The landslides triggered by the December 2009 meteorological events in North-Western Tuscany (Italy): first results

Heavy precipitation (rainfall and also abundant snowfalls from December 18 to 21 at sea level as well) occurred in North- Western Tuscany in December 2009. Such precipitation caused floods and landslides in the Serchio River Valley, in Versilia (Lucca Province) and in the Pistoia Apennine (Pistoia Province). The Serchio River was interested by a discharge up to 1900 m3/s (NARDI, 2010), breaking the embankments and causing floods in the Lucca and Pisa plains (about 30 km2 of flooded ground, Fig. 1). The abundant rainfall associated with the rapid snow melting determined limit equilibrium conditions in a lot of slopes in the study area. Many slopes collapsed during the subsequent heavy rainfall events on December 22-23 and, mainly, on December 24- 25. Altogether in December 2009 the total rainfall amount was very high. The highest values were reached at the Campagrina raingauge (Apuan Alps, 997.8 mm) and at the Pracchia raingauge (Pistoia Apennine, 676.8 mm). Throughout the considered area the total rainfall of December 2009 corresponds to the 30% of the annual average precipitations. Unfortunately, the lack of a snowgauge in the regional monitoring network does not enable accurate snowfalls estimation. However, the rapid snow melting might have had a significant role in determining the slope instability conditions. This phenomenon was also caused by rapid increase of the minimum temperature (from -3.2 °C on December 20 to +11.9 °C on December 23 recorded at the thermometric station of Seravezza, Versilia) and by subsequent heavy rainfall (Fig. 2). The most damaging rainfall event occurred during the night between the 24 and 25 December. From 9 p.m. on 23 to 11 a.m. on 25 uninterrupted rainfalls were recorded. In particular, the precipitation reached the maximum intensity from 12 p.m. on 24 to 3 a.m. on December 25, recording 263.8 mm and 164.6 mm at the Campagrina and Fabbriche di Vallico stations, respectively. A similar event occurred in the Pistoia Apennine where, from 12 p.m. on December 24 to 4 a.m. on 25, 169.2 mm were recorded at the Pracchia raingauge. The December 2009 events caused over 600 landslides in the North-Western Tuscany. These landslides were mostly classified as rapid shallow movements (complex, translational debris slide- flow), also known as soil slip-debris flows (Fig. 4). Such landslides are typical of particularly intense rainfall in this area (D’AMATO AVANZI et alii, 2002, 2004). However, in some cases different types of movements also occurred (rock and debris translational slides and rotational-translational slides; Fig. 5) A lot of landslides were of first generation, and mainly involved the debris-colluvial covers of the slopes (few decimetres to some meters thick). In rare cases, the landslides involved the most deformed and fractured portion of the bedrock. In general, the landslides bedrock is mainly constituted by sandstone (Macigno Fm.), metamorphic sandstone and siltstone (Pseudomacigno Fm.) and by phyllite and schist (Apuan Metamorphic Complex). In conclusion, the December 2009 landslides were determined by multiple and concomitant triggering factors. The rapid snow melting might have played an important role (at present not quantified) in determining instability conditions and collapses of the slopes, where the rainfalls were more intense. In addition, several landslides, still triggered by the extreme events, involved the road network, whose surface water drainage system is often obsolete, inadequate or absent.