CHAPTER 10
C O N C L U S IO N S : C R IT IC A L R E V IE W O F T H E E X P E R IE N C E S A N D C O N S E Q U E N C E S
FOR FU TU R E W ORK
By
H.
W
e id e m a n n
D eutsches H ydrographisches In stitu t, B ernhard-N ocht Str. 78, 2 -H am b u rg 4, F ederal R epublic of G erm any
The implementation of an experiment of smaller
scale than RHENO, using even only one surveying
vessel, involves the solution of a number of practical
problems, but most of them are small compared with
those arising with a multi-nation-ship survey. Some of
these problems have been mentioned above (see 1.3);
there is the finding of an appropriate time for the
availability of several vessels, the m atter of technical
and linguistic inter-communication during the experi­
ment and last but not least, the compilation of a
multi-author report.
The experience of R H EN O suggests that in many
respects the order of magnitude of this experiment
was at a boundary, not only when thinking of the
total cost. Higher dye quantities would enlarge the
survey areas and, as a consequence, require still more
vessels in order to maintain the resolution of the
surveys, thus increasing the intercommunication diffi­
culties and the problem of planning the survey pro­
gramme in the light of knowledge of the situation as
it develops. If, on the other hand, not enough vessels
are available, larger areas would mean longer survey
tracks, thus each individual survey taking more time.
In this case the fiction of a “quasi-synoptic” survey is
no longer valid because shape and local concentration
change too much between the beginning and the end
of each survey, as can be demonstrated when crossing
the same “point” of the patch a second time several
hours later.
The attempt to reduce the survey courses to a situ­
ation corresponding to a “central time” making use of
current observations becomes more and more unrea­
listic with increasing areas; this is very obvious when
looking at the near-surface current measurements
which were strongly divergent during most of the
RH EN O time (see 3.4, 6.5, 9.3).
Considering these fundamental sources of uncer­
tainties it does not seem very promising to try to im­
prove the survey precision by increasing the relative
survey density. The lack of information between the
single profile lines always leaves ample space for indi­
vidually different interpretations (see 3.2), and be­
cause nobody knows the shape, size and orientation of
the patch before the survey is accomplished, it is im­
possible to design an “ideal” or optimum survey pat­
tern in advance.
Nevertheless, improvements should be possible when
better techniques for two-dimensional survey profiles
are available, e.g. by towing a vertically undulating
sensor vehicle at the same time with the near-surface
measurement at a constant depth. Survey results ob­
tained by such methods would probably yield con­
siderably improved insights into the three-dimensional
behaviour of the eddy diffusion processes compared
with the method used now (see chapters 4 and 5).
The discussions in Chapter 9 made it very clear that
the region selected for RH EN O was in some aspects
not ideal; certainly it had been chosen because of a
fairly central position, of weak tides, of well defined
stratifications above a bottom of more or less uniform
depth, but the altering regime of Baltic and North Sea
currents and water masses produced a variable system
of additional shear, eddies or divergences all of which
probably had a considerable influence on the diffu­
sion process. The observed diffusion parameters were
hence neither due to pure tide motions nor to pure
wind influences as Ekman currents or surface waves,
but also to that eddy system and thus to a mixture of
all these components in varying rates.
When making experiments in a natural environment
the chance of finding “clean” or “pure” conditions
anywhere or at any time will be minimal. Attempts to
derive laws or constants, even descriptive formulas,
can hence have only restricted success (see Chapter 6).
It has to be remembered that all diffusion theories are
fundamentally statistical, that means that great num ­
bers of events, large scales compared with the single
process scale, or long observation times are necessary
to establish laws, but that the individual event may
yield quite differing results. Nevertheless, the method
Conclusions : C ritical review of th e experiences
used in Chapter 6.4 to introduce certain empirical
parameters which are determined numerically by “best
fitting” methods, and to correlate these parameters
with time series of the environmental situation seems
to be very promising. Probably this, or a similar, me­
thod when applied to a great number of experiments
under widely varying environmental conditions will
be a reasonable way to attain quantitative approaches
for the influence of the individual components of the
environment on the diffusion process.
It will remain, however, a very difficult problem to
estimate the influence of factors like the time lag or
phase difference between energy input and observed
effect, including the energy dissipation chain from
currents and large eddies down to small-scale turbu­
lence and to molecular heating processes.
The general application of the results of experi­
ments of this kind to practical pollution problems is
limited within boundaries given by many of the above
mentioned facts, such as technical or evaluation defi­
ciencies, or statistically insufficient confidence, etc.
O ther restrictions are dependent on the nature of the
pollutants under consideration: oil slicks on the sur­
face or insoluble mineral sludges at the sea bottom
obey completely different laws from soluble substan­
ces. These latter, on the other hand, often stem from
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bottom deposits, corroding waste containers, etc., and
their dispersion is thus not comparable with that of
surface sources as simulated by our type of experiment.
In principle it seems possible and necessary to adapt
the experimental technique now to near-bottom dye
releases and surveys, using available in-situ-fluorometers and some telemetering devices for the bottom cur­
rents, the knowledge of which is absolutely indispens­
able for estimating where to search for the tracer, as
long as its mass remains invisible below the thermocline. When trying to design a similar method for the
deep sea floor the difficulties will increase consider­
ably, mainly due to problems of precise location, con­
trol and steering of the sensor vehicle many kilome­
tres below the surface vessel - if no research sub­
marine for deep sea work is available. But getting
precise data on deep sea mixing and transport pro­
cesses is of highly increasing practical interest, because
the deep sea floor will most probably more and more
be considered world-wide as a site for dumping of
waste materials.
Near-bottom tracer experiments in shallow waters
and, as soon as technically possible, also in the deep
sea seem, as a consequence, to deserve highest priori­
ties within future research work.