Study on the role of lateral unsaturated flow in triggering slope failure under varying boundary water level conditions

•This study examined the effects of lateral unsaturated flow on shallow landslides triggering in situations of rising water level at the uphill/downhill boundary.•Lateral unsaturated flow is important for slope stability assessment of loamy hillslopes but can be ignored for sandy hillslopes.•The effect of lateral unsaturated flow highly depends on slope angle and the rising rate of uphill boundary water level.•Intensive rainfall may weaken the role of lateral unsaturated flow in activating slope failure. Lateral unsaturated flow was found to contribute little to rainfall-induced shallow landslides, but it can be important when the landslides are triggered under conditions of varying boundary water level. By considering a hillslope with an impermeable bedrock and rising water level at the uphill/downhill boundary, this study explored how lateral unsaturated flow would affect slope stability. Three different Hillslope-Storage Boussinesq models that differ in their treatment of the unsaturated flow were coupled with the same infinite slope stability model to predict and compare the safety factor and instability time. Specific to the situations considered, this study discovered that, despite the inclusion of lateral unsaturated flow effect, the hillslope remains stable when water level at the downhill boundary rises. Under the condition of increasing uphill boundary water level, the consideration of lateral unsaturated flow leads to earlier slope failure for loamy hillslopes with a moderate slope. Also, the lateral unsaturated flow may not induce slope failure for mildly sloping loamy hillslopes and plays an ignorable role when landslides occur fast in very steep hillslopes. By contrast, the lateral unsaturated flow does not affect much the failure of sandy hillslopes. Moreover, when water level at the uphill boundary increases faster, the lateral unsaturated flow exerts a greater influence on initiating slope failure. Further examination shows that intensive rainfall may weaken the effect of lateral unsaturated flow on shallow landslides triggering under the condition of rising uphill boundary water level.