Hazard characteristics and causes of the “7.22” 2021 debris flow in Shenshuicao gully, Qilian Mountains, NW China

Debris flows are capable of rapidly mobilizing and transferring large volumes of sediment downstream, where they can cause severe damage to infrastructure and loss of life. In China, debris flow hazards are widely distributed, but mainly concentrated in the western mountainous area, which has experienced a significant increase in large debris flow occurrences in recent years because of the influence of extreme climate and earthquakes. Our study focused on the Qilian Mountains, on the NE Qinghai–Tibet Plateau, where the geological and meteorological setting provides suitable conditions for the generation of large debris flows: high altitude mountains and deep canyons, strong tectonic uplift, abundant crustal materials formed by earthquakes and weathering, extreme rainfall events, and intense freeze‒thaw cycles. At 18:00 on July 22, 2021, an extreme rainstorm in SE Qilian County, China (up to 50.6 mm of precipitation in 7 h) generated a debris flow in Shenshuicao gully that was the most severe geological hazard to hit the region in 20 years. The debris flow burst out into Shenshuicao gully and destroyed two highway bridges (1–40-m prestressed reinforced concrete T-beam), causing significant economic losses, although fortunately no casualties were reported. Here, we analyzed the “7.22” debris flow using remote sensing, uncrewed aerial vehicle (UAV) surveying, on-site investigation and sampling, and laboratory analysis to better understand the predisposing and triggering factors of the debris flow. Our findings provide guidance for the prevention and control of sudden debris flow hazards in the Qilian Mountains.