Long-term settlement of icy debris flow deposits: Process, underlying mechanisms and analysis model

Rock-ice avalanches and debris flows transport substantial amounts of fresh debris-ice mixture from high-elevation cold regions to warmer lowlands. These fresh deposits undergo pronounced settlement upon ice melt, driving landscape evolution and potential secondary hazards. Yet the evolution and underlying mechanisms of thaw-induced deformation are known. This study aims to (1) elucidate the thaw-settlement process in fresh debris-ice mixtures, (2) reveal the governing mechanisms, and (3) develop a model to predict thaw-induced deformation. The Sedongpu hazard in 2018 in the Yarlung Tsangpo Grand Canyon, Southeastern Tibet, was analyzed as a benchmark via remote sensing, field investigations, and laboratory tests. Results show that (150 ± 6.3) × 10⁶ m³ of sediments detached from the 9° valley floor. The mass flow fan blocked the river, forming a barrier dam 55 × 10⁶ m³ in volume. The average deposit thickness was 70 m. The primary settlement of the deposit was completed within two years, with a vertical strain of 32.5%. The settlement mechanism differs fundamentally from that of ice-cemented soils. The ice in fresh deposits exists as discrete interstitial fragments rather than bonding soil particles, forming a loadbearing ice-soil skeleton. Melting of ice slags triggers skeleton collapse and particle rearrangements, resulting in large deformation. The process follows a three-stage pattern: rapid initial, uniform, and decelerated settlement. We developed a novel ice-content-dependent settlement model that captures both the magnitude and temporal dynamics of thaw-induced settlement, validated against field and experimental data. The findings provide insights into the evolution of debris-ice fans and scientific basis for hazard mitigation.