Modelling shallow landslide susceptibility by means of a subsurface flow path connectivity index and estimates of soil depth spatial distribution

Abstract

Abstract. Topographic index-based hydrological models have gained wide use to describe the hydrological control on the triggering of rainfall-induced shallow landslides at the catchment scale. A common assumption in these models is that a spatially continuous water table occurs simultaneously across the catchment. However, during a rainfall event isolated patches of subsurface saturation form above an impeding layer and their hydrological connectivity is a necessary condition for lateral flow initiation at a point on the hillslope.Here, a new hydrological model is presented, which allows us to account for the concept of hydrological connectivity while keeping the simplicity of the topographic index approach. A dynamic topographic index is used to describe the transient lateral flow that is established at a hillslope element when the rainfall amount exceeds a threshold value allowing for (a) development of a perched water table above an impeding layer, and (b) hydrological connectivity between the hillslope element and its own upslope contributing area. A spatially variable soil depth is the main control of hydrological connectivity in the model. The hydrological model is coupled with the infinite slope stability model and with a scaling model for the rainfall frequency-duration relationship to determine the return period of the critical rainfall needed to cause instability on three catchments located in the Italian Alps, where a survey of soil depth spatial distribution is available. The model is compared with a quasi-dynamic model in which the dynamic nature of the hydrological connectivity is neglected. The results show a better performance of the new model in predicting observed shallow landslides, implying that soil depth spatial variability and connectivity bear a significant control on shallow landsliding.

Sensitivity analysis and calibration of a dynamic physically based slope stability model

“…Lanni et al, 2013) or soil thickness (e.g. Catani et al, 2010;Segoni et al, 2012), is still one of the most difficult and laborious parameters to measure on a catchment scale, yet it is crucial for physically based modelling of slope stability (Dietrich et al, 1995;Lanni et al, 2012;Segoni et al, 2012). It is defined as the thickness of unconsolidated material covering the earth's surface, i.e.…”

“…Regolith depth can be assessed by (i) direct measurements (e.g. Lanni et al, 2012;Wiegand et al, 2013), (ii) means of geophysics (e.g. Davis and Annan, 1989;Sass, 2007) and (iii) modelling (e.g.…”

“…Furthermore, the depth of past landslides can be derived from multitemporal, remotely sensed elevation data (Zieher et al, 2016). For regolith depth mapping, regression models correlating regolith depth to either elevation, slope angle or other derivatives were used in previous case studies on shallow landslide susceptibility (Baum et al, 2010;Lanni et al, 2012;Salciarini et al, 2006;Segoni et al, 2012). For the assessment of regolith depth in the Laternser valley, 126 dynamic cone penetration tests (DCPTs) were conducted along four transects.…”

The role of uncertainty in bedrock depth and hydraulic properties on the stability of a variably-saturated slope

“…Using a detailed depth to bedrock map of a small catchment in Japan and different parametrizations of the soil hydraulic functions, these authors found that saturation develops predominantly at the soil-bedrock interface. The bedrock surface connects sparsely saturated regions [28], and thus determines, along with regolith thickness, the water pressure within the soil mantle's pores. Indeed, depth to bedrock is a key variable that controls subsurface flow [29], and triggers landslides during rainfall events [30][31][32].…”

“…Resulting topographic maps often do not do justice to complex field measurements of the bedrock depth, which often demonstrate significant spatial variability [3,33] with a geometry that is difficult to characterize adequately with some closed-form mathematical expression, while hydraulic and strength parameters can vary abruptly at the soil-bedrock interface. Whereas some authors have used high-resolution bedrock depth maps to assess hillslope stability [28,32,34,35], existing studies in the literature do not properly recognize the effect of bedrock depth uncertainty on slope stability assessments.…”