, Ws 80 mday-I) was transported further into the water column, compared to the case of UIOO of 0.1 ms- I (Fig. 3d). Consequently, a smaller amount of
this matter accumulated close to the seafloor. Similar to the results of Newberger
and Caldwell (1981), the logarithmic concentration distributions in experiments
2-5, verify our proposed approach to investigate near-bed aggregate distributions with a simple diffusion-advection equation. The profiles resembled the expected distributions of distinct particles suggested by Rouse (1937).
For the two hydrodynamic regimes, the concentration distribution for the
three aggregate size classes differed significantly (Fig. 4a, b). In the faster flow
(Fig. 4b, u lOO = 0.5 ms- I ), concentrations near to the seafloor were lower than in
the slower flow (Fig. 4a, u lOO = 0.1 ms- I ) and the maximal vertical transport distance was larger. In the faster flow, concentrations of the three classes were similar above 0.1 m above the seafloor, in the slower flow they were similar above
0.5 m. Thus, in the faster flow (ulOO = 0.5 ms- I ), each size class contributed between 0.3 and 0.8% of the total initially added suspended matter at distinct
heights above the seafloor, within the last 100 m above the seafloor (Fig. 4d). The
relative contribution of each aggregate size class to the total amount of suspended matter at a distinct depth was nearly similar for all three classes.
In the more tranquil hydrodynamic regime 1 (ulOO = 0.1 ms- I ) a single size class
controlled the grain size distribution at different heights above the seafloor (Fig.
195
solid line]. For all three size classes the constant Ac(z) produced a rather homogeneous distribution of concentrations to about 1 m above the seafloor, but significantly lower than the initial conditions. Smaller aggregates (settling velocity of 30
m day-I) diffused further into the water column, whereas the larger and heavier
aggregates (settling velocities of 50 and 80 m day-I) stayed closer to the seafloor.
In contrast, both height-dependent, Azc(z) and AZd(z) parameterisations (Table
1, experiments 2-5) generated logarithmic concentration profiles of all aggregate
size classes and both hydrodynamic regimes (u lOO of 0.1 and 0.5 ms- I , Fig. 3a-f).
In all four experiments (Table 1, experiments 2-5) the concentration distributions of the observed aggregate size classes were the same for the two approaches
of the parameterisations of eddy diffusion coefficients AzcCz) and AZd(z) (Fig. 3bf). However, preliminary results of simulations with strong discontinuities of the
diffusion coefficient with rapidly decreasing values above the upper limit of the
boundary displayed accumulations of small aggregates at the limit of the boundary layer. Mass accumulation especially of small aggregates at pycnoclines, is a
well-known phenomenon (McCave 1986). In the model, the sharp gradient of the
diffusion coefficient at the upper limit of the BBL could influence the aggregate
behaviour like a strong density gradient in the field. Therefore, we decided to use
a modified diffusion coefficient with decreasing values above the height of the
boundary layer [Eq. (6)] in all experiments including aggregate interaction.
The magnitude of the diffusion coefficient strongly influences the vertical
transport distance of all aggregate classes. Comparable to the Rouse parameter
(u*k/w s )' in the present model the interaction of the diffusion coefficient and the
settling velocity Ws determines the concentration profile of each aggregate size
class. Accordingly, at a u lOO of 0.5 ms- I , more material of the largest aggregate
class (Fig. 3c, Ws 80 mday-I) was transported further into the water column, compared to the case of UIOO of 0.1 ms- I (Fig. 3d). Consequently, a smaller amount of
this matter accumulated close to the seafloor. Similar to the results of Newberger
and Caldwell (1981), the logarithmic concentration distributions in experiments
2-5, verify our proposed approach to investigate near-bed aggregate distributions with a simple diffusion-advection equation. The profiles resembled the expected distributions of distinct particles suggested by Rouse (1937).
For the two hydrodynamic regimes, the concentration distribution for the
three aggregate size classes differed significantly (Fig. 4a, b). In the faster flow
(Fig. 4b, u lOO = 0.5 ms- I ), concentrations near to the seafloor were lower than in
the slower flow (Fig. 4a, u lOO = 0.1 ms- I ) and the maximal vertical transport distance was larger. In the faster flow, concentrations of the three classes were similar above 0.1 m above the seafloor, in the slower flow they were similar above
0.5 m. Thus, in the faster flow (ulOO = 0.5 ms- I ), each size class contributed between 0.3 and 0.8% of the total initially added suspended matter at distinct
heights above the seafloor, within the last 100 m above the seafloor (Fig. 4d). The
relative contribution of each aggregate size class to the total amount of suspended matter at a distinct depth was nearly similar for all three classes.
In the more tranquil hydrodynamic regime 1 (ulOO = 0.1 ms- I ) a single size class
controlled the grain size distribution at different heights above the seafloor (Fig.