The river blockage characteristics of “8·20” large-scale debris flow and the hazard prediction in Banzi gully in Miansi Town，Wenchuan County

After Wenchuan earthquake, many large debris flow have occur in Banzi Gully. Especially in 2019, the "8·20" debris flow caused serious damage to the roads, bridges and villages in the gully mouth, mail river was seriously squeeaed to the other side and there is a big risk of blocking the river in the future. Based on the field investigation and comprehensive analysis of the basic characteristics and formation conditions of the debris flow, the characteristics of river blockage are analyzed, the parameters of river blockage under different debris flow frequency are calculated, and the possible impact of dam-breaking flood under each frequency on Miansi Town is predicted. The results show that debris flow with frequencies of 2%, 5% and 10% are less likely to cause river blockage of Min River, and if there was a river blocking event , the town of Miansi would not be exposed to dam-breaking flood. The debris flow with a frequency of 1% are likely to cause river blockage. The debris flow material with a volume of 57.38×10⁴ m³ can reach the Min River to form a dam with a height of about 51.61 m. Under the action of the main river flood (1890.0 m³/s), the dam will be broken, and the peak flow rate of the flood was 5935.49 m3/s, which dropped to 2312.25 m³/s while arrive at Miansi Town. Since the flood depth (4.00 m) is higher than the height of the protective embankment (3.5 m), the flood is likely to damage the houses near the protective embankment of Miansi Town.This paper provided a reference for the analysis of the characteristics of large-scale debris flow blockage in the future, as well as the possible impact of the collapse flood on downstream towns.