The US-based World Resources Institute think-tank and four Dutch research groups estimated that some 21 million people worldwide were affected by river flooding in a year.
"That number could increase to 54 million in 2030 due to climate change and socio-economic development," a report by Institute for Environmental Studies of the VU University Amsterdam said.
In India, approximately 4.84 million people get affected by floods in a typical year, the study said.
These countries are all considered least developed or developing. Roughly 167,000 people in the US, the highest-ranked high-income country, are affected every year by floods, the study said.
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India also has the highest effect on its GDP by river flooding at USD 14.3 billion, it said.
"India faces more potential change in exposed GDP than any other country. Using a middle-of-the road scenario, India's current USD 14 billion in GDP exposed annually could increase more than 10-fold to USD 154 billion in 2030," it said.
"This phenomenon would expand flood-prone areas, and make floods more likely to occur in those areas more often," the study said.