Model Simulation ofTemporal Variability of Suspended Matter in the North Sea
43
provide for horizontal diffusion. Diffusion of tracer particles is simulated by a
random walk process, such as described in Stronach et al. (1993). The coefficients of vertical diffusion are provided by the water circulation model, a typical
value near the water surface during stormy weather is 0.03 m 2 S-I.
The settling velocity of SPM depends on the SPM's origin. SPM from the
North Atlantic Ocean settles with 2 x 10- 3 mm s-l, SPM eroded from the sea bed
settles with 3 x 10- 1 mm s-l. SPM from the English Channel, from rivers, cliff
erosion and dumping is divided into three fractions with settling velocities of 1
x 10- 1 ,2 X 10- 2 and 4 x 10- 3 mm s-l. The information about settling velocities
comes from in situ measurements (PuIs et al. 1995), it is deduced from mud accumulation rates (e.g. Johnson and Elkins 1979) or is obtained by tuning the
model results via comparison with observed time series of SPM concentrations.
Deposition of SPM takes place (1) by settling and (2) by water filtration by
bottom dwelling suspension feeders. Deposition by settling is possible only if the
bottom shear velocity, u*, is below a threshold shear velocity for deposition
(Krone 1962), which is between 0.8 and 2.8 cm S-I, depending on the SPM's settling velocity. A typical water filtration rate, being valid for sandy bottom in the
southern North Sea (seasonal average) is 100 litres m- 2 day-l (Rachor, pers.
comm.).
Most of the SPM deposited by settling forms a mud blanket on the bed surface. If this mud blanket is not resuspended, it is gradually consumed by bottomdwelling deposit feeders and stored in the upper 5 cm of the sediment bed, now
being an integral part of the bed. SPM deposited by suspension feeders is immediately stored in the sub-bottom layer 0-5 cm.
Vertical mixing in the sediment bed by bioturbation (0-20 cm sub-bottom
depth) is simulated by simple diffusion (Boudreau 1986). The effective diffusion
coefficient is 10 cm 2 a-I at the bed surface and decreases linearly down to 1 cm 2
a-I with increasing sub-bottom depth.
2.4
SPM Time Series
To give an impression of the short-term variability of the SPM regime in the
southern North Sea, Fig. 3 shows time series of the bed shear velocity, u*, and of
depth-averaged SPM concentrations (total SPM and eroded SPM). The time series is simulated for December 1988 at "site N' (see Fig. 2), a location in the
Southern Bight, water depth 27 m. The bed shear velocity shows four maxima
per day which reflects the maxima of the M2 tidal currents. During storms, surface water waves generate a substantial increase of u*, e.g. on December 6-7,
1988. The semi diurnal variability of the total SPM concentration (on December
1-2 and December 20-22) results from tidal advection of a concentration gradient past the site. Obviously, this spatial gradient exists only for non-eroded SPM,
because eroded SPM does not show the semidiurnal variability.
When u* exceeds the threshold shear velocity for erosion (2.8 cm S-I), fine
sediment is picked up from the bed by removing the fine sediment (as long as it
43
provide for horizontal diffusion. Diffusion of tracer particles is simulated by a
random walk process, such as described in Stronach et al. (1993). The coefficients of vertical diffusion are provided by the water circulation model, a typical
value near the water surface during stormy weather is 0.03 m 2 S-I.
The settling velocity of SPM depends on the SPM's origin. SPM from the
North Atlantic Ocean settles with 2 x 10- 3 mm s-l, SPM eroded from the sea bed
settles with 3 x 10- 1 mm s-l. SPM from the English Channel, from rivers, cliff
erosion and dumping is divided into three fractions with settling velocities of 1
x 10- 1 ,2 X 10- 2 and 4 x 10- 3 mm s-l. The information about settling velocities
comes from in situ measurements (PuIs et al. 1995), it is deduced from mud accumulation rates (e.g. Johnson and Elkins 1979) or is obtained by tuning the
model results via comparison with observed time series of SPM concentrations.
Deposition of SPM takes place (1) by settling and (2) by water filtration by
bottom dwelling suspension feeders. Deposition by settling is possible only if the
bottom shear velocity, u*, is below a threshold shear velocity for deposition
(Krone 1962), which is between 0.8 and 2.8 cm S-I, depending on the SPM's settling velocity. A typical water filtration rate, being valid for sandy bottom in the
southern North Sea (seasonal average) is 100 litres m- 2 day-l (Rachor, pers.
comm.).
Most of the SPM deposited by settling forms a mud blanket on the bed surface. If this mud blanket is not resuspended, it is gradually consumed by bottomdwelling deposit feeders and stored in the upper 5 cm of the sediment bed, now
being an integral part of the bed. SPM deposited by suspension feeders is immediately stored in the sub-bottom layer 0-5 cm.
Vertical mixing in the sediment bed by bioturbation (0-20 cm sub-bottom
depth) is simulated by simple diffusion (Boudreau 1986). The effective diffusion
coefficient is 10 cm 2 a-I at the bed surface and decreases linearly down to 1 cm 2
a-I with increasing sub-bottom depth.
2.4
SPM Time Series
To give an impression of the short-term variability of the SPM regime in the
southern North Sea, Fig. 3 shows time series of the bed shear velocity, u*, and of
depth-averaged SPM concentrations (total SPM and eroded SPM). The time series is simulated for December 1988 at "site N' (see Fig. 2), a location in the
Southern Bight, water depth 27 m. The bed shear velocity shows four maxima
per day which reflects the maxima of the M2 tidal currents. During storms, surface water waves generate a substantial increase of u*, e.g. on December 6-7,
1988. The semi diurnal variability of the total SPM concentration (on December
1-2 and December 20-22) results from tidal advection of a concentration gradient past the site. Obviously, this spatial gradient exists only for non-eroded SPM,
because eroded SPM does not show the semidiurnal variability.
When u* exceeds the threshold shear velocity for erosion (2.8 cm S-I), fine
sediment is picked up from the bed by removing the fine sediment (as long as it