Flood risks in delta cities are increasing, study finds

(Press-News.org) New research shows how the combination of extreme climate events, sea-level rise and land subsidence could create larger and deeper floods in coastal cities in future. 
 
The study focused on Shanghai, in China, which is threatened with flooding by large and strong typhoons, or tropical storms, producing storm surge and waves. 
 
When these events coincide with other causes of flooding, such as high water flows in the Yangtze River, they can combine to create even more catastrophic floods, as happened with Typhoon Winnie in 1997. 
 
The study was carried out by researchers from the University of East Anglia (UEA), Shanghai Normal University and the University of Southampton, together with other institutions in China, the United States and the Netherlands. 
 
It assessed all the causes of flooding in Shanghai and found that if considering climate, sea-level rise and land subsidence, by 2100 the floods of Shanghai could expand in size by up to 80 per cent and be much deeper. 
 
The authors say that to avoid disaster a major adaptation effort is required, which will almost certainly include raising defences and constructing mobile flood barriers, like those seen at the Thames Barrier in London. 
 
However, they warn there is also the risk of “catastrophic failure” of defences due to rising water levels, especially due to the combination of subsidence, sea-level rise and higher surges during typhoons, as occurred in New Orleans during Hurricane Katrina in 2005. 
 
They say this danger is not fully appreciated and must be considered in adaptation in Shanghai and other deltaic cities, with a layered rather than single line of defence needed. 
 
The study, published today in the journal One Earth, is the first comprehensive analysis of flooding in a delta city. 
 
“These findings have wider implications for all coastal cities and especially those built on deltas like Shanghai,” said lead UK author Prof Robert Nicholls, of the Tyndall Centre for Climate Change Research at UEA and University of Southampton. “Such analyses are critical to anticipate and support the significant adaptation needs in these cities.” 
 
Low-lying deltas host some of the world’s fastest-growing cities and vital economic centres, but they are increasingly vulnerable to flooding from tropical and extratropical cyclones. 
 
Floods are driven by combinations of tide, storm surge, wave, river flows and rain. The most extreme floods occur due to the simultaneous combination of different sources of flood, such as a high river flow and a storm at the same time. 
 
“The likelihood and magnitude of floods are often underestimated as these combined floods are not considered,” said Prof Nicholls. “Further climate change and land subsidence - all deltas sink - is increasing the likelihood of flooding. Therefore, the threat is growing in all coastal cities and especially delta cities where all these issues occur.” 
 
The team used an atmosphere, ocean, and coast model (AOCM) of the Shanghai region that for the first time includes all the flood drivers. Taking 10 historic typhoon events that produced significant floods, they simulated how they will change over the next 75 years to 2100, with different amounts of climate change and land subsidence. 
 
Lead author Prof Min Zhang, of Shanghai Normal University, said: “We find that the area flooded in a typhoon by an extreme, one in 200-year event - an event that should be considered in disaster risk management and flood planning - could increase by up to 80 percent in 2100”. 
 
“The response to this challenge will almost certainly be raising of defences as Shanghai and most delta cities are already defended. However, rising water levels, especially due to the combination of subsidence, sea-level rise and higher surges during typhoons raise the prospect of catastrophic failure and large, deep floods if the defences fail.” 
 
Prof Nicholls added: “This so-called ‘polder effect’ when defences fail is not fully appreciated. It must be carefully considered in adaptation planning in Shanghai and other deltaic cities. Rather than depending on a single line of defences, layered defence is needed to make these cities more resilient today and into the future.” 
 
‘Growing Compound Flood Risk Driven by Both Climate Change and Land Subsidence Challenges Flood Risk Reduction in Major Delta Cities’, is published in the journal One Earth.

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