Wednesday 20 July 2022

Flash flooding kills at least twelve in western China.

At least twelve people have died, and thousands more have been forced to flee their homes, in a series of flash floods across western China. Six people have been confirmed dead and another twelve are missing in Sichuan Province, where 13 000 people have been evacuated from homes in low lying areas. In the city of Longnan, which lies in southern Gansu Province, slightly to the northeast of Sichuan, another six people have died and 3000 people have been forced to abandon their homes.

Cars trapped by floodwater in Lanzhou, Gansu Province, China. China News Service/Getty Images.

The flooding was triggered by 36 hours of near-continuous rainfall which fell over Friday 15-Saturday 16 July 2022, part of a season of extreme weather being suffered by China (and many other parts of the world). Much of China has been hit by an extreme heat wave, with many areas suffering heat in excess of 40°C. This has caused a wide range of problems, including making western China, which has a monsoonal climate, more vulnerable to flash flooding. 

Extreme heat waves can kill off herbaceous vegetation, which tends to stabilise environments against such events, and cause upper soil layers to bake dry, forming a solid crust which does not absorb water readily, leading it to flow over the soil surface. At the same time the high temperatures have extended over the South China Sea, raising the level of evaporation there, and therefore the amount of water being carried northward by southerly winds prevalent in the region at this time of year. As these warm, water-laden winds hit the uplands of the southern Tibetan Plateau, in western Sichuan and southern Gansu, they are pushed upwards where they encounter cooler air, leading them to rapidly drop their moisture as rainfall, leading to flash flooding events.

Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.

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