Objective Benggang is the most severe soil erosion type in the red soil hilly regions of southern China, but due to its abrupt nature, long-term quantitative studies on its evolutionary processes remain scarce. This study aims to reveal the medium- to long-term erosion process characteristics and internal mechanisms of an active Benggang.

Methods This study focused on a typical Benggang in the middle stage of development located in Tongcheng county. Through RTK in-situ monitoring, combined with spatial analysis and erosion volume calculations, the twelve-year evolution patterns of the Benggang were quantified. Four sequential high-precision RTK surveys were carried out between 2010 and 2022. Spatiotemporal analysis of geomorphic changes was subsequently performed using the Cut/Fill tool in the ArcGIS platform to quantify volumetric changes and calculate key erosion parameters across different monitoring intervals.

Results The results revealed a distinct three-stage evolutionary pattern, including collapse-driven acceleration, rapid hydraulic response, and prolonged slow erosion. This pattern indicated that Benggang erosion was a non-equilibrium and episodic process. Following a major collapse event of 149.67 m³ between June and November 2010, erosion intensity peaked within one year. During this stage, the annual average soil loss rate reached 1 733.27 m³/a, which was 17.89 times higher than the 96.91 m³/a rate in the subsequent stable stage. This stage accounted for 58% of the total soil loss over the entire twelve-year period. Spatiotemporal analysis showed that freshly collapsed colluvial deposits played a dual role. They served as the primary sediment source for hydraulic erosion, while simultaneously aggrading the channel bed, reducing the longitudinal gradient, and dissipating flow energy through redistribution. The erosion-to-deposition ratio declined sharply from 14.33 during the collapse-driven stage from June to November 2010 to 2.03 in the subsequent stage of sediment redistribution, indicating a systemic transition from alternating source-sink dynamics to a continuous sediment transport channel. Deposition volume in the second stage reached 395.57 m³, effectively reshaping the gully morphology and reducing channel slope.

Conclusions Benggang erosion is fundamentally an episodic process driven by intermittent collapse events, with sediment supply driven by collapse events and hydraulic processes controlling transport and redistribution. The newly formed colluvial deposits exhibit a critical dual functionality. They act as a short-term sediment source immediately after collapse, and subsequently function as a natural sediment trap through their redistribution, which reduces channel gradient and retards further erosion. This is a key self-regulating mechanism responsible for the sharp decline in erosion rate during the late stage. These findings advance the mechanistic understanding of coupled hydraulic-gravitational processes in Benggang erosion and sediment transport systems.