西藏天摩沟冰川遥感监测及泥石流触发成因分析

对于有冰川发育的泥石流隐患点而言, 获取冰川动态有助于分析泥石流触发成因.西藏波密县天摩沟是大型泥石流灾害隐患点, 在过去二十年中多次暴发泥石流.本文利用Planet、TanDEM-X和Sentinel-2卫星数据估计了天摩沟冰川2016—2021年间的边界变化、2000—2016年间的表面高程变化, 以及2016—2021年间的流速变化.冰川边界、表面高程和流速变化观测结果均表明天摩沟冰川在2018年7月11日大型泥石流暴发前未发生跃动或崩塌.通过分析高分辨光学影像和气象数据, 认为冰川附近山体崩滑产生的岩屑, 冰川前方的冰碛物和散冰, 以及主沟中上游沟岸滑坡产生的松散物共同组成2018年7月11日泥石流的启动物源, 灾前连续两天降水使得物源含水量达到饱和, 土体抗剪强度降低, 灾害当天持续小雨诱发高位物源启动, 形成泥石流灾害.天摩沟冰川对泥石流发育的作用在于贡献了部分物源和水源.其流速明显高于周边冰川, 主干峰值流速可达0.80±0.02 m·d^-1.冰川高速流动使得高位冰碛物堆积速度快, 而冰川融水使得泥石流降雨启动阈值更低.天摩沟在2018年7月之后没有再发生大规模泥石流, 但是主沟中上游及中游两侧坡体陡峭且岩性松散, 在雨水和冰雪融水作用下极易垮塌.无人机像片显示目前主沟中游沟底已经累积一定量的松散物.同时天摩沟冰川的流动会持续增加高位冰碛物体积.如果沟内物源累积到一定程度, 强降雨仍会诱发泥石流.

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.