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Articolo pubblicato il 17-11-2006
Autore: Luigi Cavaleri
Director ISMAR
S.Polo 1364
30125 Venice, Italy
Numero 33-34 - Anno 3
17 November 2006
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November 4, 1966 – what could have happened
On November 4, 1966 Venice suffered its worst documented flood.
A very strong sirocco (S-E) wind blew continuously for more than 30 hours forcing the
sea in front of Venice to grow till the record level of 194 cm above the reference zero level.
In practice there was no warning of the intensity of the event and the town suffered
tremendous damages.
Forty years after the event people still ask if such an event could happen again and,
if so, if we were now capable to issue a useful forecast. The reply, in particular to
the last question, has been recently given by a study carried out at the Institute of
Marine Sciences (ISMAR) of the Italian National Research Council, in Venice.
Today the situation is quite different with respect to 1966. Now we receive huge amounts
of data from satellites, ships, airplanes, meteorological stations. We have available
fast and powerful computers and sophisticated numerical models to represent the dynamical
evolution of the atmosphere and the sea.
Hence we have asked ourselves the following
question: should we in 1966 have had at disposal the present technology, what could
we have done? In practice we acted as the meteorologist in 1966 with only the data
at his disposal, but with the present computers and models, and checked what the results
would have been. The output was a real surprise.
The results showed that not only we could simulate correctly a posteriori the event,
but that it could have been forecast till six days in advance. In practice on October
29 we could have already known that something exceptional was preparing in the atmosphere.
This is summarised in the diagram of Figure 1 where each colour represents the forecast
issued on a different day. It is clear that the basic information was available since six
days in advance. The present huge availability of data, orders of magnitude larger than in
1966, adds confidence to this conclusion.
Figure 1 – Time series of the sea level in Venice during the flood of November 4, 1966.
The dark line shows the recorded data. The other ones show the evaluation a posteriori
and the forecasts issued “today” with the data available at the time in the days before the event.
It is important to realise that in 1966 the conditions could have been much worse.
At a given time the actual sea level is given by the superposition of two different
tidal components: the astronomical one, well defined also for the future, and the
meteorological one associated to the wind and pressure distribution over the area
of interest.
Therefore the peak value reached at a certain location depends on the
relative phase between the two components. On November 4 the meteorological peak was
reached (see Figure 2) a few hours after the astronomical one.
Figure 2 – Superposition (continuous line – recorded data) of the astronomical
(dotted line) and meteorological (dashed line) tides during the event of November 4, 1966.
This implies that, should the storm have happened five hours in advance, the tidal
peak would not have been 194 cm, but almost 240 cm. The damage to Venice would have been
much more extensive.
The storm did not casue only a big flood in Venice, but it also led to an extreme
storm in the Adriatic Sea. Figure 3 shows the distribution of the significant
wave height in this basin at the peak of the storm. The isolines, at one metre
intervals, show the areas with different heights, identified also by the colour
intensity.
Please note the larger than eight metres values in the area in front of Venice,
upper left in the figure. It is worthwhile to specify that the significant wave
height provides a sort of high average of the wave heights present in the sea.
Within the statistical distribution we must expect to find at a certain position
a wave height larger than 18-1.9 times the significant wave one. This implies
that in the area facing the Venetian littoral there were single waves till 15 metre height.
Figure 3 – Distribution of the significant wave height in the Adriatic Sea at the peak
of the November 4, 1966 storm. Isolines at one metre intervals.
It is worthwhile to make an estimate of the probability that an event like the
November 4 one could happen again. It is estimated that a storm with that intensity
happens on the average once every 100-150 years. Combined with the probability that the
meteorological and astronomical peaks happen at the same time, this puts the return
time of the 240 cm event around 1000-2000 years.
However, we should consider that
high tidal levels can be reached also with less intense storms, but in phase with
the astronomical tide. Hence again we find that an event like the one of November 1966
can on the average happen once every century.
A further problem is given by the rising sea level. This process is happening
on a planetary scale, and the most recent estimates indicate an increase of 3 mm/year.
Even without any further acceleration of the process, it follows that within a century
the sea will be 30 cm higher than now. This implies that the number of exceptional
sea level events is due to grow.
It is worthwhile to point to a singular, not yet fully explained, process.
During the last thirty years of the past century the Mediterranean Sea, hence
also the Adriatic Sea, have not paralleled the ocean in its sea level growth.
It turns out that, still within its natural statistical variability, during this
period the average sea level in Venice has been almost constant. Clearly such a
situation cannot last forever.
As a matter of fact there is a measured evidence that
in the last few years the average sea level in Venice has been rapidly growing.
This strongly affects the number of floods the town has to suffer.
In Figure 4 we show
a) a common situation during a relatively limited flood in Venice,
b) the corresponding virtual situation on November 4, 1966,
c) what could have happened had the storm hit five hours in advance.
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Autore: Luigi Cavaleri
Director ISMAR
S.Polo 1364
30125 Venice, Italy
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