Recurrent Landslide-Derived Debris Flows in the Yizhong River Basin: Geomorphic Controls, Structural Predisposition, and Rainfall-Driven Instability
Abstract
The Yizhong River Basin, located in an alpine gorge on the southeastern Tibetan Plateau, experiences recurrent landslide-derived debris flows originating from moraine-covered source areas. This study integrates unmanned aerial vehicle (UAV)-based geomorphic interpretation, field geological profiling, laboratory direct shear tests, rainfall analysis, and sequential SEEP/W–FLAC3D modeling to examine source-area preconditioning and rainfall-related slope response. Present-day UAV-derived profiles and field observations support a two-layer configuration in which unconsolidated moraine deposits overlie fractured bedrock and are consistent with an inferred sequence of headward incision, gully deepening, and lateral expansion; this sequence is interpreted conceptually rather than as a chronologically resolved reconstruction. The moraine cover represents a substantial high-elevation sediment source, with an approximate potential volume on the order of 3.9 × 105 m3. Laboratory wetting from the natural state (6.3% water content) to a controlled wet state (16.8%) reduced cohesion from 24.5 to 15.3 kPa and the internal friction angle from 30.2° to 26.4°. Under an idealized 8 h equal-total rainfall scenario, the sequential seepage–stability model yielded a factor-of-safety decrease from 1.33 to 0.92 and a maximum downward displacement of 22.3 cm, with deformation concentrated near the mapped collapse–slide zone. The recurrent activity is therefore interpreted as a feedback pathway linking moraine-slope instability, gully erosion, sediment recharge, and renewed failure. The results provide a case-based assessment of how moraine–bedrock preconditioning and rainfall-related weakening may jointly contribute to recurrent source-area instability, highlighting the combined roles of geological structure, sediment supply, and hydrological weakening in debris-flow initiation and recurrence.