The Disastrous 23 July 2009 Debris Flow in Xiangshui Gully, Kangding County, Southwestern China

On 23 July 2009, a catastrophic debris flows were triggered by heavy rainfall in Xiangshui gully, Kangding county, southwestern China. This debris flow originating shortly after a rainstorm with an intensity of 28 mm per hour transported a total volume of more than 480×103 m3 debris, depositing the poorly sorted sediment including boulders up to 2-3 m in diameter both onto an existing debris fans and into the river. Our primary objective for this study was to analyze the characteristics of the triggering rainfall and the debris supply conditions, and to estimate debris-flow volume, mean velocity, and discharge. A comparison with adjacent rain-gage records indicates that debris flows in this setting can be produced in response to as little as 17 mm/hour or 3.5 mm/10-minute of rainfall intensity with relatively lower amount of cumulative antecedent rainfall. The field measurement and the interpretation of the Worldview image indicate that abundant landslides occurred on steep slopes within areas underlain by highly weathered granite. Using empirical equations that combine flow depth and channel slope, the mean velocity and discharge of the debris flow were estimated to be 9.2 m/s and 2150 m3/s, respectively. The results contribute to a better understanding of the conditions leading to catastrophic debris flows.