Analysis of the control effect of faults on material sources of debris flows in the red bed region of eastern Tibet: A case study of Waqu middle reaches in Gongjue County, Tibet

Waqu, a second-order tributary of the Jinsha River, is located in Gongjue County within the red bed region of Changdu, eastern Tibet. The middle reaches of the basin form a high-mountain canyon zone, serving as a distinctive locus of concentrated rainfall-induced debris flow development area in the county. The density, scale, and activity of debris flow in this area surpass the county’s average level, and the debris flows show a strong correlation with fault structures. Based on field investigation and UAV aerial photography, this paper discusses and scrutinizes the developmental characteristics and principal influencing factors of debris flow within Waqu basin. The findings are as follows: (1) 19 small to medium-scale rainfall-induced debris flows are intensively developed in the middle reaches of the Waqu River, with a development density of 0.84 flows/km, significantly higher than the country-wide average of 0.009 flows/km². Debris flow on the left bank are characterized by higher development density, drainage area, main channel length, and deposition volume compared to those on the right bank, showing distinct differences in distribution. (2) Debris flow sources on the right bank of the middle reaches are primarily composed of weathered detrital material, distributed throughout the entire watershed. On the left bank, debris flows are dominated by rockslide debris and weathered detrital material, primarily concentrated in the upper and middle reaches of the valley, with a larger overall volume compared to the right bank. The proportion of debris flow sources within the fractured zone of the fault is significant, making it a primary source for debris flows on the left bank. (3) Fault structures are the most significant factor influencing the concentrated development and distinct distribution of debris flows in the middle reaches of the Waqu river, primarily in controlling source conditions. (4) Faults control debris flow source conditions by altering rock mass structure, controlling stratum distribution, modifying microtopography, and changing groundwater conditions, leading to significant differences in source formation rates and volumes between the two banks, which is the fundamental cause of the differential distribution of debris flows.