Chronic flooding drives cumulative risk in 11,422 urban centres

Rapid urbanisation and climate change are increasing flood exposure in cities, but global assessments commonly analyse infrequent, high-magnitude events such as the 1-in-100-year flood. Here, we provide the first global analysis of cumulative chronic flood exposure for 11,422 urban centres. We do this by integrating high-resolution chronic flood hazard data (1-in-5-year flood) with population and gross domestic product (GDP) data and compare the estimated cumulative impacts of these events to the 1-100-year event. Assuming stationarity, we estimate 170 million instances of population exposure to a 1-in-5-year flood, over a hypothetical 100-year period, representing US$1.69 trillion of GDP exposed. Low and lower-middle-income countries experience disproportionate exposure, with the greatest concentrations in small and medium-sized urban centres across poorer regions of Sub-Saharan Africa and East and South-Eastern Asia. In short, those most exposed to cumulative chronic flooding are also those most likely to be adversely impacted. As flood exposure and societal inequality increase, the current ‘big flood’ framing of risk overlooks the insidious consequences of cumulative chronic flood exposure. Cumulative chronic flooding is a globally underappreciated but quantitatively significant source risk for urban dwellers that warrants further research.