Unprecedented flooding in Porto Alegre Metropolitan Region (Southern Brazil) in May 2024: Causes, risks, and impacts

Abstract

On May 2, 2024, Porto Alegre in Rio Grande do Sul, Brazil, experienced a historic flood surpassing the catastrophic 1941 event. The flood's peak water level of 535 cm was 59 cm higher than in 1941, driven by extreme precipitation anomalies and land use changes in the Regional Metropolitan Area of Porto Alegre, with rainfall accumulations up to 500 mm in May 2024. Severe urban flooding impacted riverbanks and low-lying areas, causing significant property damage, displacement, and disruption of essential services. Agricultural lands and grasslands were extensively affected. Detailed maps from May 3 to May 8, 2024, showed floodwaters rising from 5.5 m to 8 m. Validation sample points ensured data accuracy, and flood model validation metrics (NSE = 0.63, R2 = 0.71, RMSE = 0.51 m) demonstrated the model's predictive accuracy. Water level fluctuations at Cais Guaíba station revealed rapid rise and recession periods typical of major floods. This study highlights the urgent need for effective flood mitigation strategies, informed by accurate modeling and risk assessment, to protect vulnerable communities and infrastructure. Enhancing resilience through improved urban planning, infrastructure investment, and environmental conservation is crucial to safeguarding Porto Alegre and the broader RMPOA region from future extreme weather events. These findings offer valuable insights for other flood-prone tropical and developing regions, where the combined effects of urban expansion and intensifying climate extremes similarly increase flood risk.