50
W. PuIs· T. Pohlmann· J. Siindermann
as numbers. SPM samples were taken 2 m below the sea surface, 2 m above the
seabed (in rough weather: about 5 m above seabed) and at about mid-depth. A
total of 4672 water samples were taken (Dyer and Moffat 1998). These samples
were analysed for SPM concentrations by filtering through GF/C glass microfibre
filters.
For having an impression of the intensity of the NSP observations, Fig. 10
shows the survey periods covered by the NSP cruises. This is done in a plot with
a time series of the (simulated) bed shear velocity at site A (Fig. 2).
The SPM concentrations measured during the NSP cruises were used for
comparison with the computed results of the numerical SPM transport model.
4.2
Mean SPM Concentrations
Figure 11 shows the spatial distribution of the temporal means of the depth averaged SPM concentrations collected during the NSP core period August 1988October 1989. The plotted data were derived as follows: in a first step, the temporal means of the depth-averaged SPM concentrations were calculated separately for each station. The number of vertical means entering into this temporal
averaging procedure is given in Fig. 11 for each station. The temporal averaging
is done in logarithmic terms. The reason for using a logarithmic scale is shown
by an example: In linear terms, an increase from 1 to 2 mg 1-1 has the same "importance" as an increase from 10 to 11 mg 1-1. For an SPM regime with concentrations in the order of 1 mg 1-1, however, an increase by 1 mg 1-1 is substantial,
while for a regime with concentrations in the order of 10 mg 1-1, the same increase is of subordinate importance. The importance of single SPM concentrations with respect to the general concentration level (i.e. the SPM regime) at a
specific site is more adequately described by using a logarithmic scale than a linear scale.
After temporal averaging over the NSP study period, there is one depth averaged SPM concentration for each station. In a second step these data are spatially
inter- or extrapolated (in logarithmic terms) in order to obtain a contour plot as
shown in Fig. 1l. In larger areas with no data (that means: no stations), the
shown shading is doubtful, of course. This is especially true for the stationless
area in the centre of the southern North Sea and in coastal areas.
The computed data corresponding to the measured data is obtained as follows. For each measured (depth averaged) SPM concentration, there is the information about the time of sampling t and the station number S. When the simulation run passes the time t, the depth-averaged SPM concentration computed at
station S is recorded and stored in a file. After having simulated the whole NSP
period, the file contains for each measured depth-averaged SPM concentration
a corresponding computed value. For the contour plot of computed SPM concentrations, shown in Fig. 12, the computed SPM concentrations are handled in
the same way as the measured data.
W. PuIs· T. Pohlmann· J. Siindermann
as numbers. SPM samples were taken 2 m below the sea surface, 2 m above the
seabed (in rough weather: about 5 m above seabed) and at about mid-depth. A
total of 4672 water samples were taken (Dyer and Moffat 1998). These samples
were analysed for SPM concentrations by filtering through GF/C glass microfibre
filters.
For having an impression of the intensity of the NSP observations, Fig. 10
shows the survey periods covered by the NSP cruises. This is done in a plot with
a time series of the (simulated) bed shear velocity at site A (Fig. 2).
The SPM concentrations measured during the NSP cruises were used for
comparison with the computed results of the numerical SPM transport model.
4.2
Mean SPM Concentrations
Figure 11 shows the spatial distribution of the temporal means of the depth averaged SPM concentrations collected during the NSP core period August 1988October 1989. The plotted data were derived as follows: in a first step, the temporal means of the depth-averaged SPM concentrations were calculated separately for each station. The number of vertical means entering into this temporal
averaging procedure is given in Fig. 11 for each station. The temporal averaging
is done in logarithmic terms. The reason for using a logarithmic scale is shown
by an example: In linear terms, an increase from 1 to 2 mg 1-1 has the same "importance" as an increase from 10 to 11 mg 1-1. For an SPM regime with concentrations in the order of 1 mg 1-1, however, an increase by 1 mg 1-1 is substantial,
while for a regime with concentrations in the order of 10 mg 1-1, the same increase is of subordinate importance. The importance of single SPM concentrations with respect to the general concentration level (i.e. the SPM regime) at a
specific site is more adequately described by using a logarithmic scale than a linear scale.
After temporal averaging over the NSP study period, there is one depth averaged SPM concentration for each station. In a second step these data are spatially
inter- or extrapolated (in logarithmic terms) in order to obtain a contour plot as
shown in Fig. 1l. In larger areas with no data (that means: no stations), the
shown shading is doubtful, of course. This is especially true for the stationless
area in the centre of the southern North Sea and in coastal areas.
The computed data corresponding to the measured data is obtained as follows. For each measured (depth averaged) SPM concentration, there is the information about the time of sampling t and the station number S. When the simulation run passes the time t, the depth-averaged SPM concentration computed at
station S is recorded and stored in a file. After having simulated the whole NSP
period, the file contains for each measured depth-averaged SPM concentration
a corresponding computed value. For the contour plot of computed SPM concentrations, shown in Fig. 12, the computed SPM concentrations are handled in
the same way as the measured data.