摘要:
西藏芒康县金沙江上游雪隆囊河谷史前时期(全新世晚期)发生了一次明显的堰塞事件,形成了一个湖水体积约3.1×108 m3的大型堰塞湖。该堰塞湖形成后期发生溃决并引发异常大洪水,这一溃决事件发生在大约1 117 A.D.。地震诱发山体滑坡可能是金沙江发生堰塞的直接原因。在雪隆囊古堰塞坝体的下游一侧到其下游3.5 km的范围内,发现大量由砾石、砂和少量黏土组成的混杂堆积体,判定其为滑坡堰塞湖的溃坝堆积,是滑坡坝体及上游河床物质在坝体溃决后快速堆积形成。整套溃坝堆积体具有支撑—叠置构造、叠瓦构造和杂基构造等沉积特征,还具有一种特殊的沉积构造:即在垂向剖面上发育粗砾石层与细砂砾层的韵律互层,但剖面中缺少砾或砂的透镜体。这种沉积构造("互层构造")是溃坝堆积相区别于冲—洪积相、泥石流相等的一种重要判别标志。采用水力学模型反演确定雪隆囊古滑坡堰塞湖溃决洪水的平均流速为7.48 m/s,最大洪峰流量为10 786 m3/s。雪隆囊溃坝堆积体沉积特征及其环境的研究,不但有助于揭示古洪水事件发生的过程和机制,同时对于认识金沙江上游地区的环境演变也具有重要意义。
Abstract:
A distinct damming incident occurred at the prehistoric period(Late Holocene) in the upper Jinsha River in the Mangkang County, Xizang province, China. The volume of the dammed lake is about 3.1×108 m3. The dammed lake breached and caused a big outburst flood at about 1 117 A.D. We can infer that the paleoseismics induced major landslide may be the direct cause triggering the river damming. In the range between the downstream side of the ancient dam body to its downstream 3.5 km, abundance of diamicton consisting of gravels, sands and minor silts and clays was discovered. The sedimental structure of the dam break-outburst sediments includes gravel support-stacked structure, gravel imbricate structure and matrix structure, and so on. Besides, it has a special sedimental structure: rhythmites interbedded structure composed of coarser gravel layer and finer sand and gravel layer, no sand lens are formed in the layers. This special structure can be an important origin criteria of the dam break-outburst sediments. Based on back analysis using the hydraulic model, the average flow rate velocity of the outburst flood is 7.48 m/s and the maximum peak discharge is 10 786 m3/s. The research on the sedimentary features and its environment of the dam break-outburst sediments can not only be served to help revealing the process and mechanism of ancient flood incidents, but also be significant of knowing the environmetal evolution in the upper Jinsha River area.
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