Subsidence control reduces flood risk in China’s coastal communities

Monday, 28 November, 2022

New research suggests that implementation of a national policy of subsidence control would greatly reduce the impacts on sea level rise for people living in coastal areas of China.

Coastal residents are concentrated in subsiding areas and the effect of land subsidence on relative sea level rise is much higher in China than it is globally.

The study, led by scientists at the University of East Anglia, in the UK, and in China, is the first to analyse the benefits of subsidence control for coastal flooding in the country. The team says the issues raised in the work, published in the journal Nature Communications, also have “global significance”.

The researchers built on a global study on relative sea level rise and their impacts. This found south, south-east and east Asia are especially prone to land subsidence issues, both natural and human-induced – relating to rapid urbanisation, economic development and industrialization, and over-exploitation of groundwater – and are experiencing high relative sea level rise.

This new study focuses on China in much more detail and considers 36 coastal cities that are subsiding in large part due to human factors.

They warn that subsidence will continue to be a risk in many countries unless there is a broad-scale policy response utilizing subsidence control combined with coastal adaptation. The method used in China could be applied in other countries in Asia with similar issues.

Prof Robert Nicholls, Director of the UK’s Tyndall Centre for Climate Change Research and Chair of Climate Adaptation at UEA, said: “Our results emphasise that subsidence is a significant threat to China’s coastal environment and human well-being. It has the largest coastal population in the world and a high subsidence potential along much of its coast due to the presence of many deltas and alluvial plains.

“We found each Chinese coastal resident experienced on average relative sea level rise of 11 to 20 mm a year in 2015. This is three to five times higher than climate-induced sea level rise and is largely due to subsidence caused by groundwater withdrawal.

“In 2050, assuming these subsidence rates continue, land area, population and assets exposed to the 100-year coastal flood event is 20%-39%, 17%-37% and 18%-39% higher, respectively, than assuming climate change alone.

“Realistic subsidence control measures, especially greatly reducing groundwater withdrawal from sedimentary coastal lowlands, can avoid up to two-thirds of this additional growth in exposure with adaptation required to address the residual.”.

China has experienced human-induced land subsidence for decades. To reduce its adverse impacts, a national response policy ‘The prevention and control planning of land subsidence in China’ was implemented in 2012.

Land subsidence control has since been successfully implemented in many coastal cities and this study shows that there would be great benefits from truly national implementation.

Dr Jiayi Fang, Associate Professor at the Hangzhou Normal University, China, said: “Traditionally, subsidence is considered to be a local problem. This research demonstrates subsidence has national implications and we found a strong need to continue upgrading coastal protection measures and implement subsidence control in coastal China.”

The research team assessed four components of relative sea-level change – climate induced sea-level change, the effects of glacier weight removal causing land uplift or sinking, estimates of river delta subsidence and subsidence in cities.

A higher resolution, more detailed, national database for the Chinese coasts, with human-induced information such dikes and city subsidence, was built. Coastal flood risks and possible response strategies for China, including estimates of present rates of relative sea level rise and flood exposure and risk to 2050, were analysed.

Prof Nicholls said: “We hope that our analysis improves the understanding of the importance of subsidence control, where appropriate, and climate adaptation actions. A wider range of adaptation measures across protection, accommodation, retreat and advance could be considered in coastal management policy worldwide.”

The research was led by the Hangzhou Normal University (Hangzhou, China) and University of East Anglia (UK) in collaboration with the University of Southampton (UK), Global Climate Forum, Berlin (Germany), Humboldt-University, Berlin (Germany), Kiel University (Germany), Shanghai Normal University (Shanghai, China), East China Normal University (Shanghai, China) and Beijing Normal University (Beijing, China).

It was funded by the National Key R & D Program of China, National Natural Science Foundation of China, the European Horizon 2020 Framework Programme and Seventh Framework Programme.

‘Benefits of subsidence control for coastal flooding in China’ was published in Nature Communications on November 14, 2022.

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