Dynamic Formation Processes and Characteristics of “28 June 2012” Debris Flow in Aizi Valley, Ningnan County, Sichuan Province, China

Abstract

On 28 June 2012, a large debris flow occurred in Aizi gully, Ningnan County, Sichuan Province, China. It was the most significant debris flow disaster at a construction site in 2012 in China, which left 40 people dead or missing. Field investigations and model calculations were conducted following the event in order to determine dynamic parameters and execute engineering mitigation measures. Analyzing the collected data, it is found that earthquake and drought play a dominant role in debris flow initiation, and from 1 January 1986 to 31 December 2010, 43 earthquakes with a magnitude greater than 3.0 occurred around the study area. Through calculation it was found that the inducive radius of 16 earthquakes was larger than the distance between the epicenter and the study area, and it proves that the study area is frequently affected by seismic activities. Furthermore, on the basis of calculation of rainfall anomaly index H from January 2011 to June 2012, it can be seen that the rainfall of 2011 is extremely low. The percentage of the rainfall anomaly index is almost less than −34%, which indicates that the drought in the basin is serious. Under the influences of repeated seismic activities and continuous droughts, the structure of soil was destroyed and its strength and permeability changed significantly, providing favorable conditions for debris flow initiation. On 28 June 2012, heavy rainfall with the intensity of 23.3 mm/h occurred, and the total amount of precipitation reached 66.1 mm before 6am of 28 June 2012. Debris flow was induced due to the triggering effect of the torrential rain. Studying the causes of the large-scale debris flows in construction sites can significantly facilitate the prevention and mitigation of future debris flow disasters, as well as reduce the potential of hazards caused by debris flows in major engineering areas.