Historical research as a tool in estimating hydrogeological hazard in a typical small alpine-like area: the example of the Versilia River basin (Apuan Alps, Italy)

Morphological and landscape features (broad, beautiful sandy beaches close to high and impressive mountains) make the Versilia area one of the most important tourist areas in Italy, located in north-western Tuscany and facing the Ligurian Sea. Nevertheless, this particular geographic configuration contributes to a high frequency of intense meteorological phenomena. This paper presents the most significant results of an investigation into historical events (floods and landslides) occurring in the last centuries in the Versilia River basin. The main purpose is to contribute to hydrogeological hazard assessment in the Apuan area as well as to collect useful data in order to make a catalogue of disastrous events. The research confirms the vulnerability of the territory being studied (186 damaging events from 1328 to 2009), which was emphasized by the tragic hydrogeological catastrophe of June 19, 1996 (13 fatalities). High severity events can also be identified in 1636, 1774, 1846, 1885 and 1902, together with many less intense events, which nonetheless had significant consequences. The damaging events show both a tendency to recur in the same areas, as well as a significant rise in frequency during the last centuries. These, in turn, probably depend on concurring factors: increased number and reliability of information sources; increased attention to the damaging phenomena; expansion of the elements at risk; possible climate changes. In the Versilia River basin, the average frequency is 1 damaging event every 3.7 years. An event similar to June 1996 should have a recurrence time of about 110 years. Moreover, the data forms the basis for a preliminary, but significant, classification of these identified events. This classification is based on event severity, deduced or estimated from the information gathered.