Flash flood in Tuscany

Flash floods are quite common in Tuscany, Italy. Eighty-six disastrous events have occurred between 1918 and 1990, causing 582 flood events along 170 different rivers, affecting 375 towns and villages in the region.

During the flood in 1996 the village of Cardoso was nearly totally destroyed by the water that rushed through the Versilia valley. Thirteen people died and there was severe destruction.

The maximum rainfall intensity was about 88mm in 30 minutes, which is one of the highest rainfall intensities ever recorded in Italy (total precipitation at Pomezzana was 470mm). For comparison, a table showing extreme rainfall intensities around the world is presented below.

The synoptic situation in the morning hours of 19 June was characterised by a low pressure system over Sweden with a frontal disturbance extending from Finland to the Alps (see above satellite loop). The upper-level flow over Italy was from the north-west with a weak low-level depression over the Gulf of Genoa.

Low-level winds over large parts of Northern Italy were from south to south-west. The explosive development in the morning hours in the Apuan Alps was probably a combination of a favourable synoptic situation with relatively warm and moist air at low levels overrun by cool, dry air at medium to high levels and orographic lifting (uplift of the low-level flow coming from the south-west along the steep slopes of the Apuan Alps).

The animation above clearly shows a stationary convective system over the Apuan Alps that is being regenerated several times in the period between 01:30 and 16:00 UTC, which explains the high rainfall. A similar explosive development can be observed about 12 hours later in Eastern Italy and Slovenia, producing large hail and, locally, more than 100mm of rainfall.

Figure 3 is a graph of the half-hourly and accumulated precipitation at Pomezzana on 19 June 1996 (from Mazzoni et al., 1997, data from Ufficio Idrografico e Mareografico Nazionale, Italy).

The drastic consequences of this severe precipitation event can be seen in the pictures below which were taken a few days after in the Apuan Alps.

As a result of this severe precipitation event the following casualties and damage were reported:

• Thirteen fatalities and one missing person.
• About 4,000 homes damaged in the communes of Pietrasanta Stazzema, Seravezza, Forte dei Marmi.
• Hundreds of industrial, commercial and agricultural buildings damaged.
• Tenth of a kilometre of major highway destroyed. The Aurelia highway closed to traffic for two weeks. The railway line between Genoa and Rome blocked for about 10 days.
• Stazzema-Cardoso semi-destroyed. Several mountain villages remained cut off for many days (e.g. Farnocchia, Pruno, Volegno and Pomezzana).

An analysis of the temporal distribution of such disastrous events indicates that most occur in the autumn when there are frequent atmospheric disturbances, mainly of Atlantic origin, (including cyclones generated in the Gulf of Genoa) and because of the peculiar influence of the Mediterranean on the Tuscan climate.

Figure 4 shows peaks in the frequency of disastrous 24-hour rainfall events in Northern Italy (this example is for the Dolomites) lie in the months of April and November, which also happen to be the months of maximum cyclonic activity in the Gulf of Genoa. On the other hand, the peak occurence of strong 15-minutes rainfall events lies in the summer months when severe convection triggered by orography and diabatic heating is dominant.

References

Mazzoni, G.B., Olivieri, M. and M. Ratti, 1997: L'evento alluvionale del 19 Giugno 1996 in Alta Versilia. Nimbus, Società Meteorologica Subalpina, Torino (Italy), 13–14, 136–140.
Liljequist, G.H. and K. Cehak, 1984: Allgemeine Meteorologie. Vieweg Verlag, Braunschweig, Germany, 396 pp