Towards establishing climatic thresholds for slope instability: Use of a physically-based combined soil hydrology-slope stability model

Shallow landslides are often linked to high magnitude rainstorms. Research has attempted to establish threshold rainfall totals that trigger shallow landslides, based mainly on field evidence. Complications arise because not all regolith has the same hydrological behaviour, and research frequently fails to take this into account. This paper uses a combination of field and modelling approaches to explore the triggering rainfall thresholds for shallow failures in deforested hill country of New Zealand. It emphasises the role of variations in regolith hydrology, focussing on unsaturated and saturated zone responses. By using a modelling approach, detailed variations in pore pressure (positive and negative) responses are investigated, developing ideas initially derived from field evidence. This paper defines and develops earlier research that establishes values for maximum and minimum probability thresholds for shallow landslides, and provides a more generalised model that can be applied more widely. Hydrological mechanisms for shallow landslides are investigated in greater detail than previously possible using a Combined Hydrology And Stability Model (CHASM(TM)).