10
J.F.R. Gower
was not aware of any flooding from the sea. In 2000, we saw the problem. Tides
in the Mediterranean are small and are often ignored by boaters. In Marseilles, for
example, the tide range is only about 20 cm. In Venice at the head of the Adriatic,
tides are larger, but the range is still less than a metre. The phenomenon of “acqua
alta” or high water in Venice is related more to winds blowing up the Adriatic, piling
water in a “surge” which can be at least a metre higher. Lower barometric pressure
associated with a storm will also raise the level by up to 30 cm, 10 cm for each
10 mb drop in pressure.
Venice also has to take seriously any drop in the level of the land on which
the city is built. Since 1930, the area surrounding Venice has sunk about 20 cm
due to the extraction of arterial water. Thanks partly to scientific work by the
ISDGM (Instituto per lo Studio della Dinamica delle Grandi Masse, now ISMAR,
Istituto di Scienze Marine), the host institute of these conferences, this extraction
has now been halted, but the loss of height remains (Carbognin and Gatto, personal
communication 5 ).
Floods have affected Venice throughout recorded history. Today, a tide of 90 cm
above the standard Venice reference will start to cover St Mark’s Square, but will
cause few other problems. A tide of 110 cm will put 12% of the surface area of
Venice under water. When this level is expected, forecast warnings are sounded
(Fig. 1.9). At 130 cm, 70% of the city is flooded, and at 150 cm this rises to 96%.
The floods do not usually last long, dropping as the high tide passes. The water
is usually clear and drains away with relatively little damage, unlike a river flood,
Fig. 1.9 The number of times in each year that water level at the Venice tide station has exceeded
110 cm, the level at which an alarm is sounded 6
5 See http://iahs.info/redbooks/a151/iahs_151_0321.pdf
6 Data from www.comune.venezia.it/maree
J.F.R. Gower
was not aware of any flooding from the sea. In 2000, we saw the problem. Tides
in the Mediterranean are small and are often ignored by boaters. In Marseilles, for
example, the tide range is only about 20 cm. In Venice at the head of the Adriatic,
tides are larger, but the range is still less than a metre. The phenomenon of “acqua
alta” or high water in Venice is related more to winds blowing up the Adriatic, piling
water in a “surge” which can be at least a metre higher. Lower barometric pressure
associated with a storm will also raise the level by up to 30 cm, 10 cm for each
10 mb drop in pressure.
Venice also has to take seriously any drop in the level of the land on which
the city is built. Since 1930, the area surrounding Venice has sunk about 20 cm
due to the extraction of arterial water. Thanks partly to scientific work by the
ISDGM (Instituto per lo Studio della Dinamica delle Grandi Masse, now ISMAR,
Istituto di Scienze Marine), the host institute of these conferences, this extraction
has now been halted, but the loss of height remains (Carbognin and Gatto, personal
communication 5 ).
Floods have affected Venice throughout recorded history. Today, a tide of 90 cm
above the standard Venice reference will start to cover St Mark’s Square, but will
cause few other problems. A tide of 110 cm will put 12% of the surface area of
Venice under water. When this level is expected, forecast warnings are sounded
(Fig. 1.9). At 130 cm, 70% of the city is flooded, and at 150 cm this rises to 96%.
The floods do not usually last long, dropping as the high tide passes. The water
is usually clear and drains away with relatively little damage, unlike a river flood,
Fig. 1.9 The number of times in each year that water level at the Venice tide station has exceeded
110 cm, the level at which an alarm is sounded 6
5 See http://iahs.info/redbooks/a151/iahs_151_0321.pdf
6 Data from www.comune.venezia.it/maree