A Method for Extracting Three-Dimensional Velocity Fields in Rock Glaciers by Combining InSAR and Aspect-Parallel Flow Models

Objectives: The Bayu Hydropower Station, a key infrastructure project located in the Yarlung Zangbo River basin of the Tibetan Plateau, lies within an area characterized by widespread rock glacier development. As ice-debris mixtures that exhibit slow and persistent downslope movement, rock glaciers serve as sensitive indicators of permafrost and climate conditions in high-mountain environments. Under accelerating global climate warming, elevated temperatures and altered hydrological regimes are expected to intensify the kinematic behavior of these landforms. Enhanced creep rates, internal destabilization, or sudden detachment of rock glacier lobes could trigger rapid landform-changing events such as debris flows or landslides. Therefore, this study aims to reveal the long-term dynamic changes and seasonal response patterns of the rock glaciers under climate warming conditions. This may deepen the understanding of their movement mechanisms and provide a scientific basis for major project site selection, geological hazard risk assessment, and early warning in plateau permafrost regions, which holds significant theoretical and practical value. Methods: We propose an integrated model combining ascending and descending Sentinel-1A Interferometric Synthetic Aperture Radar (InSAR) time-series observations with an Aspect-Parallel Flow (APF) constraint to estimate three-dimensional surface deformation velocities of rock glaciers over time. The Small Baseline Subset InSAR (SBAS-InSAR) technique was applied separately to 230 ascending-track and 199 descending-track Sentinel-1A SAR images acquired between March 2017 and November 2024 to obtain line-of-sight (LOS) deformation time series. The APF constraint, which assumes surface movement aligns with the slope direction in the horizontal plane, was introduced to resolve the inherent pathological issues when converting two-dimensional LOS measurements into three-dimensional deformation components. A smoothed 30-m TanDEM-X Digital Elevation Model (DEM) was employed to remove terrain phase, construct the APF constraint matrix, and ultimately obtain the three-dimensional velocity field and time series. Results: We identified 31 rock glaciers within the study area, predominantly located on northwest facing slopes within an elevation band of 4800-5300 m, with a mean area of approximately 0.24 km². The 3D velocity fields revealed that the rock glacier group is in a state of continuous creep. Maximum annual rates within the rock glacier group were: 150.95 mm/a in the east-west direction, 273.42 mm/a in the north-south direction, and -172.54 mm/a in the vertical direction. Based on the spatial distribution of the rock glaciers, three sub-regions with distinct creep characteristics were identified: Region I exhibits a pronounced flow trend and sensitivity to climatic factors; Region II shows the most intense creep, primarily characterized by northeastward downward movement; Region III is the most stable, with minimal deformation. Further time-series analysis of selected feature points within the regions revealed that creep in Regions I and III exhibited pronounced seasonal fluctuations superimposed on longterm stable trends, while movement in Region II remained relatively stable. Finally, Gray-Scale Analysis and Variational Mode Decomposition based on three-dimensional results indicated that the flow of the rock glaciers on the western slope of the study area exhibits periodic fluctuations primarily controlled by precipitation, demonstrating seasonal activity patterns. Conclusions: This study successfully developed and applied a novel model integrating multi-track InSAR time-series analysis with an APF constraint for monitoring long-term, three-dimensional creep of rock glaciers. The model effectively captured the temporal-spatial dynamics of the rock glacier group upstream of the Bayu Hydropower Station, demonstrating its validity for geological hazard deformation monitoring in complex alpine terrain. Experiments conducted using this model on the study area revealed: The rock glaciers are actively creeping, with significant spatial variability in magnitude and direction. Precipitation is the primary external climatic driver modulating the seasonal velocity fluctuations of rock glaciers on the western slope in this region, exerting a more direct influence than temperature. Their movement exhibits a compound signal of a persistent long-term creep trend and a seasonal cycle, with the deformation response lagging behind climatic forcing due to subsurface thermal and hydrological processes. The methodology and results provide a new, effective approach for assessing potential geological hazard related to rock glacier instability, offering critical insights for the site selection and safety assessment of major engineering projects in high-altitude permafrost regions.