Formation Mechanism Of Giant Debris Flow: A Case Study In Zhouqu, China

A giant debris flow occurred in Zhouqu County, Gannan Tibetan Autonomous Prefecture, Gunsu Province, in the evening of 8 August 2010, causing 1765 deaths and missing, with enormous property losses. It ruined 4321 houses and caused 22,667 homeless. The stricken area at Sanyanyu debris flow was 50.0 hm(2) (including 3 hm(2) urban area and 47hm(2) farmland). A dammed lake 2 km in length was formed in Bailong River by the debris flow deposit with 8-10 m high, which blocked the river. The main urban area of Zhouqu city was inundated for one month. The major causes of debris flow formation are the abundance of unconsolidated materials, rough channel slope and motivating conditions (rainstorm and outburst flood). In Sanyanyu, there is still over 500 million m(3) unconsolidated soil. Among them, 200 million m(3) is easy to erode and thus generate debris flow. There is a series of debris barriers, with the height ranging from 40 to 280 m, which blocked the channel. The debris dams breached during the heavy rain, the recorded rainfall of 77.3 mm in one hour at nearby Dongshan station, and formed the outburst flood. The field survey released that flood in upstream eroded the debris barriers and unconsolidated soil bed in channel and developed into debris flow. The laboratory physical experiments indicated that the major mechanism of giant natural debris flows formation is scale amplification caused by cascading landslide dam failures and mass supply of eroded channel bed. Moreover, the new models to calculate the scale enlargement of debris-flow peak discharge that accounts for cascading landslide dam failure and entrainment of sediment materials have been developed by physical experiments.