2012--Debris flow in Aizi Gully, Sichuan Province

A disastrous debris flow event occurred at Aizi Gully in Ningnan County, Sichuan Province, between 5 and 6 AM on June 28, 2012. It destroyed a three-storied dormitory, resulting in 40 deaths, most of whom were construction workers building the key project Baihetan Waterpower Station.

The location and topography of Aizi gully

The gully has a long scoop shape which facilitates runoff confluence. The average longitudinal slope of the channel beds in the two gullies is about 155%. Furthermore, there are several steep steps and cliff sides which magnified the debris flow volume and velocity.

The debris flows were triggered by a local intensive rainfall of 26.9 mm/h after one month of antecedent rainfall. The daily rainfall from 27 June to 28 June was 75 mm.

The formation area was the sub-catchment belonging to the upstream right tributary. Massive material flows from hill slopes and small gullies were concentrated within a short duration of approximate 30 minutes. Than the peak discharge was amplified by 3 to 4 times to 826 m3s-1 because meandering channels and narrow passes of the main stream concentrated the flows, and a great mass of debris from colluvium, diluvium, and sediment deposits in the downstream channel was eroded. The event is characterized by a short duration of formation and propagation, moderate scale but high peak discharge, and inter-occurrence of flooding and debris flows.

Several hydropower stations are located downstream along the Jinsha River in one of most susceptible regions for landslides and debris flows in China.

A lesson from this tragedy is the need for hydropower engineers to pay special attention to potential low-frequency debris flow ravines such as Aizi Ravine.

 

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Loose materials along the channel