Deutsche Hydrographische Zeitschrift - German Journal of Hydrography 
472 
5.3 Tidal currents 
For both data sets we made a tidal analysis 
and determined the hourly mean tidal currents 
from 6 hours before until 6 hours after high water 
(HW) in Helgoland (port of reference) for spring 
and neap tides. There is a very good agreement 
between both data sets. Figure 6 shows the 
spring tidal stream figures for the near-surface, 
mid-depth, and near-bottom layers. The same 
good agreement is observed at neap tide. At the 
surface, the tidal stream is alternating in an 
ADCP 
east-west direction. Towards the bottom, the tidal 
stream figure becomes more circular, represent 
ing a nearly continuous anti-clockwise rotation of 
the tidal stream in the near-bottom layer. 
Energy density spectra show significant 
peaks at periods of 12.4 (M 2 ), 6.2 (M 4 , MS 4 ), and 
4.2 (M 6 , 2MS 6 ) hours. With respect to the M 2 -tide, 
good agreement is found between the ADCP and 
model data. A difference is only observed in the 
meridional component of the near-surface layer 
(see Table 5). The other tidal constituents exhibit 
significant differences between both data sets. 
model 
0 50 
i i i 
cm/s 
Fig. 6 Mean tidal stream figures evaluated from ADCP data (left) and model data (right). The vectors give the hourly 
averaged spring tidal currents from 6 hours before (-6) until 6 hours after (+6) high water (HW) in Helgoland. 
The bold numbers give the sampling depth, i.e. the centre of the ADCP bin, respectively model layer.