Deutsche Hydrographische Zeitschrift - German Journal of Hydrography 
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158 
U a (cm/s] 
Fig.7: Thickness of the Ekman layer /i L . estimated ac 
cording to [4] and percentage frequency distribu 
tion of i/ f (1 cm/s intervals) as observed at MK1 
and MK2 at 504 mab (52 days), and at D1 at 200 
mab (861 days). i/ f is averaged over the respec 
tive 4 preceding days which is the time scale for 
the maintenance of the Ekman layer. 
extrapolations from a few measurement positions 
may be unreliable for a larger area. 
Because of the manganese nodule coverage, 
there will be no viscous sublayer, i. e. the flow will 
be turbulent directly above the sea bed. The loga 
rithmic layer is of the order of 1 metre. The height of 
the Ekman layer varies according to [4]. The time 
scale of the EL is of the order of half a pendulum 
day (Bowden [1978]), i.e. 106 hours at the SEDI- 
PERU site and 98 hours at the DEA site. Therefore, 
not any increase in the current speed will cause an 
expansion of the EL, but the speed level must be 
maintained for about 4 to 5 days. 
Figure 7 shows the percentage frequency dis 
tribution of the mean (geostrophic) flow U e for MK1 
and MK2 at 503 mab - which is clearly outside the 
BBL - and for D1 at 200 mab. The mean velocity 
magnitude used for this diagram is a mean over the 
respective 4 preceding days, i. e. the velocity which 
is available for maintaining the Ekman layer. During 
periods of slow bottom currents (MK1, MK2) we can 
expect an Ekman layer height of between 10 and 30 
m. The D1 time series which covers a much broader 
time interval (861 days) with several events of rela 
tively strong currents shows that h c can reach 
values up to 60 m during such events. Figure 8 pre 
sents a time series of U., (4 day average) and h c at 
D1. On average /¡ c amounts to 20 m, but there are 
also periods of 1 to 2-month duration with values of 
about 40 m and even higher values. 
Armi and Millard [1976] found a correlation 
between BML height and the daily mean value of 
the ‘outer’ flow. The recordings at MK1 between 50 
and 503 mab ended a few days before the CTD pro 
files shown in Figure 6 were taken, but the daily 
mean value at 0.6, 6, and 13 mab gives a current 
speed of less than 1 cm/s. Therefore, the weak 
BML’s we observe in the profiles can be regarded 
as a relict of an old BML which has been esta 
blished during an earlier period of stronger flow. 
Fig.8: Daily mean values of Ekman layer height h c (top) und mean geostrophic velocity U e at D1 200 mab (bottom). 
U is averaged over the respective 4 preceding days which is the time scale for the maintenance of the Ekman 
layer.