Historical data and flood/landslide hazard: Main results in the Versilia River basin (Tuscany, Italy)

The Versilia, in north-western Tuscany, is one of the most known tourist regions in Italy, situated between the Ligurian Sea to the West and the Apuan Alps to the East. The last are a mountain chain which reaches almost 2,000 m in elevation and is located a few kilometres from the coastline. This particular geographic position favours in Versilia frequent and intense rainfalls, which sometimes cause severe damage and destruction. The Versilia River basin drains the southern portion of the Apuan Alps (Fig. 1). The importance of the historical research in the flood and landslide hazard determination is recognized by several authors (e.g. GUZZETTI et alii, 1994; GUZZETTI, 2000; AMANTI et alii, 2001; GLADE, 2001; CARRARA et alii, 2003). In this short paper, the most significant results of a historical archive investigation on floods and landslides in the Versilia River basin are synthesized. The main purpose is to contribute to the assessment of the hydrogeological hazard in the Apuan- Versilian area as well as to collect useful data to realize a catalogue of disastrous events. The research allowed collecting much information about landslides (in particular shallow landslides, typical phenomena triggered by heavy rainstorms in this area) and floods, sometimes consisting in a mere description of the damage, in particular for the most remote events. Recently, the availability of objective data (rain amount, hydrometric level of rivers and torrents, etc.) allowed quantifying the event intensity by mean of physical parameters. The results confirmed hazard and vulnerability of the studied territory, which was emphasized by the tragic June 19, 1996 hydrogeological catastrophe (14 deaths and about 500 millions of Euro, D’AMATO AVANZI et alii, 2004). Recently, significant damaging events occurred also in December 2009. Other high severity events were identified in 1636, 1774, 1846, 1885 and 1902, together with a lot of less intense events, causing however sensible geomorphologic effects and damage. Altogether from 1328 to 2009, 186 sensible events were found (the average frequency resulted of 1 sensible event every 3.7 years). The spatial and temporal distribution of the meteorological events showed a tendency to recur in the same areas and a pronounced rise in frequency during the last centuries. This likely depends on many concurring factors, among which there are: increased number and reliability of the information sources; increased attention to the damaging phenomena; spreading of the elements at risk in hazardous areas; possible climate changes. Referring to the study area, the historical research allowed individuating the most hazardous period of the year, in which the flood/landslide event probability raises: the three-month period between September and November appears to be particularly at risk (Fig. 2). Furthermore, the collected data allowed obtaining a preliminary, but significant classification of the identified events. The classification was based on the event severity, deduced or estimated by the collected information on intensity and damage, partially following the suggestions of the French DRM - DÉLÉGATION AUX RISQUES MAJEURS (1990). Four intensity classes were individuated, in which the E4 class is comparable with the 1996 event, which can be regarded as a reference event: E1: low intensity E2: medium intensity E3: high intensity E4: very high intensity The results (Fig. 3) showed that an event characterized by intensity comparable with that of the June 19, 1996 event could have a recurrence time of about 100 years. In Fig. 4 is represented an image showing damage produced by a disastrous event, probably comparable with the 1996 one. On the contrary, other hydrologic/pluviometric studies estimated the recurrence time of such an event at a few hundreds of years. We can infer that in the approach in evaluating the hazard of a territory the historical approach provides useful and effective data.