Distributed transient response modeling of rainfall-triggered shallow landslide for susceptibility assessment in Ribeira Quente valley (S. Miguel Island, Azores)

-89In volcanic areas, landslides are one of the main causes of destruction and fatalities (Costa 1984), namely where steep slopes cut pyroclastic materials. The high recurrence rate of extreme rainfall events in some of these regions makes them authentic “clock bombs” concerning slope instability. In Europe, important rainfall-triggered landslides events occur in volcanic areas like in Campania Region, Southern Italy (Cascini et al. 2000, Frattini et al. 2004, Calcaterra et al. 2007), and in S. Miguel island, namely in Povoação County (Valadão et al. 2002, Marques et al. 2008). The trigger mechanism of a landslide and the landslide subsequent behavior are conditioned by the rheological material proprieties as well as by the, hydrologic system inherent to the slope. The decisive precipitation amount needed to determine slope failure depends on the morphology, on the mechanical and hydrological properties of the soil and on the vegetation that cover of the slope (Crosta 1998, van Asch et al. 1999). ABSTRACT: In the last 15 years, several heavy rainstorms have occurred in Povoação County (S. Miguel Island, Azores), namely in Ribeira Quente Valley. These rainfall events have triggered hundreds of shallow landslides that killed tens of people and have been responsible for direct and indirect damages amounting to tens of millions of Euros. On the 6 March 2005 an intense rainfall episode, up to 160 mm of rain in less than 24 h, triggered several shallow landslides and caused 3 victims and damaged/blocked roads. The Ribeira Quente Valley has an area of 9.5 km and is mainly constituted of pyroclastic materials (pumice ash and lapilli), that were mostly produced by the Furnas Volcano explosive eruptions. To provide an assessment of slope-failure conditions for the 6 of March 2005 rainfall event, it was applied a distributed transient response model for slope stability analysis. The adopted methodology is a modified version of Iverson ́s (2000) transient response model, which couple an infinite slope stability analysis with an analytic solution of the Richard`s equation for vertical water infiltration in quasi-saturated soil. The validation was made on two different scales: (1) at a slope scale, using two distinct test sites where landslides were triggered; and (2) at the basin scale, using the entire landslide database and generalizing the modelling input parameters for the regional spatialization of results. At the slope scale, the obtained results are very accurate, and it was possible to predict the time of the slope failures. At the basin scale, the obtained results are very conservative, even though the model predicts all the observed landslide locations. The used methodology revealed to be a reasonable tool for landslide forecast for both temporal and spatial distributions. In the future, the model components will be integrated into a GIS based system that will publish the FS values to a WebGIS platform, based on near real time ground rainfall monitoring. This application will allow the evaluation of scenarios considering the variation of the pressure head response, related to transient rainfall regime. The resultant computational platform combined with regional empirical rainfall triggered landslides threshold can be incorporated in a common server with the Regional Civil Protection for emergency planning purposes.