SHALLOW LANDSLIDE HAZARD ZONATION BY DETERMINISTIC ANALYSIS: THE EXAMPLE OF THE APRIL 27th - 28th 2009 OLTREPO PAVESE (ITALY) STORM

Oltrepo Pavese, which is located in the Northern Apennines of Italy, extends over about 1100 km² and lies at altitudes between 200 m and 1725 m a.s.l. The outcropping lithologies consist mainly by marly arenaceous flysch and it has historically suffered from widespread damage from landslides. Recent winters have been very rainy and snowy, followed by rainy springs characterized by intense and very localized weather events. The April 27 ^th and 28 ^th 2009 the north-eastern sector of Oltrepo Pavese experienced extreme rainfall event. On April 28 ^th a rain-gauge station recorded 150 mm of rain in 48 h (20% of the annual average amount). The effects on the landscape were widespread erosion (arable and grassland), gully erosion (grassland and vineyards), shallow landslides and floods. Due to the shallow landslides in the Recoaro valley several buildings were completely destroyed and a person was found dead buried by the ruins of his house. Some days after the event a photogrammetric flight was done at low altitude and high-resolution in order to map the existing landslides. In order to understand the main causes that triggered the landslides and to produce accurate maps of landslides and of landslides hazard several field surveys were carried out and samples for laboratory investigations were collected. To model the rainfall-induced shallow landslides, was adopted a deterministic approach, the Transient Rainfall Infiltration and Grid-based Regional Slope-Stability (TRIGRS) that couples an infinite-slope stability analysis with a one-dimensional analytical solution for rainfall infiltration. Necessary geomechanical and hydrological parameters were obtained coupling samples laboratory analysis and in situ measures; soil thickness was estimated using an empirical model while distribution of rainfall intensity was analyzed by performing a spatial interpolation. Our preliminary results demonstrated a good agreement between predicted shallow landslide hazard and the inventory map derived from surveys, but more accurate results can be obtained using detailed calibration region which also include land use and consequently variation of parameters such as soil cohesion and permeability. For this purpose field surveys of permeability and of cohesion are ongoing, in order to take also account of the role played by the roots.