42
w. Puis· T. Pohlmann· J. Siindermann
3. The currents are provided by a separate 3-dimensional North Sea water circulation model (Pohlmann 1996). The water circulation model has the same
grid structure as the SPM transport model. The provided simulated currents
are:
- M2/S2 tidal currents
- daily residual currents, generated by water density gradients and wind.
The water circulation model also provides daily averaged coefficients of vertical diffusion which includes information about stratification (thermocline,
halocline).
4. Simulated 2-h surface wave data is given at 60 points in the North Sea. The
data is provided by the German Amt flir Wehrgeophysik, Traben-Trarbach
(Helbig 1985).
The near-bed currents and the surface water waves are combined to calculate
the bed shear velocity u., (PuIs et al. 1997b). For each of the 1350 horizontal grid
cells of the model domain, u. is calculated every 80 min.
For a model run starting at the time tp there is a "hot start" concerning the
distribution of SPM in the model domain and concerning the fine sediment in
the bed. This hot start is achieved by premodelling the time period of 2 years before tp using the actual currents and waves of that time period.
2.3
Simulated Processes
Maximum SPM concentrations in the North Sea are about 100 mg 1-1. This is a
comparatively small concentration which does not affect vertical mixing or SPM
settling.
Erosion of the bed sediment happens if the bottom shear velocity u. increases
above the threshold shear velocity of the sediment. The threshold shear velocity
U.ECR is 2.8 cm S-1 both for sandy and muddy sediment, which is the result of recent model tunings. The concept for the erosion of a sediment bed is: a surface
sediment layer of the thickness DE is stirred up; DE is proportional to u* 2 -
(U. E CR)2. The present mass of the sediment fraction < 20 11m in the disturbed
layer is entrained into the water column, the coarser sediment is not regarded.
Erosion is thus not simulated as a rate (i.e. a time-dependent) process, but as an
instantaneous process. Maximum DE-values are in the order of 2 cm. The increase of the SPM concentration in the water column during such an extreme
event is in the order of 100 mg 1-1.
Newly deposited SPM clothes the bed in a thin veneer. This mud blanket is resuspended instantaneously (en bloc) if the bottom shear velocity, u., increases
above the threshold shear velocity of the veneer material. According to the measured data of Jago et al. (1993) the threshold shear velocity for resuspension
should be in the order of2 cm s-l, but in the present model version the same value is used as for sediment erosion, i.e. 2.8 cm S-I.
SPM is advected by the currents provided by the water circulation model of
Pohlmann (1996).As a by-product, the spatial gradients of horizontal velocities
w. Puis· T. Pohlmann· J. Siindermann
3. The currents are provided by a separate 3-dimensional North Sea water circulation model (Pohlmann 1996). The water circulation model has the same
grid structure as the SPM transport model. The provided simulated currents
are:
- M2/S2 tidal currents
- daily residual currents, generated by water density gradients and wind.
The water circulation model also provides daily averaged coefficients of vertical diffusion which includes information about stratification (thermocline,
halocline).
4. Simulated 2-h surface wave data is given at 60 points in the North Sea. The
data is provided by the German Amt flir Wehrgeophysik, Traben-Trarbach
(Helbig 1985).
The near-bed currents and the surface water waves are combined to calculate
the bed shear velocity u., (PuIs et al. 1997b). For each of the 1350 horizontal grid
cells of the model domain, u. is calculated every 80 min.
For a model run starting at the time tp there is a "hot start" concerning the
distribution of SPM in the model domain and concerning the fine sediment in
the bed. This hot start is achieved by premodelling the time period of 2 years before tp using the actual currents and waves of that time period.
2.3
Simulated Processes
Maximum SPM concentrations in the North Sea are about 100 mg 1-1. This is a
comparatively small concentration which does not affect vertical mixing or SPM
settling.
Erosion of the bed sediment happens if the bottom shear velocity u. increases
above the threshold shear velocity of the sediment. The threshold shear velocity
U.ECR is 2.8 cm S-1 both for sandy and muddy sediment, which is the result of recent model tunings. The concept for the erosion of a sediment bed is: a surface
sediment layer of the thickness DE is stirred up; DE is proportional to u* 2 -
(U. E CR)2. The present mass of the sediment fraction < 20 11m in the disturbed
layer is entrained into the water column, the coarser sediment is not regarded.
Erosion is thus not simulated as a rate (i.e. a time-dependent) process, but as an
instantaneous process. Maximum DE-values are in the order of 2 cm. The increase of the SPM concentration in the water column during such an extreme
event is in the order of 100 mg 1-1.
Newly deposited SPM clothes the bed in a thin veneer. This mud blanket is resuspended instantaneously (en bloc) if the bottom shear velocity, u., increases
above the threshold shear velocity of the veneer material. According to the measured data of Jago et al. (1993) the threshold shear velocity for resuspension
should be in the order of2 cm s-l, but in the present model version the same value is used as for sediment erosion, i.e. 2.8 cm S-I.
SPM is advected by the currents provided by the water circulation model of
Pohlmann (1996).As a by-product, the spatial gradients of horizontal velocities