Rainfall thresholds for the triggering of soil slips and debris flows

It is widely recognised that soil slips and debris flows frequently occur in coincidence with intense rainfall. As a consequence, analysis of rainfall is the most frequently adopted approach for forecasting the occurrence of shallow landslides. The basis for a complete approach, which examines landslide triggering by rainfall, lies in the different response to rainfall that soils or rock masses show because of their physical, mechanical and hydraulic characteristics. Furthermore, morphologic and anthropogenic features control surface, sub-surface and deep groundwater flow and they can strongly influence both slope stability and triggering conditions. Different rainfall thresholds and hydrologic models have been proposed in the literature. Among these threshold relationships and hydrologic models, it is possible to find some important differences that are sometimes neglected in their application. Most of the proposed thresholds and models are related to shallow landslides along open hillslopes or hollows but they are sometimes incorrectly adopted to forecast channelled instabilities. Again, most of these relationships determined for catastrophic events with occurrence of tens or hundreds of instabilities are adopted to forecast single or recurrent slope failures. This paper uses rainfall and landslide data, available in the literature, to explain the observed differences and to illustrate possible applications. Examples of global (regional) and local rainfall thresholds are presented and discussed together with the principles to be followed for a correct collection of data.