Delineating conditionally stable areas and critical soil water content maps for initiation of rainfall-induced landslides

Initiation of rainfall-induced landslides is intricately linked to hydrological conditions, mainly soil water content (SWC), which directly reflects precipitation intensity and patterns. Initiation may occur only on areas that are susceptible to the movement, i.e., the so-called conditionally stable areas. Existing methods delineate unconditionally and conditionally stable areas in “partially saturated” soils based on topography, mechanical properties, and a steady state wetness index (WI) or depth of groundwater level. This study presents a methodology that delineates conditionally stable areas under fully unsaturated soil water conditions, i.e., in the absence of groundwater. In particular, the methodology identifies (i) the ‘partially-saturated’ conditionally stable areas previously mentioned in terms of groundwater level or positive pressure head, and (ii) an ‘unsaturated’ conditionally stable areas, assessed in terms of SWC or negative pressure head. This is obtained computing the factor of safety (FoS) by using two equations of the infinite slope model, which account for both saturated and unsaturated soil conditions. The region delineation ultimately depends on the spatial heterogeneity of topographic and hydro-mechanical properties of the terrain. Finally, for the conditionally stable areas, both ‘partially saturated’ and ‘unsaturated,’ we derive critical maps of landslide initiation, either in terms of SWC or pressure head, respectively. In order to provide efficient and easy-to-interpret maps, the methodology generates Homogeneous Soil Units (HSUs) where each unit is represented by a unique combination of slope and hydro-mechanical properties of the terrain. A unique critical value of SWC or pressure head will result for each HSU at a given hypothetical failure surface, i.e., soil depth. We apply the methodology over the Friuli Venezia Giulia region, Italy, and central Puerto Rico, where thousands of shallow landslides were triggered by Hurricane Maria in September 2017.