The 10 June 2021 debris flows at Wuxie in Zhejiang, China

On 10 June 2021 an extreme rainstorm triggered over 100 landslides, including a set of devastating debris flows, in the Wuxie scenic area in Zhejiang Province in China.  A useful paper just published in the journal Landslides (Wang et al. 2022) provides an overview of the events.

Recent Google Earth imagery has captured the landslides, which are located in the area of 20.709, 120.047, well.  In particular there are three large debris flows in the vicinity of the dam for the Wuxie Reservoir:-

Large debris flows triggered by the 10 June 2021 rainfall event at Wuxie in China.

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The debris flows were triggered by the most intense rainfall event on record for this area.  Wang et al. (2022) note that the nearest rain gauge recorded just over 200 mm of rainfall over a seven hour period, but with a peak rainfall intensity of around 110 mm in a single hour.  It is this type of extreme rainfall intensity that often triggers catastrophic debris flows.

One set of debris flows impacted upon a set of buildings that included houses, restaurants and hotels.  The Google Earth image below shows this area (but note the landslides that can also be seen in the area in the background):-

Damaging debris flows triggered by the 10 June 2021 rainfall event at Wuxie in China.

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Four people were killed by these landslides, and four houses were destroyed.  The debris flows inflicted serious economic losses too as the recreation area was closed for three months through the peak summer season.

As is so often the case, these debris flows initiated as comparatively small, shallow slips high in the catchment, triggered by saturation of the soil on steep slopes.  Fluidisation of these failures  then caused them to transition to channelised debris flows, entraining water and debris as they travelled down the gullies.  Of course, in these extreme rainfall events the gullies themselves would have been in a state of high flow.

In this case, lessons need to be learnt in terms of placing buildings at the toe of gullies with no mitigation against the threat from debris flows.  As the buildings housed restaurants and hotels, they were constructed to attract people to the site, placing them at high risk.   We see this pattern time after time.

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Reference

Wang, F., Yan, K., Nam, K. et al. 2022. The Wuxie debris flows triggered by a record-breaking rainstorm on 10 June 2021 in Zhuji City, Zhejiang Province, China. Landslides (2022). https://doi.org/10.1007/s10346-022-01903-w