Efforts to prevent the Yellow River flooding may have made it worse

Building mud barriers alongside rivers to prevent floods may have the opposite effect, suggests an analysis of flooding from the Yellow River in China.

More extreme and frequent rainfall due to global warming means river flooding is a growing threat to millions of people worldwide. While there is a large body of research on how climate change affects flood risk, the role of human activities is less clear.

To explore this, Shi-Yong Yu at Jiangsu Normal University in China and his colleagues analysed the frequency of floods on the Yellow River in northern China. This 800-kilometre-long waterway was the cradle of ancient Chinese civilisation between 4100 and 3600 years ago.

The researchers compiled a timeline of floods on the river from the past 12,000 years using historical records and data from river sediments. They found that flooding was rare between 12,000 and 7000 years ago, with an average of just four floods every 100 years.

They then compared the timeline of floods with records of human activities, such as agriculture, and found that floods became more common following the expansion of local human settlements around 4000 years ago.

In particular, the analysis revealed that flooding rates substantially increased around 1500 years ago, when people began building mud ridges along the river as flood barriers called levees, says Yu.

Flooding occurred 10 times more often in the past 1000 years compared with before the start of ancient Chinese civilisation, the researchers found. Their analysis suggests that human activities, primarily the use of artificial embankments, drove about 80 per cent of this increase in flood rates, with the rest attributable to natural changes in the climate, says Yu.

Computational modelling of the river indicates that riverside mud barriers may lead to a greater build-up of sediment at the bottom of the river. This lifts the riverbed and raises water levels, making floods more likely, says Yu.

“The work stresses the need to examine the range of human activities affecting flooding in the backdrop of climate change. This is an important message we must take on board today,” says James Best at the University of Illinois Urbana-Champaign.

Artificial embankments are no longer used to stem flooding in the Yellow River. Since the early 1980s, the Chinese government has introduced a policy to conserve wild riverside vegetation, which keeps the soil around the river stable, says Yu. This helps prevent soil from falling in and may be a better approach, he says.

But as building mud barriers is still the preferred flood prevention strategy in many parts of the world, the research suggests that other countries should also shift away from artificial embankments, says Yu. “We can learn lessons from studying the history of rivers,” he says.