Even if climate change and urbanization mean that floods are more frequent and the consequences are worse, a large proportion of humanity still lives in such areas. Worse still, there are even more. In a new study published in Nature, researchers used satellite images to to map over 900 flood eventsthat occurred between 2000 and 2018, affecting between 255 and 290 million people.
While the world population grew by 18.6 percent from 2000 to 2015, population growth in these areas of all places significantly exceeded this value – with an increase of 34.1 percent in the same period. This means that between 58 million and 86 million more people in these areas were exposed to flooding over 15 years. “It’s not particularly surprising that floods are increasing,” says Beth Tellman, co-founder of the Flood map start-ups Cloud to Street and lead author of the study. “But what I noticed was that people were moving to places where we have seen floods in the past.”
The researchers examined over 3,000 events in the Dartmouth Flood Observatory database, which records floods from recent media coverage. They then matched the events with location data and satellite images from MODIS, a detector mounted on two NASA satellites that have been taking daily images of the earth since 2000. The researchers used an algorithm to map where the flooding had occurred, which checked which pixels were covered with water and which were not. Then they added population data to see how trends in the flooded areas changed over time.
Growth in the flood area
Low- and middle-income countries have seen the fastest population growth in flood-prone areas over the past two decades – with the highest growth rates in sub-Saharan Africa and southern Asia. Socio-economic factors could explain part of this migration, Tellman says. Vulnerable groups may have no choice but to settle in floodplains, where land is cheaper and more readily available. Using satellite imagery, researchers were able to describe the effects of real floods more precisely than conventional models. Models can capture some types of flooding, such as: B. those that occur on rivers and coasts. However, for other floods caused by heavy rainfall or random events – such as dams breaking or a storm surge – satellite images provide a clearer picture.
The 913 mapped floods are still only a fraction of the tens of thousands that occur worldwide each year. “That’s just the tip of the iceberg,” says Tellman. MODIS takes pictures with a resolution of 250 meters, which is roughly the length of two soccer fields. This means that the researchers could not record minor floods or those in large cities. Clouds also impaired the image processing algorithm, and since the satellites only flew once or twice a day over a certain point on earth, they also avoided short-term floods. Newer instruments have much higher resolution and can see through clouds, says Bessie Schwarz, co-founder and CEO of Cloud to Street. Together with artificial intelligence, these instruments can now map floods even more precisely. However, to systematically map floods over time, researchers had to rely on images from a single source and use technology that has been around for a long time.
This gives scientists a clearer picture of the magnitude and human impact of the recent floods than any other source. And the results will be particularly useful for modelers trying to predict risk, says Philip Ward, who studies flood risk assessment at Vrije Universiteit Amsterdam and was not involved in the study. When researchers build a model to predict flood risk, they usually test it against maps of past floods. While many floods are mapped by local researchers or governments, they often use different methods – and some data are not publicly available. With a large data set that maps many floods using the same method, researchers can develop more accurate models. “Now you can compare apples to apples,” says Ward. “This is really valuable.”