40
W. Puis· T. Pohlmann· J. Siindermann
bed, SPM mass is subtracted from the particles. When there is erosion of fine
sediment, the SPM mass on the particles is increased.
A typical number of particles in the southern North Sea is 100 per horizontal
grid cell. This means a relative standard deviation of the estimated SPM concentration in that horizontal grid cell of ± 10 %, resulting from the randomness of
the particle number.
What follows is a brief description of the general concept of the model simulations, of the prerequisites for SPM modelling and of the simulated processes.
For a more comprehensive model description see Pohlmann and PuIs (1994).
2.1
General Concept
Sediment transport in waters is induced by near-bed water movements, generated by currents and by surface water waves. The forces on the sediment bed are
expressed by the bed shear stress 't or by the bed shear velocity u. := ('t p-l )0.5,
with p being the water density. The relationship between u. and the mean current velocity u lOO at 1 m above bed is u lOO = Cu" with C being a Chezy coefficient
in the order of 15.
In hydraulic engineering, sediment transport is usually quantified by transport formulae, relating u. to the mass of transported sediment. Transport formulae exist for sediment transported near the bed (bed load), sediment in suspension (suspended load) or the sum of both (total load), they are mostly valid
for non-cohesive sediment.
The model presented here deals only with suspended sediment. A transport
formula is not applied because such a formula implies that the SPM mass or the
SPM concentration in the water column is in equilibrium with the local, instantaneous bed shear velocity u •. SPM from the North Atlantic, for instance, settles
only 10m within 50 days. Regarding a time scale of several days, the content of
Atlantic-borne SPM in a water column is independent of u •.
The grain size of SPM in the North Sea is typically in the clay fraction or in
the fine silt fraction (Eisma and Kalf 1979; Eisma and Kalf 1987). The North Sea
model simulates the grain size fraction < 20 11m only.
In the presented model version, the model simulates terrigeneous SPM (plus
detritus) only, i.e. phytoplankton dynamics are not simulated.
The model is designed to simulate the large-scale, long-term SPM transport in
the North Sea during several years. At present, the years 1976-1995 can be simulated because both wave data and current data are available for this time period.
2.2
Prerequisites for SPM Modelling
For a realistic simulation of the transport of fine sediment in the North Sea, the
SPM model needs information about (1) bottom sediment, (2) SPM sources, (3)
currents and diffusion coefficients, and (4) surface water waves.
W. Puis· T. Pohlmann· J. Siindermann
bed, SPM mass is subtracted from the particles. When there is erosion of fine
sediment, the SPM mass on the particles is increased.
A typical number of particles in the southern North Sea is 100 per horizontal
grid cell. This means a relative standard deviation of the estimated SPM concentration in that horizontal grid cell of ± 10 %, resulting from the randomness of
the particle number.
What follows is a brief description of the general concept of the model simulations, of the prerequisites for SPM modelling and of the simulated processes.
For a more comprehensive model description see Pohlmann and PuIs (1994).
2.1
General Concept
Sediment transport in waters is induced by near-bed water movements, generated by currents and by surface water waves. The forces on the sediment bed are
expressed by the bed shear stress 't or by the bed shear velocity u. := ('t p-l )0.5,
with p being the water density. The relationship between u. and the mean current velocity u lOO at 1 m above bed is u lOO = Cu" with C being a Chezy coefficient
in the order of 15.
In hydraulic engineering, sediment transport is usually quantified by transport formulae, relating u. to the mass of transported sediment. Transport formulae exist for sediment transported near the bed (bed load), sediment in suspension (suspended load) or the sum of both (total load), they are mostly valid
for non-cohesive sediment.
The model presented here deals only with suspended sediment. A transport
formula is not applied because such a formula implies that the SPM mass or the
SPM concentration in the water column is in equilibrium with the local, instantaneous bed shear velocity u •. SPM from the North Atlantic, for instance, settles
only 10m within 50 days. Regarding a time scale of several days, the content of
Atlantic-borne SPM in a water column is independent of u •.
The grain size of SPM in the North Sea is typically in the clay fraction or in
the fine silt fraction (Eisma and Kalf 1979; Eisma and Kalf 1987). The North Sea
model simulates the grain size fraction < 20 11m only.
In the presented model version, the model simulates terrigeneous SPM (plus
detritus) only, i.e. phytoplankton dynamics are not simulated.
The model is designed to simulate the large-scale, long-term SPM transport in
the North Sea during several years. At present, the years 1976-1995 can be simulated because both wave data and current data are available for this time period.
2.2
Prerequisites for SPM Modelling
For a realistic simulation of the transport of fine sediment in the North Sea, the
SPM model needs information about (1) bottom sediment, (2) SPM sources, (3)
currents and diffusion coefficients, and (4) surface water waves.