Review of “How to mitigate flood events similar to the 1979 catastrophic floods in lower Tagus” by Diego Fernández-Nóvoa et al., 2022 (nhess-2022-243))

Summary

The study demonstrates two operation strategies for flood mitigation in the lower Tagus valley. The Tagus is the largest river on the Iberian Peninsula and the study focuses on flood mitigation by means of regulating the largest dam of the river, the Alcántara dam. The Iber+ model is employed together with a pre-selected digital elevation model (Copernicus) to conduct the simulation of river flow and operating strategies in hindsight at the example of five flood events between 1972 and 1997 with a focus on the major flooding in 1979.

Evaluation and Recommendation

The manuscript is an interesting case study to the highly important field of flood mitigation. The topic of this manuscript is suitable for the journal.

The manuscript is overall well-written and extensively referenced. Data and software that were used are freely available and data sources are referenced. Appealing maps are provided as figures to illustrate the site of investigation.

While the content might be valuable to readers that are interested in flood mitigation operation strategies, the presentation of the material requires some restructuring and elaboration. One of the major points in this respect is that the results section includes many statements that should rather be presented in the introduction and in the methods section. In the current form, the actual results are presented between many of these additional statements and references, and key insights are therefore hard to elicit. Content-wise, the line of thought can be followed but the reader gets the impression that a step beyond is missing: As addition, it would strengthen the manuscript if there were a comparison with other operating strategies that are mentioned (see, e.g. l. 273-275) but not further pursued . Alternatively, an uncertainty analysis using noise that is added to the historic data (like perturbed precipitation data) could demonstrate the applicability of the proposed operating strategies beyond the deterministic hindsight scenario, specifically w.r.t. to the suggested operation strategy 2.

The manuscript bears the potential for being a valuable contribution to the field once the points laid out here are addressed. My recommendation therefore is major revision. In my comments, I suggest changes about restructuring the article. These shall outline one way to do it and do not have to be followed strictly. Yet, an iterated presentation of the material is required.

Specific comments

• Abstract: “…several dams…” is stated, but only the major Alcántara dam is regulated under historic data, right? Please make sure to not raise wrong expectations in the abstract.
• l. 26: “…with the proposed strategies”. Briefly name what is the core idea behind the strategies already in the abstract.
• Provide a clear “Motivation” or “Objective” section. The intention of the paper becomes clear over when reading the article, but it is preferable to have the goals clearly stated at some point – this is also something the final conclusions can relate to. One important motivation that is worth mentioning might be that a thought-through operation strategy is a low-cost approach for flood mitigation compared to additional structures that have to be build.
• Section 2: Please explain why the Alcántara dam is the only one considered here and whether there are other operating strategies that include other structures as well.
• Section 3: rephrase as “Data, Models and Methods”
• Section 3: merge 3.1 – 3.4 in subsection “Data”
• Section 3: great public resources!
• L 92: please add which distance is resembled by 0.1° in km
• L .120ff: please explain in more detail why the model was operated the chosen way: why was the inlet chosen to be critical/subcritical? Why was the outlet chosen to be supercritical/critical? Are these conditions static or do they change over the time series? Why was the SCS-CN methodology by Mockus (1964) used for getting the infiltration from precipitation – are there no more recent and potentially improved alternatives?
• L 124: which sizes: side length or circumference?
• L 126: “…tries to reproduce…” Does it only try to reproduce? Of course it is a simulation, but please
• L 147: please describe why a Taylor diagram was used and how it works, i.e. that it is a tool for visualizing multi-objective optimization
• Section 4: several parts in the discussion section should clearly be moved to the introduction or methods section because they refer to the conduct of experiments/simulation rather than the presentation and discussion of the results. Large parts thereof also contain various cited references which is typically something earlier in the manuscript. The mentioned parts are e.g.: 176-178, Section 4.2, Section 4.3 (incl subsections) until line 282; l 305-324.
• L. 295ff: clearly state in the methods section that the proposed OS shall be applied to these five selected flood events
• L. 325-329: Here, the uncertainty associated to anticipate the expected volume for the coming days is mentioned. Why was this topic not addressed in an uncertainty analysis?
• L 370: maximum water velocity – if this is an important aspect, state this already in the methods or objectives section to highlight that it will be assessed in the article. Currently, it is hidden as a side note at the end of the results.
• Section 5: This is in large parts of a summary and no conclusion section. As mentioned above: a specific “Objectives” section might help here to motivate drawn conclusions, e.g. could the stated goals be met? Why, or why not? Are the routes for improvement?
• L. 398-405: Acknowledging caveats is an important scientific discussion but should be part of the “results and discussing” section and not of the “conclusions.”
• L 415: In the early part of the manuscript, it was mentioned that bad communication between Portuguese and Spanish authorities exacerbated the flood impacts in 1979. Did the the EU help here or is there improved bilateral operation? Please elaborate.

Tables and Figures

• Table 2: Specify caption, e.g. which peak flow – the incoming flood wave?
• Table 2: add percentage reductions as was done in Table 3
• Table 3: very nice overview with absolute and relative reductions!
• Figure 2 caption: please add “, respectively” to the end of the sentence
• Figure 6: please add x-axis tick labels, i.e. numbers. And specify number of day since when?

Language

• Overall, the manuscript is well-written. Please read over it again to e.g. fill in missing articles (l. 20: [the] Iber+ model; l. 32: [The] Iberian Peninsula;…) or to rephrase very long sentences (e.g. l. 336-339)
• Some rewording suggestions: l. 32: “important” à “intense”; l.49-50: “that can play an important rule in” à “for”; l. 159 “free” à “freely”

References

• Complete

I consider this report shows an excellent research about flooding mitigation in the Lower Tagus.