Monitoring the glaciers with remote sensing techniques to investigate the triggering mechanism of debris flow in Tianmo gully, Tibet

For glacierized mountain valleys with debris flow hazard, obtaining the glacier dynamic can facilitate the analyzing of the triggering mechanism of debris flow. Tianmo gully in Bomi County, Tibet is a prominent debris flow hazard. Large-scale debris flows occurred multiple times in this gully during the past two decades. In this study, Planet, TanDEM-X, and Sentinel 2 images were used to estimate the changes in glacier boundary during 2016 to 2021, the changes in glacier surface elevation during 2000 to 2016, and the changes in glacier flow velocity during 2016 to 2021. Results show that the glaciers did not surge or collapse before the large-scale debris flow occurred on 11 July 2018. By analyzing the optical images and meteorological data, we deemed that the rock debris surrounding the glaciers that was generated by rock collapse, the moraine and scattered ice in front of the glaciers, and the loose material generated by landslides in the middle-to-upstream part of the main gully, composed the triggering mass of the debris flow on 11 July 2018, and two consecutive days of heavy rainfall just before the debris flow saturated the moisture content of the mass and reduced the shear strength of the soil. Continuous light rain of the day directly induced the creeping of mass in high places, triggering the debris flow. Glaciers partly contributed mass and water source to the debris flow hazard in the Tianmo gully. The flow velocity of the glacier in Tianmo gully is notably higher than that of surrounding glaciers. The peak velocity of the glacier trunk is up to 0. 80 0. 02 m . d (1), which makes the glacial moraine accumulate fast. Meanwhile, the glacier meltwater lowers the threshold of rainfall to trigger the debris flow. No large-scale debris flow has occurred since July 2018. However, the side slopes in the mid-to-upstream and midstream parts of the main gully are steep and loose, which renders them prone to collapsing or sliding under the influence of rain and glacier meltwater. Unmanned aerial vehicle images show that a certain amount of loose material has already gathered at the bottom of the midstream part of the main gully. Moreover, the glacier flow will continuously increase the volume of the moraine. If the mass resource accumulates to a certain extent, heavy rainfall will trigger debris flow in the Tianmo gully again.