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:

• The introduction lacks clear structure and research questions derived from an overview of existing research in the field. The main messages that the authors want to convey in each paragraph are difficult to follow. For example:
  • A lot of emphasis is placed on the effect of climate change on flood extremes (e.g. L33–46), despite this not being a topic addressed by the study.
  • The authors emphasise that hydrodynamic models are used for such studies and present existing applications (L66–78). However, they do not highlight what is missing or how the present study differs from or builds on these approaches.
  • The main objective of the study is presented as follows: ”understanding of flooding mechanisms in South Brazil” (L79) , yet little is said beforehand to explain why this is necessary and why it has not been done before. Some explanations that are not central to the introduction are provided at line 61: “After the disaster, many questions were raised regarding the function of the natural system: the relevance of the upstream rivers, the slopes generated by water inflows and even if extra outlets in the lagoon to the sea would not have avoided the flooding at upstream areas (Hunt et al., 2024; Silva et al., 2024a).”.
  • This study analyses potential mitigation measures and flooding synchronicity, but the introduction provides no background to explain why this is relevant, what has been done to assess this in other studies, or how their approach or analyses are novel in that regard.

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.

• The method section lacks clear structure, making it difficult to follow. It would have been useful to include a diagram presenting an overview of the different experiments to help readers understand the study. Furthermore, many methodological points are introduced in the Results section, making it difficult to link the different experiments to the study's objective. For instance, the synchronisation experiment is only partially explained in section 4.2.2 of the results. While this experiment may seem trivial to some readers, I believe it would benefit from more detailed explanations of the exact methods employed.
• Figures 10 and 11 are difficult to understand. The quality of the panels on the right is poor, the lines are thin and close together, and there are many sub-panels with little space between them. It is therefore difficult to understand how the figures can support the analyses presented.
• There is no distinct discussion section, which highlights that the research questions are unclear and the study has not been compared to existing literature. In order to justify the recommendations presented in Section 4.5, the authors must discuss their results in more depth and demonstrate how they have addressed their research questions.
• The flood synchronisation experiment could be very interesting if the author provided more motivation. Why was this experiment conducted? Maybe I missed the reason somewhere. Is it physically 'reasonable'? Were these rivers sometimes synchronised for flooding in the historical period? What motivated the different methodological choices?
• The accuracy of the model needs to be put into perspective. For example, how does “an average BIAS of -0.47 meters between the water level peak in the stations” (L228) relate to flooding in the Guaíba River, which has “an average depth of 2 meters” (L95)? Does this mean that, in some cases, the model would not produce simulations exceeding a certain impact threshold?
• The authors mention that the Manning coefficient was calibrated: L148: “Initial values of Manning’s roughness coefficient were derived from the literature, followed by manual calibration for the study period to ensure optimal accuracy.”. The authors mention this aspect as a potential source of uncertainty in Section 4.4. Shouldn’t a sensitivity analysis be performed outside the calibration period to evaluate the transferability of the results to other periods and flood events? Tuning the parameters could make the model more accurate for this flood event by compensating for other sources of uncertainty.
• I noticed many typos and issues with the way things were worded. I am not a native speaker but these issues sometimes made the text difficult to read. I have listed a few examples below:
  • “Flood becomes a major concern” L32
  • “However, the relationship between climate change and flood is complex, with impacts vary regionally and influenced by multiple factors” L36-37
  • “in instance” L42
  • “as consequence” L49
  • “wate” L101
  • “manually” L119
  • “The first main result is the model validation itself, which calculate values were compared to level gauges…” L201 needs to be reformulated.
  • “We accessed” L309
  • “testes” L311
  • “from studies as flood mitigation measures” L326
  • “The analysis was based using 2D hydrodynamic modelling” L415

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.