汶川地震区文家沟泥石流成因模式分析

Anaysis on Genetic Model of Wenjiagou Debris Flows in Wenchuan Earthquake Area,Sichuan

文家沟2008～2010年期间8次泥石流事件是在地震滑坡堆积体上因持续强降雨渗透变形溃决和后续侵蚀产生的,不同于一般的沟谷型和坡面型泥石流。文家沟滑坡堆积体上新生的泥石流沟共冲出松散固体物质总体积约180×104m3,2010年的"8.13"泥石流事件是其中规模最大的一次,冲出的松散固体物质体积约在115×104m3。文家沟泥石流的成因模式是,强降雨过程在滑坡堆积体上先期出现"渗流管涌、暂态壅水、溃决滑塌"的造沟作用模式,后期出现"溯源侵蚀、冲刷刨蚀、侧蚀坍塌、混合奔流(搅拌机)"的扩沟作用模式。2008年的"6.21"和2010年的"7.31"泥石流事件主要起因于前者,其他事件主要起因于后者。松散堆积体因排泄持续降雨入渗的能力不足而造成地下水滞留和水位升高是导致斜坡体稳定性降低的内在原因。当地下水壅高水位面达到水平时,堆积体内渗透动水压力达到最大,堆积斜坡的稳定性最低,成为堆积体表层发生滑塌溃决的临界条件。

The eight events of debris flow happened in Wenjiagou catchment in the Wenchuan earthquake area, Sichuan, are different from either the general ravine type or the slope slipping type of debris flow, they are resulted from rainfall-induced infiltrating deformation-failure and erosion of the earthquake--landslide mass. The total volume of the eight events of debris flow from the Wenjiagou earthquake--landslide mass is about 180 ×10^4m^3, in which, the maximum one with the volume of 115 ×10^4m^3 happened in Aug. 13, 2010. The mechanism of the Wenjiagou debris flows is that of rainfall-induced seepage and piping, temporary dam water, bursting mass sliding of landslide mass in the early valley-forming stage, and headward erosion, washing and digging away, sideerosion and collapse, mixing flow （similarly to the stirring machine action） in the late valley-enlarging stage. The two events of debris flow happened in June 21,2008, and July 31,2010, are resulted from the mechanism of the early valley-forming stage, and the others are resulted from the mechanism of the late valley-enlarging stage. The inherent mechanism of the slope failure and induced debris flow is that the capacity of rainfall infiltration is not enough to cause groundwater stagnant and water-level increasing in losing landslide mass. When temporary dam water is increasing up to the horizontal level, the hydrodynamic pressure is up to the maximum value, then the slope stability decreases to the lowest, and finally, the critical condition of failure slide will be appeared in the surface of landslide mass.