Exposed yet unmapped: evidence of differential flood exposure in deprived urban areas using citizen science.

Rapid urbanization in Sub-Saharan Africa (SSA) has intensified flood risks, disproportionately affecting deprived urban areas (DUAs). Yet, these areas remain systematically unmapped in existing assessments, particularly in data-scarce environments. This study is among the first to center DUAs within a comparative flood exposure assessment across SSA cities, while integrating a lightweight low-cost flood model, global remote sensing datasets and citizen science methods. We find that DUAs are up to 200% more exposed to flooding, with frequent shallow floods (up to 10 cm) causing significant, yet often overlooked, impacts. Of these, over 50% concern property damage, disease outbreaks and infrastructure failures. Surprisingly, secondary cities sometimes surpass primaries in absolute exposure. We also highlight the spatial biases from built-up surface datasets, in turn overestimating exposure in peri-urban areas and omitting dense DUAs. Our findings challenge conventional ways of assessing flood risk and emphasize that local knowledge is indispensable in fostering urban resilience.