140 
Dl. hydrogr. Z. 44, 1091/92. H. 3. Klein el al., Currents; German Bight 
During the first period the scalar speed i decreased strongly between 18 inab and bottom. 
At 12 niab v amounts to 35%, at 1 mab 16%, and al 0.3 ntab 129i of the speed al 18 mab. 
Due to the tides the stability of the current, indicated by the stability factor SF = (v/v) * 100, 
is generally weak, only al 18 ntab a slight stabilisation due to the winddrift is noticeable. At 
all four levels the eddy kinetic energy K t exceeds the mean kinetic cnerg) Km". 
The storm significantly enhanced the downward energy transfer. Al 12 mab the mean scalar 
speed still amounts to 64% of its value at 18 mab. at I mab 38 % and at 0.3 mab 14%. Compared 
to the first period, al 18 mab we obsene an amplification of the mean scalar speed of only 
8 c k. At 12 mab this increase amounts to 93% and at I mab even 145%. Below I mab bottom 
friction reduced the increase of the mean speed to 22% (0.3 mab). but its relative amplification 
is stronger than at 18 mab. This downward transfer of kinetic energy causes roughly a halving 
o f the vertical shear between 18 and 12 mab, but a fivefold amplification of the near-bottom 
shear between 1 and 0.3 mab. Furthermore, the strong winddrift within the whole water column 
amplified the stabilty factor and considerably reduced the k E Jk M -rnlio. Al 0.3 mab the storm 
enhanced k yt and k E by a factor of 57 and 2, respectively. This energy surplus - together with 
an intensified energy input front surface waves - is available for resuspension processes. 
5 Resuspension of sediments 
The erosion and transport of sediment is influenced by unidirectional and oscillatory flows. 
Compared to the period of oscillatory wave motions, tidal currents can be considered as quusi- 
unidirectional. In the TOPEX area the sediment grain size D ranges between 0.125 and 0.500 mm 
(3 to I <l>, fine to medium sands) with finer sediments on the ridge and the coarsest sediments 
in the troughs (S w i ft ct al. 119181). Grain sizes greater 0.5 mm can be found at a few randomly 
distributed spots. The unidirectional near-bottom How exerts a stress ton the sediment particles 
which depends upon current speed and sediment grain size D. Al a critical stage the particles 
are removed from the bottom and can be transported by the local currents. The stress can be 
related to the flow at 1 mab, n l00 : 
T - C Km p 0i mu) 
The frictional drag coefficient C| (K i amounts to about 3x 10‘ 3 (2x 10' 3 to 4 x 10' 3 ) for 
hydrodynamical rough flows, i.c., for //| (W > 15 cm/s (K omar |1976|), p is the density of 
seawater. Table 2 summarizes »khi values for different weather conditions and different locations 
during TOPEX-I. 
The threshold stress for initial sediment movement r, can be evaluated from the relative 
stress 01: 
(p, - p) gD 
The density of the quartz grains p s amounts to 2.65 g/cm 3 , g is the acceleration of gravity. 
The 0,-values (or the different grain sizes were taken front Komar | 1976, page 1091). The 
bold curve in fig. 7 represents r, as function of D, the second curve r (//mo). To mobilize 
sediments with a diameter of 0.125 mm, for example, the stress must exceed about 13 g/ems 2 . 
i. e., //kjo must be greater than 62 cm/s (broken line). During the storm //mu reached values up 
to 100 cm/s over the ridge and about 80cm/s in the trough. Maximum grain sizes which can 
be resuspended are 0.48 and 0.33 mm, respectively. According to this model, nearly all locally 
occuring grain sizes can be resuspended over the ridge. 
11 K\i = 0.5 (ir * v 2> , Ki = 0.5 (//V + v’ v’). » and v are the east-west and north-south components of the 
velocity vector.