Evaluation of rainfall thresholds for triggering shallow landslides on the Genoa municipality area (Italy): the case study of the Bisagno valley

Abstract

In urbanized areas rainfall-induced landslides very often expose the population to serious risk and their occurrence can cause considerable economic losses.Because of its peculiar geological, geomorphological and climatic conditions the Genoese area, frequently affected by heavy intensity rainfall events, presents a very high susceptibility to floods and rainfall induced shallow landslides. The aim of this paper is to define the rainfall thresholds for triggering shallow landslides for a portion of the Genoa Municipality area with homogeneous geological and geomorphological characteristics: the Bisagno Valley. In the study area the occurrence of shallow landslides is favoured by the presence of steep slopes and by the changes in the land-use management occurred during the last century.An empirical approach has been applied. By using the data recorded by one rain gauge during rainfall events triggering instabilities between 1991 and 2002, it was possible to assess the rainfall thresholds for different seasons and to state the importance of the cumulative amount of precipitations in the previous period (15 days). In addition, a threshold for shallow landslides, in terms of mean intensity and duration was defined.The comparison between the obtained results and those defined by other authors for other contexts confirms the shallow landslides high susceptibility of the study area.The support of data relate to new shallow landslides could, how- ever allow to improve the threshold curves trend.