Global urban growth in high flood risk areas outpaces settlement in safe areas

In an article published in the journal Nature this month, researchers have noted that human settlements are expanding into hazardous flood zones faster than in safe areas. The researchers analysed statistics on urban settlement patterns between 1985 and 2015 and compared this to the evolution of flood exposure of settlements over time. 

The paper shows that urban settlements grew by 85% in the 30-year period to a total surface area of 1.28 million km². By contrast, settlement in areas exposed to high flood risk grew by 122%, indicating that high-risk settlement grew 60% faster than safe settlement. The researchers argue that despite the increased exposure to extreme weather events, countries and urban areas are not sufficiently adapting to climate change but, on the contrary, increasing the exposure to flood hazards.

The study made use of a combination of high resolution global flood hazard data produced by Fathom and the World Settlement Footprint Evolution data  produced by the German Aerospace Centre. By combining these data sets, the researchers were able to track both the speed and the shape of urban expansion from small rural settlements through to large urban conglomerations. They showed that urban settlements covered 693,000 km² in 1985 and covered 128,000,000 km² in 2015.  There was however a disproportionate degree of settlement in high-risk areas.

It is only recently that high-resolution flood hazard maps and settlement footprint data has become available, suggesting that this information needs to be systematically integrated into urban planning systems. Previous estimated of flood risk exposure have been made, but these studies were not sufficiently detailed either in their spatial resolution or had large time gaps between observations. The continuously evolving urban shapes need to be tracked accurately and regularly in order to mitigate flood risk.

There is some evidence from case studies of the dynamics that underlie settlement in these areas: when safe spaces are already occupied, the researchers argue, new developments can occur disproportionately in riverbeds, on flood plains and in wetlands. In making settlement decisions, there is a trade off to be made between market accessibility and potential on the one hand, and risk exposure on the other hand. In addition, information on these risks may be lacking, the market cost of this new land may not reflect the associated risks, and people may have a bias towards locating in areas close to the water.

This dynamic appears to be especially prominent in Asia. Evidence from India shows that low income rural-urban migrants arriving in Mumbai are faced with high density settlements and large price differences between areas, forcing new arrivals into previously avoided areas. Similarly in Ho Chi Minh city, informal, poor settlements are systematically exposed to higher flood risks than the rest of the city.