典型高山峡谷泥石流堆积扇发育过程及特征——以藏东南帕隆藏布流域天摩沟为例

Development and Geomorphic Characteristics of a Typical Debris Flow Fan in Alpine Valley:A Case Study of the Tianmo Gully in the Parlung Tsangpo Basin,Southeast Tibet,China

高山峡谷区泥石流动力地质作用强烈,是区域地貌变化的重要驱动力。泥石流堆积扇是支流(支沟)和主河(主沟)地貌外营力交互作用的地带,其形态变化反映了泥石流沟道自身水沙动态和地貌过程,也有助于理解高山河谷地貌的长期演变趋势。研究堆积扇发育过程和驱动因素有助于确定高山区河谷地貌演变成因和趋势。然而,受限于野外自然条件和数据资料,对高山区泥石流堆积扇发育过程的观测研究仍较少。本文以中国藏东南帕隆藏布流域天摩沟为研究对象,结合遥感影像、DEM数据、无人机航拍、RTK地形测量以及野外勘探调查,分析4次泥石流事件中天摩沟沟口堆积扇发育过程及形态特征。结果表明:(1)泥石流堆积扇发育过程受泥石流事件与一般水流过程共同影响。泥石流事件造成堆积扇宏观形态剧烈变化(扇面淤积抬升、扇体规模扩大等);一般水流过程则在泥石流间歇期缓慢塑造堆积扇沟道(沟床冲刷下切、流道局部平面摆动等)。(2)泥石流堆积扇发育与主河(帕隆藏布)相互影响。堆积扇外缘形态受主河径流调整和塑造,扇体沟道溯源侵蚀下切受控于主河河床侵蚀基准变化;主河局部地貌由原本相对窄深的单一线型发育成较为游荡的辫状河道。本研究可为泥石流沉积学、泥石流堆积扇工程选址及泥石流灾害风险防控等提供一定的科学数据支撑。

Debris flows in alpine-valley areas play a crucial role in regional geomorphologic evolution by severe dynamic geological effects.A debris flow fan is an interactive result of external geomorphic agencies occurring at a confluence reach between main river(main gully)and its tributary(branch gully);its morphological change reflects water-sediment dynamics and geomorphological process of a debris flow channel itself,and also helps to understand the long-term geomorphological evolution trend of alpine valley.Investigating the development process and driving factors of a debris flow fan helps to determine the geomorphic genesis and evolution of an alpine-valley landscape.Unfortunately,due to poor field working conditions and insufficient technical data,there were still few observation-based research to be conducted regarding to the development processes of debris flow fans in alpine-valley areas.In this research,a debris flow fan(named after Tianmo fan)developed at the outlet of the Tianmo Gully,a tributary of the Parlung Tsangpo River in the Southeast Tibet,China was aimed for a case study of debris flow fan evolution.It used remote-sensing images,DEM data,UAV aerial photography,RTK topographic survey,and other fieldwork to outline the development process and geomorphic characteristics of the fan after subjected to four times of debris flow events.It found(1)the formation and developing process of the debris flow fan was controlled by both episodic debris flow events and perennial stream flow.Debris flow events caused dramatic changes in macro-morphology of debris flow fans,such as silting fan surface,expanding fan body;in the case of debris flow fan eroded by perennial stream flow,it had been scoured and reshaped during an intermittent period between debris-flow events,demonstrated by incision of gully bed or lateral migration of flow channels.(2)Strong interactions existed between the development of the Tianmo fan and the Parlung Tsangpo River.The shape of the outer edge of the Tianmo fan was scoured and reshaped by the runoff of the Parlung Tsangpo River,and the headward erosion was controlled by base level of erosion.Consequently,the local landform of the main river has developed from a relatively narrow and deep single channel into kind of wandering braided river channels.The study can provide some scientific data support for sedimentology of debris flow,engineering site selection in debris flow fan and risk prevention and control of debris flow disaster.