A severe rainstorm that should only happen once every 50 years caused widespread flooding in Sichuan between 8-10 July. Torrential flood water caused severe damage to buildings, washed out bridges, and set off a landslide that buried up to 40 people in Zhongxing Township. Ya’an city (which suffered from an earthquake in April) was flooded, affecting almost 5,000 households; direct economic losses are estimated at 768 million yuan (US$125 million). In total the fierce rainstorms have affected more than 500,000 people, 36,800 people of whom have had to be relocated.
Sichuan is prone to seismic activity, which makes the slopes of its hills and mountains unstable. Deforestation has also caused severe soil erosion. As a result, large amounts of loose sediment can become deposited in river channels, particularly during heavy precipitation.
During the July flood the town of Beichuan was the victim of this trend – there has been continuous deposits of eroded sediment in the surrounding catchments and channels of the Qing Jing River that flows through Beichuan. The river flood protection standard had also been downgraded from being capable of dealing with heavy storms (of the kind only supposed to occur once every 20 years) to being capable of responding to the smaller floods that occur every few years. As a result flood water rose above embankments and inundated the town.
The way forward
Though the Sichuan flood in July was the product of a rainstorm that should happen only twice a century, such extreme weather events may occur more frequently as a result of climate change.
To mitigate flood risk, it is important to integrate flood management with science. We need to understand the hydrological and geomorphological processes from all parts of river catchment in Sichuan; only then will we be able to identify causes in specific locations, such as due to sediment accumulations from hill slopes, or lack of drainage capacities in urban areas, and then respond accordingly.
Understanding the socio-economic conditions is equally important, such as the characteristics of the population and their economic assets at risk. Sometimes it may be more cost-effective to relocate a whole village to some safer location as flood protection measures are normally expensive.
Finally, towns and districts should be equipped with flood warning systems, as we have learnt from many previous painful lessons that dykes and banks might be breached in extreme scenarios. Adequate and accurate warning systems can encourage preparedness, and more importantly give people time to leave dangerous areas, saving their life and some personal assets.