Model Simulation ofTemporal Variability of Suspended Matter in the North Sea
39
al. 1997a}. This amount has accumulated during the last 100 years. Within the
last decade, the input of lead into the North Sea was substantially reduced; but
there is still a lead impact on the German Bight environment because of the lead
in the sediment, which leads to the question of how long the lead in the sediment
will remain in the German Bight. One reduction mechanism is the erosion of
fine sediment by storms, the following transport with the current over a certain
distance and finally the redeposition of the fine sediment. Stepwise, fine sediment will be transported further "until it eventually reaches a point where the
base of storm-induced waves will not reach the seabed any more. In the case of
the North Sea this point can be found in the Skagerrak" (cited from Liebezeit
1991).
This chapter deals with the temporal variability of the SPM regime in the
North Sea, especially the southern North Sea. The temporal variability is calculated (I) for the data collected by the North Sea Project (NSP) of the British Natural Environment Council (NERC), and (2) for the data computed by a 3-dimensional SPM transport model. The comparison of the model results with measured data gives an impression of the model's quality.
2
SPM Transport Model
The model domain of the SPM transport model for the North Sea is shown in
Fig. 1. The horizontal grid size, indicated by the zigzagging coastlines, is 20 km.
The thickness of the vertical layers is 5 m from the surface down to 50 m (i.e. 10
layers); below 50 m the layer thickness increases. The maximum number oflayers is 19. The horizontal grid lines subdivide the model domain into l350 horizontal grid cells. The additional subdivision by the vertical layers results in
l3851 single grid boxes. The time step for the model simulations presented here
is 80 min.
The sea bed below each horizontal grid cell has a thickness of 20 cm, it is vertically divided into 43 layers. The thickness of the layers increases from 0.1 mm
just below the sediment surface up to 24 mm in the deeper parts.
Most 3-dimensional models for SPM transport are based on solving a 3-dimensional transport-diffusion equation using the finite difference method
where SPM is treated as a continuum (e.g. O'Connor and Nicholson 1988). In
contrast to such an approach, the SPM transport model presented here uses a
stochastic, or Monte-Carlo, procedure. A description of a tracer particle transport method is for instance given by AI-Rabeh and Gunay (1992).
The movement of SPM in the model domain is simulated by the migration
and the random walk of tracer particles (Lagrangian tracer technique). Up to
100 000 particles are used, each being loaded with a certain mass of SPM. The
SPM masses on the single particles must not be identical, each particle can have
a different settling velocity. During a model run these particles are injected at
grid points where SPM input takes place. A particle carries only the mass of SPM
from one single source, e.g. "Rhine river". When SPM is deposited on the sea
39
al. 1997a}. This amount has accumulated during the last 100 years. Within the
last decade, the input of lead into the North Sea was substantially reduced; but
there is still a lead impact on the German Bight environment because of the lead
in the sediment, which leads to the question of how long the lead in the sediment
will remain in the German Bight. One reduction mechanism is the erosion of
fine sediment by storms, the following transport with the current over a certain
distance and finally the redeposition of the fine sediment. Stepwise, fine sediment will be transported further "until it eventually reaches a point where the
base of storm-induced waves will not reach the seabed any more. In the case of
the North Sea this point can be found in the Skagerrak" (cited from Liebezeit
1991).
This chapter deals with the temporal variability of the SPM regime in the
North Sea, especially the southern North Sea. The temporal variability is calculated (I) for the data collected by the North Sea Project (NSP) of the British Natural Environment Council (NERC), and (2) for the data computed by a 3-dimensional SPM transport model. The comparison of the model results with measured data gives an impression of the model's quality.
2
SPM Transport Model
The model domain of the SPM transport model for the North Sea is shown in
Fig. 1. The horizontal grid size, indicated by the zigzagging coastlines, is 20 km.
The thickness of the vertical layers is 5 m from the surface down to 50 m (i.e. 10
layers); below 50 m the layer thickness increases. The maximum number oflayers is 19. The horizontal grid lines subdivide the model domain into l350 horizontal grid cells. The additional subdivision by the vertical layers results in
l3851 single grid boxes. The time step for the model simulations presented here
is 80 min.
The sea bed below each horizontal grid cell has a thickness of 20 cm, it is vertically divided into 43 layers. The thickness of the layers increases from 0.1 mm
just below the sediment surface up to 24 mm in the deeper parts.
Most 3-dimensional models for SPM transport are based on solving a 3-dimensional transport-diffusion equation using the finite difference method
where SPM is treated as a continuum (e.g. O'Connor and Nicholson 1988). In
contrast to such an approach, the SPM transport model presented here uses a
stochastic, or Monte-Carlo, procedure. A description of a tracer particle transport method is for instance given by AI-Rabeh and Gunay (1992).
The movement of SPM in the model domain is simulated by the migration
and the random walk of tracer particles (Lagrangian tracer technique). Up to
100 000 particles are used, each being loaded with a certain mass of SPM. The
SPM masses on the single particles must not be identical, each particle can have
a different settling velocity. During a model run these particles are injected at
grid points where SPM input takes place. A particle carries only the mass of SPM
from one single source, e.g. "Rhine river". When SPM is deposited on the sea