Abstract: Background Benggang represents the most severe form of soil erosion in the hilly red-soil region of southern China, characterised by extremely high erosion rates and rapid development that pose substantial threats to ecological integrity and land resources. Owing to the abrupt and violent nature of Benggang erosion, quantitative investigations into the natural development processes and erosion patterns of active benggang under field conditions remain limited. MethodsEmploying a mid-stage benggang in Tongcheng County as the study site, four RTK-based topographic surveys were conducted over 12 year; high-resolution DEM were generated and integrated with spatial cut-and-fill analyses to quantify geomorphic change. Results The findings demonstrate that Benggang erosion does not proceed at a constant rate but instead evolves through three distinct phases—collapse-driven acceleration, rapid hydraulic response, and prolonged slow erosion. Following a major collapse, the annual soil loss peaked at 1733.27 m³/a within the first year, a rate approximately 17.89 times higher than that observed during the subsequent stable phase and accounting for 58% of the cumulative 12 year loss. Erosion and deposition were spatially and temporally synchronous; newly generated colluvial deposits served as both an immediate sediment source and a temporary storage element, reducing channel gradient and ultimately triggering a pronounced decline in erosion intensity.Conclusions The erosion-deposition sequence of Benggang exhibits distinct stage-dependent characteristics, with colluvial deposits playing a critical regulatory role. These outcomes advance the mechanistic understanding of coupled hydraulic-gravitational processes in Benggang erosion and provide a scientific foundation for the development of targeted prevention and management strategies.