Typhoon, rainfall and trees jointly cause landslides in coastal regions

Typhoon-induced landslides are the primary natural hazard in hilly coastal regions of South East Asia and can potentially devastate coastal communities and infrastructure. They occur at approximately the same time as typhoon strikes, showing complex failure mechanisms under the coupled action of wind, rainfall and trees. Therefore, special assessments should be performed for such geohazards. Here we elucidated the general features of typhoon-induced landslides in southeast coastal China in terms of their geological characteristics, spatial and temporal distribution, and vegetation cover characteristics. We further established a framework to investigate their possible failure mechanisms coupled to wind, rainfall and trees. Specifically, the mechanical response of trees in a typhoon was estimated through a combination of the autoregressive method (AR) and a multi-degree-of-freedom tree swaying model, while the trees and rainfall contributions were determined by calculating the cohesion attributable to roots and the soil shear strength reduction caused by the water content variation. Our results suggest that the root reinforcement disappears when the sliding surface is below the root embedded depth, and the impact of wind load is significant during the slope stability analysis. The Force 14 wind (mean wind velocity up to 45 m/s) reduces the factor of safety (Fs) of the slope by 20% when blowing downhill, and its combined contribution with rainwater infiltration will lead to a higher risk of slope failures (Fs reduction of 45% at the base of the root-soil composite). Additionally, typhoon-induced shallow slope failures are prone to develop at the base of the root-soil composite or the rock-soil interface. Our model results suggest that the impacts of wind and vegetation on the generation of landslides during a typhoon can be significant and deserve more attention in hazard assessment studies.