Characteristics and chain-generated processes of extreme rainfall-induced geohazards in the upper reaches of the Juma River

From July 29 to August 2, 2023, a historically rare extreme rainfall event occurred in North China, which triggered a large number of geohazards in the mountainous areas of western Beijing and Hebei, resulting in serious casualties and property losses. This study analyzed the characteristics of geohazards under the extreme rainfall of “23•7” in Beijing-Hebei region, using on-site investigation, UAV aerial photography, and numerical simulation methods. The analysis focused on the geohazard characteristics in the upper reaches of the Juma River, examining the formation and evolution process and movement characteristics of the geohazards chain in Xita village. The results show that the sudden geohazards in the Beijing-Hebei region exhibited a clustered pattern, with frequent shallow surface collapses and landslides, which often became the source of mudslides, forming of a complete disaster chain of shallow surface landslide-mudslide-river blockage-failure together. The geological disaster chain in the upper reaches of the river shows the characteristics of long chain evolution and multiple sources of material accumulation. The length of a single disaster chain can reach 1.2～1.5 km, with the sources of shallow avalanches and slides replenishing both in the formation area and circulation area of the disaster chain, amplifying the scale of the disaster. Numerical simulation results show that the movement speed of the geohazards chain in Xita Village reaches up to 9.1 m/s. The maximum accumulation thickness in the two ditches was 7.1 m and 6.2 m when activated individually, and 8.6 m in the case of simultaneous activation, with the movement process showing typical multi-source accretion and amplification. This study provides valuable insights for risk prevention and management of sudden geohazard chains under extreme rainfall in northern mountainous areas.