the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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.
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Status: final response (author comments only)
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RC1: 'Review of egusphere-2025-1285', Anonymous Referee #1, 24 Jul 2025
- AC1: 'Reply on RC1', Leonardo Laipelt, 30 Sep 2025
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RC2: 'Comment on egusphere-2025-1285', Anonymous Referee #2, 19 Aug 2025
The manuscript shows promising results, but I have several comments that need to be addressed (e.g., methodological improvements, restructuring of some sections, etc.) before the manuscript can be considered for publication. You will find my comments in the attached document. I hope they help you improve on the great work you have done so far.
- AC2: 'Reply on RC2', Leonardo Laipelt, 30 Sep 2025
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Review of “Mechanisms and scenarios of the unprecedent flooding event in South Brazil 2024” by Laipelt et al.
This study uses a 2D hydrodynamic modelling framework to evaluate the hydraulic mechanisms driving the 2024 flooding event in southern Brazil. First, an evaluation of the modelling approach is conducted using different data sources. The authors then perform modelling experiments to: determine which rivers contributed most to flooding in RMPA; understand the consequences of potential synchronous flooding in the two main rivers; and determine whether flood control measures could have reduced river levels. It is an interesting topic, and I understand that the authors put a lot of effort into evaluating their approach using different data sources. However, in my opinion, the paper is not well written and fails to explain how the research is novel, what the research questions are, and how the results and framework compare with those of other studies in the field. Furthermore, the main goals of the study are unclear and the methodology lacks the overarching structure required to achieve them. I provide more detailed comments below to demonstrate this point. The manuscript would need to be reshaped and rewritten to make a valuable contribution to HESS.
Detailed comments:
I suggest that the authors completely reshape and rewrite the introduction to focus on their main analyses and questions, providing a clearer justification for their study. This could be achieved by focusing on four main apsects: 1) model evaluation using different sources of data, 2) the hydraulic mechanisms/drivers of flooding, 3) flooding synchronicity, and 4) the evaluation of mitigation measures.