Objective In view of the still limited research on the cause analysis and disaster-causing mechanisms of debris flows driven by extreme rainfall in northern mountainous areas, this study took the "23·7" debris flow in the Nan'an Basin, Nanjiao township, Fangshan district, Beijing as an example to reveal the coupled topographic, source, and rainfall conditions and the disaster-causing mechanisms, thereby providing a basis for disaster prevention and mitigation in similar regions.

Methods Combined with rainfall monitoring data, field investigation and laboratory tests were adopted in this study. The particle size distribution, uniformity coefficient, and curvature coefficient of loose source sediments in debris flow gully were emphatically analyzed, and the causative conditions and disaster-causing mechanisms of debris flows were further clarified.

Results The disaster-causing mechanism of the "23·7" debris flow in Nan'an Basin presented a triple coupling effect of topographic facilitation, source sediment supply, and hydrodynamic triggering. 1) The main gully of Nan'an Basin was characterized by large longitudinal slope, well-developed branch gullies, and flat-wide gully morphology, which led to concentrated and rapid confluence and easy formation of high flood peak discharge, forming the topographic foundation for debris flow initiation and transportation. A large number of loose deposits dominated by gravelly soil were distributed in the gullies with poor gradation and weak inter-particle bonding, which constituted the material foundation for debris flow occurrence. 2) This disaster was classified as a dilute, gully storm-type, coarse clastic debris flow. All indicators of the extreme heavy rainfall during the "23·7" event far exceeded the triggering thresholds of debris flows in Beijing. The combination of short-duration intense rainfall and saturated soil significantly reduced the shear strength of the source sediment and generated strong scouring and erosion capacity, which directly triggered the debris flow.

Conclusions The disaster-causing mechanisms of debris flows in northern mountainous areas differ markedly from those in southwestern China. The source sediment is predominantly loose debris previously accumulated in gullies, the hydrodynamic condition is extremely short-duration intense rainfall, and the flow type is a coarse-debris-dominated dilute debris flow. Such debris flows occur only during extreme rainfall events and are highly episodic.