The 27 July 2011 debris flows at Umyeonsan, Seoul, Korea

On 27 July 2011, a total of 33 debris flows occurred in Umyeonsan, Seoul, Korea, resulting in 16 fatalities and extensive damage to houses, roads, and other properties. The debris flows originated during an intense rainstorm with a peak intensity of 112.5 mm/h and a cumulative rainfall of 306.5 mm over 16 h. The debris flows were triggered by a combination of two processes: (1) shallow translational landslides in colluvium at the ridge crest along a fault plane associated with the Chugaryeong Fault Zone and (2) the progressive entrainment of loose material due to surface water runoff in the channel. The ensuing debris flows extensively eroded the colluvial deposits overlying the bedrock along their paths and transported large quantities of soil and woody debris. Two watersheds with catastrophic debris flows in the study area were chosen for a forensic investigation to analyze the geomorphological features of the debris flow gullies and to study several factors influencing the debris flows. The debris flow activity was found to be primarily related to rainfall, as well as to the loose, compressible colluvium overlying the gneiss bedrock, the watershed topography with >30° slopes, and the vegetation.