Regolith stripping and the control of shallow translational hillslope failure: application of a two-dimensional coupled soil hydrology-slope stability model, Hawkeʹs Bay, New Zealand

Rainfall-triggered regolith landslides constitute a dominant erosional process in New Zealand hill country. Conversion of forest to pasture about 150 years ago decreased the size of event required to trigger slope failure resulting in greater landslide activity. Recent research indicates that shallow landslides then cause progressive regolith stripping with redeposition at the slope base. The exposed bedrock is less permeable than the pre-existing regolith cover and the redeposited soil has a higher unit weight. Hence, alterations in both hydrological and geotechnical conditions result, changing the triggering thresholds for further failure. In this paper three phases of regolith stripping are investigated using a process-based model, to define variations in both the triggering storms as well as the failure mechanisms under these altered hydrological and geotechnical conditions. Over time, as the regolith is stripped progressively upslope, the threshold for slope failure also changes such that the landscape becomes more stable. However, there is a varying sensitivity to storm type for each of the three phases, which adds complexity to the relationship between regolith stripping and slope failure. Model results suggest different hydrological conditions for slope instability, depending on the degree of stripping and redeposition, adding further complexity. Future advances in process-based modelling are required for detailed investigation of temporal changes in landscape susceptibility to slope failure, and uncertainty in both model parameterisation as well as process representation should be emphasised when applying such models to the long term.