岷江上游叠溪溃坝堆积体的沉积特征与历史溃决洪峰流量

Sedimentary Characteristics of the Stacked Body of the Broken Diexi Dam in the Upper Reaches of the Minjiang River,China and Its Historical Peak Flow

岷江上游叠溪河谷段在地质历史时期发生了一次大型岩质古地震山体滑坡事件,并堵塞岷江形成一个大型古堰塞湖。堰塞湖形成后在晚更新世晚期(约27 ka BP)发生了溃决,并在其下游形成长约5 km的天然溃坝堆积体。基于野外地质调查,采用测量统计、取样分析及理论计算等方法,研究了叠溪古堰塞湖溃坝堆积体的沉积特征,并重建了溃决洪水的最大洪峰流量。叠溪溃坝堆积体主要由砾石、卵石、砂及少量粉/黏土组成,剖面中发育叠瓦、孔洞、块状、杂基、支撑—叠置及韵律互层等构造特征。从上游至下游,溃坝堆积体的出露厚度逐渐变薄,砾石碎屑成分表现出由粗变细的变化趋势。采用水力学理论方法重建了溃决洪水的最大洪峰流量,溃口处洪水平均流速和最大洪峰流量分别为17.23 m/s和49821.28 m^3/s,如此大规模的堰塞湖溃决洪水在世界范围内也是十分罕见的。叠溪溃坝堆积体的沉积特征与溃决洪峰流量重建,对于了解滑坡堰塞湖坝体溃决过程、溃决洪水演进机制以及山区地质环境演化规律等方面具有重要意义。

During the geological history,a large-scale shock-induced rock landslide occurred at Diexi in the upper reaches of the Minjiang River and blocked the Minjiang River to form a large dammed lake. After the formation of the dammed lake,later it broke in the Late Pleistocene（ about 27 ka BP） and formed a natural dam-break mass downstream about 5 km long. Based on the field geological survey,the sedimentary characteristics of the Diexi dam were studied by means of measurement statistics,sampling analysis and theoretical calculation,and the maximum outburst flood discharge was reconstructed. The gravel,pebble,sand and a small amount of powder/clay were the main components of Diexi dam-break deposit. Structural features such as imbrication,caviation,bolder structures,miscellaneous groups,supporting-stacking and rhythmic interbed were developed in the profile. From the upstream to the downstream,the exposed thickness of the dam body gradually becomes thinner,and the composition of the gravel debris changed from coarse to fine. The maximal peak flow of the outburst flood was reconstructed by using the theory of hydraulics. The average flood peak velocity at the breach and the maximum peak flow were estimated as 17. 23 m/s and 49821. 28 m^3/s, respectively. Therefore, it is rare case worldwide. The sedimentary characteristics and the reconstruction of outburst flood discharge in Diexi dam are of great significance in understanding the process of failure of landslide dam,the evolution mechanism of flush flood and the evolution of the geological environment in mountainous areas.