Formation mechanism and disaster characteristics of debris flow in the Tianmo gully in Tibet

In September of 2007, July of 2010, September of 2010 and July of 2018, four large or giant debris flows took place in the Tianmo gully in Bomi county of Linzhi in Tibet. Four debris flow activities all blocked the Parlung River to varying degrees, buried the 318 national highway and destroyed bridges. Dammed lakes flooded village roads and caused downstream bank collapses. They caused great damage for the local residents’ life and property, especially the traffic trunk roads. Based on the above four debris flow activities, this paper uses high resolution remote sensing images to interpret the disaster-pregnant environment. Formation mechanism and disaster characteristics of different debris flows are analyzed in combination with the UAV aerial photography and ground investigation. The results show that (1) glaciers in the Tianmo gully is rich, and its interannual change is big. Formation of the primary debris flows in the Tianmo gully is rockfall (debris flows in 2007 and 2010) and dam breaking (debris flow in 2010). The Tianmo gully is characterized by glacial debris flow and rainfall debris flow at the same time. (2) After three large-scale debris flows in 2007 and 2010, the slope material source reserve in the Tianmo valley increased by 19.6 %, and most of the activated moraine and landslide material sources were converted into debris deposits in the debris flow gully or out of the gully mouth. (3) In 2018, the mass density of debris flow in the Tianmo gully is 2.10 g/cm3, and it is a viscous debris flow. The debris flow has a strong impact force and squeezes the Parlung river channel for many times. The maximum deviation distance in history is 190 m. (4) The debris flow on July 11 of 2018 is formed by rockfall on the right side of the main gully source caused by intensified glacier melting under the rain-fall condition, which has multiple bands. The volume of the main debris flow is 18×104 m3, and buried the G318 for nearly 220 m. The possibility of large-scale debris flows still exist in the future. It is suggested that prevention and controlling of debris flows are effective ways to reduce damage caused by debris flows.