Flood-prone areas are hotspots for urban development

Countries worldwide have been building villages, towns and cities in flood plains faster than in areas less likely to be inundated, shows a paper published today in Nature 1. This trend risks driving up the human and economic costs of climate change. “In a time when human settlements should be adapting to climate change, many countries are actually rapidly increasing their exposure to floods,” says Jun Rentschler, an economist at the World Bank in Washington DC who led the study. “This is a concerning trend, especially as climate change is intensifying flood disasters worldwide.” Rentschler and his co-authors found that the extent of human settlements — ranging from villages to megacities — expanded globally by 85.4% between 1985 and 2015. However, development in areas exposed to the biggest flood hazards — those where the worst floods can see water rise higher than 150 centimetres — increased by 122%, far outpacing the growth in flood-safe areas. The team’s work referred to floods resulting from rising rivers and torrential rain, as well as coastal flooding caused by storm surges and sea-level changes. Some 36,500 square kilometres — roughly 6% of all land developed — was in the ‘very high’ flood-hazard category. A further 76,400 square kilometres was in the ‘high’ flood-hazard bracket — between 50 and 150 centimetres. Although the authors deemed flood exposure substantial worldwide, the East Asia and Pacific region was the most exposed. In 2015, some 18.4% of all settlements in the area were prone to flooding, the highest portion globally. North America and sub-Saharan Africa had the lowest exposure, at 4.5% and 4.6% of settlements, respectively. The figures were most striking for China. Over the three decades from 1985, the extent of settlements in the highest flood-hazard category more than tripled, the authors wrote. The work confirms results from previous studies. Yu Kongjian, founder of Beijing-based landscape architecture firm Turenscape, proposed China’s ‘sponge city’ concept for urban flood management, in which concrete surfaces are replaced with green, permeable areas that can absorb water. His work found that in the three decades before 2010, 70% of new development in China was in flood plains or high-risk flood zones. Yu explains that amongst other reasons, in China, land in flood zones is cheaper for local governments to acquire than is land with no flood risk, and it is therefore more profitable to sell to developers. Marginal becoming desirable Development on flood plains is driven mainly by land scarcity, Rentschler says. He says that in many areas, much of the safe land suitable for development is already occupied, forcing new development into areas that were previously avoided because of flood risk. “Countries face trade-offs between economic opportunities and disaster risk,” he notes, highlighting that this is a factor in the expansion of major port cities, beach-front communities and tourism hubs in flood-prone coastal areas. He adds that development in flood zones can also be an “uninformed choice” when flood risk assessment is poor or urban-planning regulations are inadequate. The paper’s findings also put a spotlight on the need for the world to speed up adaptation to the impacts of climate change. Wang Xiaojun, founder of People of Asia for Climate Solutions, a non-profit organization based in Manila, was surprised to learn the extent of the settlement that had occurred in flood-prone regions in recent decades, when “awareness of the climate crisis among governments and the public has been growing much higher”. “We need more visionary world leaders to take their people towards a future of safety and security instead of uncertainty and even danger,” he says. Rentschler agrees that community policy and local patterns of urbanization are key to reducing people’s exposure to climate risk. “City mayors and local authorities play a crucial role in our response to climate change,” he says. Looking to the future, Rentschler and his colleagues plan to continue monitoring climate risk and urbanization trends around the world. They hope to identify high-risk developments early on and publicize success stories, thus “helping to build people’s resilience to climate change”.