Mechanisms and scenarios of the unprecedent flooding event in South Brazil 2024

Abstract. In May 2024, an extraordinary precipitation event in southern Brazil triggered record floods in South Brazil, specially over a complex system that includes rivers as Jacuí and Taquari, draining into Guaíba and Patos Lagoon. It resulted in unprecedented impacts on local population and infrastructure. Considering past observations and projections indicating an increase in flood events in the region due to climate change, understanding the flooding processes in the region is essential for better preparing cities for future events like the May 2024 flood. In this context, hydrodynamic modelling serves as an important tool for reproducing and analysing this past extreme event. This study aims to assess the detailed hydrodynamic mechanisms and processes that occurred during this historical flood event and scenarios of direct interventions for flood control that came into the public debate after the event. The focus is on the most populated areas at the Metropolitan region of Porto Alegre (RMPA) capital city. We calibrate and validate a two-dimensional hydrodynamic model to accurately replicate the May 2024 flood. The results demonstrated that the model accurately represents the 2024 May flood, with average NSE, RMSE and BIAS of 0.82, 0.71 meters and -0.47 meters, respectively, across the main rivers in the basin. Furthermore, the flood extent simulation represented 83 % of the affected area, as compared to high-resolution satellite images. Our analysis of the mechanisms that influenced the event showed that the Taquari River was the main responsible for the peak in the RMPA, while the Jacuí River contributed the most to the duration of the flood. The synchronization of the flood peaks from both rivers could have increased water levels by 0.82 meters. Evaluated hydraulic interventions for flood mitigation demonstrated that the effectiveness of the proposed measures varied by location, with usually low influence in the RMPA water levels (lower than 0.38 m). Major lessons related to the behaviour of river-lagoon hydrodynamic systems and to the relevance of structural measures for such cases are discussed, which are of broader interest for future research and decision making around the globe.