Characteristics and hazard prediction of large-scale debris flow of Xiaojia Gully in Yingxiu Town, Sichuan Province, China

The Wenchuan earthquake of May 12, 2008 produced large amounts of loose solid material that can lead to the development of devastating debris flows. In 5 years since the earthquake, large numbers of large-scale debris flows occurred and caused great damage to the resettled population and the reconstruction efforts. The reconstructed Yingxiu Town suffered debris flow and its secondary disaster in the past 5 years, and will be in the risk range of debris flow hazard in the future. It is very important to predict debris flow and its secondary hazard (especially the dam-breaking flood) near the town. The debris flow of the Xiaojia Gully was described and analyzed as a typical large-scale case in the Yingxiu area. The study aimed to describe the debris flow event, investigate the sediment supply conditions, analyze the characteristics of the debris flow deposition, calculate the river-blocking parameters, and predict the damage of dam-breaking flood to Yingxiu Town under different debris flow frequencies. Debris flows in Xiaojia Gully were well developed and occurred several times, resulting in frequent interruption of traffic on the S303 road. The interpretation of satellite images showed that landslide deposition, channel deposition, and soil erosion sediment were three kinds of sediment supply types for debris flow formation. 188 sediment supply sites in total were identified in the catchment. The total area for the supply of sediment for debris flows is about 0.99 km2, giving a total volume of 532.09 × 104 m3. If the Yuzi River is blocked by debris flow material, the dam-breaking flood can cause potential damage to the newly constructed Yingxiu Town. The calculation results reveal that debris flows with a frequency of 5% and 10% are impossible to cause river-blockage, and consequently, the Yingxiu Town is safe in these situations. However, debris flows with a frequency of 1% and 2% may cause river blockage and the dam-breaking flood can cause damage to the Yingxiu Town.