Rapid urbanisation and rising sea levels will expose coastal cities to costly and dangerous flooding, according to a new study.

Global flood damage could cost $US1 trillion ($1.11 trillion) a year by 2050, the report by World Bank economist Stephane Hallegatte and the Organisation for Economic Co-operation and Development says.

Most coastal city defences are not designed to cope with climate change and rising sea levels, and substantial investment and better planning are required.

The study examined maps and databases of population and world assets, storm frequency data and damage cost models for 136 large coastal cities.

In terms of overall damage cost, the cities at greatest risk are Miami, New York, New Orleans, Tampa and Boston in the US, Guangzhou and Shenzhen in China, Mumbai in India and Nagoya and Osaka in Japan.

Cities where flood risk will increase most are not necessarily those at high risk today, the report warns.

Those facing the greatest increase in risk by 2050 are Alexandria in Egypt, Barranquilla in Colombia, Naples in Italy, Sapporo in Japan and Santo Domingo in the Dominican Republic.

“Coastal defences reduce the risk of floods today, but they also attract population and assets in protected areas and thus put them at risk in case the defence fails, or if an event overwhelms it,” Mr Hallegatte said.

“If they are not upgraded regularly and proactively as risk increases with climate change and subsidence, defences can magnify – not reduce – the vulnerability of some cities.”

Better crisis management and contingency planning, including early warning systems and evacuation plans, are also needed.

In cities where flood damage is uncommon, spending on defences can be politically unpopular, the report warns.