Background At the beginning of October 2021, the northwest China suffered torrential rainfall and several loess landslides were induced in Northwest China. This work presents a typical case (the Zhongzhai landslide) reactivated during this period in Niangniangba town, Tianshui, Gansu, China. The retrogressive failure was triggered again in the early morning of 5 October 2021 after a succession of torrential rain, and buried 2 houses and damaged another 2 houses. In this work, the geometric feature, soil composition, evolution history and failure process were preliminary described based on field investigation, history satellite images, and unmanned aerial vehicle photogrammetry. Results The Zhongzhai landslide is mainly composed of Malan loess and the underlain silty slate, and the sliding surface is localized at their interface. The tensile cracks near the main scarp provide favorable channels for rainfall precipitation and the permeability difference between loess and silty slate enhances the liquefaction occurrence. The counter-tilt silty slate benefits water impoundment and exacerbates the flow-like slope failure. The lag effect is obvious in the Zhongzhai landslide due to the rainfall infiltration process and pore pressure generation. Conclusions Although the Zhongzhai landslide is characterized with small scar and shallow sliding surface, it provides a classic case both for evolution history investigation of loess landslide and successful emergency evacuation. Based on our preliminary study, the unstable mass at the source of the Zhongzhai landslide with an area of approximately 3600 m2 that was slightly displaced in this event and has a high possibility to slide in future. Meanwhile, the Zhongzhai landslide is a retrogressive landslide and progressive failure occurred in the 2021 case, further analysis concerning the relationship among rainfall intensity, infiltration rate and pore water generation, as well as motion feature and runout prediction will be conducted.

Debris flows induced by heavy rainfall are a major threat in Northwest and Southwest China, due to its abrupt occurrence and long runout. In light of this, this work presents the runout simulation and risk assessment of the Boshuigou debris flow under different rainfall conditions in Wudu district, Gansu Province, Northwest China. Based on field reconnaissance, the geomorphological feature and main source of the Boshuigou debris flow were described. With the application of the FLO-2D simulation, the potential flow depth and flow extent of the Boshuigou debris flow under 100-year return-period rainfall and 50-year return-period rainfall were calculated. The maximum flow velocities of the Boshuigou debris flow under the 100-year return-period rainfall and 50-year return-period rainfall were 5.46 and 5.18 m/s, respectively. Accordingly, the maximum flow depths were 5.85 and 5.57 m. Then, the hazard zonation was conducted in combination of the construction and other properties within the potential impact zone, and the risk assessment of the Boshuigou debris flow under the 100-year return-period rainfall and 50-year return-period rainfall was finally completed. This work presents a method for debris flow risk assessment considering the solid source and water flow, which can provide a basic reference for mitigation and reduction of geohazards induced by torrential rainfall.

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