Landslides and flash floods induced by the storm of 22nd November 2011 in northeastern Sicily

On 22nd November 2011, after extreme rainfall (more than 350 mm fell in 10 h), thousands of shallow landslides evolving into debris flows occurred in several torrent catchments of Peloritani ridge, northeastern Sicily, carrying a large amount of wood. A rapid increase in torrent discharge occurred during the storm, inducing floods of the Longano and Mela torrents, also amplified by wood obstructions of bridges. A detailed reconstruction of the main geo-hydrological events has been done. Landslides have been mapped by satellite photos and checked by field surveys, and were found to be particularly concentrated in the Longano, Mela and Saponara catchments. The timing of landslide initiation has been estimated by technical report data, eyewitnesses, and by calibration of the Leaky Barrel Model [Wilson, Wiezoreck, Environ Eng Geosci 1(1):11–27, 1995]. The recorded amount of rainfall has been transformed into the torrent discharge by the HEC-HMS model, using a detailed digital elevation model (DEM) with resolution of 2 × 2 m, and the torrent hydrograph has been obtained corresponding to several river sections. In the urban area of Barcellona Pozzo di Gotto, the torrent discharge reached its peak before the arrival of the large trunks, indicating that the main cause of the inundation in the urban area was the bridge obstruction. The aim of this study is to provide a detailed case history of landslides inducing floods, and their relationship with the use of simple hydrological models, which could help in estimating the risk of inundation caused by a large amount of wood in unmonitored catchments.