金沙江“11·3”白格堰塞湖溃决洪水事件在奔子栏-石鼓段的地貌作用和沉积特征

The geomorphic process and sedimentary characteristics of the“11·3”Baige Dammed Lake outburst flood event in the upper reaches of the Jinsha River from Benzilan to Shihku

古堰塞湖溃决洪水事件的重建是当前地学研究的热点问题之一,寻找足够的可参考的现代溃决洪水事件案例是顺利开展这项工作的基础。2018年11月13日发生在金沙江干流的白格堰塞湖超万年一遇的溃决洪水事件(学术界称之为“11·3”白格堰塞湖溃决洪水事件)就是一个难得的样本。这次溃决事件发生在枯水期,洪峰完全由溃决洪水产生,没有叠加其他来源,对评估流域地貌和沉积体系对堰塞溃决事件的响应有很好的参考价值。本文以溃决洪水事件受灾最为严重的奔子栏-石鼓段为研究区,通过详细的野外调查和初步的水力学估算发现“11·3”白格堰塞湖溃决洪水事件在奔子栏-石鼓段的地貌作用主要表现为洪水淹没区的岸坡塌岸和沉积物堆积,未发现明显的基岩侵蚀。沉积物主要由分选良好的具水平纹层的砂组成。受金沙江较低的河床比降影响,洪水产生的基底剪切应力较弱在27~142 N/m^(2),不能悬浮和搬运直径5 cm以上的砾石,也不能产生明显的磨蚀和冲(撞)击作用。在发生塌岸的部分段落,洪水沉积物中有砾石坠入,甚至会出现类似浊流沉积的层序。这些现象的发现对深入理解堰塞湖溃决洪水的复杂地貌过程和沉积特征有重要参考意义。

Reconstruction of paleo-dammed lake outburst floods is one of the hotspots in current Earth science research.Collecting enough reference cases of modern outburst floods is the basis for this research.The outburst flood on November 13,2008,referred to as the“11·3”Baige Dammed Lake outburst flood,is a once-in-1000-year event when the outburst flood discharged from the Baige Dammed Lake,blocking the mainstream of the Jinsha River.As the flood event occurred during the dry season,the flood peak was solely from the outburst flood;therefore,this rare case provides a great reference for evaluating the impact of dammed lake outburst flood on the basin landform and sedimentary system.In this flood event,the upper reaches of the Jinsha River from Benzilan to Shihku,the focus area of this study,was most devastated.Based on detailed field investigation and preliminary hydrodynamic estimation,we found that the geomorphic process of the outburst flood was mainly manifested in bank slope collapse and sediment accumulation,with no significant bedrock erosion observed in the flooded area.The outburst flood sediments are mainly composed of well sorted,horizontally laminated sand.Due to relatively low slope gradient of the Jinsha River,the bed shear stresses exerted by the outburst flood,ranging from^(2)7 to 142 N/m^(2),were relatively weak and could not suspend and transport gravels larger than 5 cm,nor could it produce obvious abrasion and impact marks.Some collapsed banks show gravels falling from the bank slope into the outburst flood sediments;even turbidity sequences appear.These phenomena provide new geomorphologic evidence for further understanding the complex geomorphological process and sedimentary characteristics of dammed lake outburst floods.