These findings are based on a new method to calculate the flood vulnerability of cities, developed by a team of researchers from the Netherlands and the University of Leeds,
Researchers used their index to analyse the vulnerability to coastal flooding of nine cities built on river deltas: Casablanca in Morocco, Kolkata in India, Dhaka in Bangladesh, Buenos Aires in Argentina, Osaka in Japan, Shanghai in China, Manila in the Philippines, Marseille in France and Rotterdam in the Netherlands.
The results reveal that the highly prosperous megapolis of Shanghai, in China, is more vulnerable than much poorer cities such as Dhaka in Bangladesh.
Manila in the Philippines and Kolkata in India are also highly vulnerable, largely because of their huge populations and degree of exposure to storms.
The vulnerability index also revealed that Dhaka, which sits just metres above current sea levels, is regularly hit by tropical cyclones and floods, yet it has few defences in place and little resilience.
"Vulnerability is a complex issue," explained Nigel Wright, professor from the University of Leeds, who led the study.
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"It is not just about your exposure to flooding, but the effect it actually has on communities and business and how much a major flood disrupts economic activity," he said in a university statement.
"Our index looks at how cities are prepared for the worst -- for example, do they have flood defences, do they have buildings that are easy to clean up and repair after the flood? It is important to know how quickly a city can recover from a major flood," said Wright.
Shanghai is particularly vulnerable because it is exposed to powerful storm surges and the land is subsiding as sea levels rise.
"A 1-in-100 year flood in Shanghai would lead to widespread damage, with serious consequences for the city, across China and, through wider economic links, for the whole world," he said.
The index incorporates 19 components, including measures of the level of economic activity in a city, its speed of recovery, and social issues such as the number of flood shelters, the awareness of people about flood risks, and the number of disabled people in the population.