Catastrophic Debris Flows on July 10th 2013 along the Min River in Areas Seriously-hit by the Wenchuan Earthquake

Over 240 debris flows occurred in hill-slopes, gullies ( indicated those with single-channel) and watersheds (indicated those with tributaries and channels) on July loth 2013 in the Wenchuan county, and caused 29 casualties and about 633x10(6) USD losses. This work aimed to analyze characteristics, hazards and causes of these events and explore mitigating measures based on field investigation and remote sensing images interpretation. The debris flows contained clay content of 0.1%similar to 3.56%, having densities of 1.72 similar to 2.14 t/m(3), velocities of 5.0 similar to 12.7 m/s, discharges of 335 similar to 2353 m(3)/s and sediment yields of 0.10 similar to 1.26x10(6) m(3), and also numerously occurred in large watersheds with the area over 10 km(2). Large debris flows formed 3 hazard-chains in slopes, gullies, watersheds and rivers, which all evolved in dammed lakes and outburst flood, and 26 dammed lakes and 10 newly ones were generated along the rivers of Min and Yuzi. The remarkable spatial difference of loose solid materials accumulation and intense rainfall, with the cumulative of about or more than 150 mm and the hourly of over 16mm, caused debris flows in the sections from Yingxiu to Miansi and Gengda. The damages on buildings, reconstructions, highways, factories and hydro power station originated from the impacting, scouring, burying of debris flows, the submerging of dammed lake and the scouring of outburst flood, and the huge losses came from the ruinous destructions of control engineering works of debris flows as well as the irrational location and low-resistant capabilities of reconstructions. For hazards mitigating of debris flows in long term, the feasible measures for short term, including risk-reassessing of foregone and potential hazard sites, regional alarming system establishing and integrated control in disastrous sites, and middle-long term, including improving reconstruction standard, rationally disposing river channel bed rise and selecting appropriate reconstruction time and plans, were strongly suggested.