A new study shows that the percentage of the global population at risk from flooding has risen by almost a quarter since the year 2000.
Satellite images were used to document the rise, which is far greater than had been predicted by computer models.
The analysis shows that migration and a growing number of flood events are behind the rapid increase.
By 2030, millions more will experience increased flooding due to climate and demographic change, the authors say.
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Flooding is the environmental disaster that impacts more people than any other, say researchers.
That view has echoed around the world in recent weeks, with huge inundations destroying lives and property
in Germany and China, record downpours overwhelmed defences, amid arguments about levels of preparation.
One of the challenges with flooding, according to researchers, is that most maps of where the waters will likely penetrate are based on models.
These simulate floods based on information such as elevation, rainfall and data from ground sensors.