Review on egusphere-2023-2420, HESS Opinions: The Sword of Damocles of the Impossible Flood, by Montanari et al.

The authors present a manuscript on extremely large floods and associated risk. They argue that these kind of “impossible” floods are not considered in classical risk management and therefore, bottom-up approaches should be adopted to account for societal risks. These should include education and awareness-raising to avoid associated disasters, apart from including them in engineering risk management strategies.

Overall, the manuscript is clearly structured, succinct and very well written – a joy to read! Nevertheless, some points need clarification before the manuscript may become acceptable for final publication in HESS.

Section 2, lines 38-41: The authors state that “communities often design their management strategies based on historical events” and such “impossible events include floods whose probability of occurrence is considered too small to act, but also events that go beyond any imagination.” Additionally, large flood events may be not covered by underlying regulations – the design events authors refer to later on are an example, as e.g. given in the EU Flood Directive (Commission of the European Communities, 2007) and related national law. Authors are kindly requested to make a few statements here.

Section 2, line 50f.: The challenge of dynamics in discharge (observation in the past ≠ system behaviour in the future) have been extensively discussed (see e.g., Sofia et al., 2017). The authors may wish to comment on their assumption that this is not considered in contemporary flood risk management, also against previous works of the authors of the present manuscript (Blöschl et al., 2019; Blöschl, 2022).

Section 3, line 140: What is meant here with the term “residual risk”? Failure of technical mitigation? Events larger than expected? The issue of not remembering historical (or even younger) flood events and associated losses has been a topic for decades (Kuhlicke et al., 2020), however, experienced flood damage in the past does not necessarily mean good governance in the future. The authors may wish to comment on these issues specifically in terms of the “recency bias” (line 141).

Section 3, line 151f.: The decision of whether or not to prepare for an event is not necessarily (or only) a political one. Fellows from social sciences have a long tradition in studying the willingness of homeowners to prepare for undesirable events by e.g. local structural protection or retrofitting of elements at risk. Examples from Europe include but are not limited to the overview of Kuhlicke (2020) or Attems et al. (2020). As such, the authors may wish to expand their vision here.

Section 4, the concept of risk: Surprisingly the authors make a distinction here between “economic risk” (eqn. 1) and “social risk (eqn. 2), which is not made clear in the text. Consistent with ISO Guide 73 and ISO 14091 (International Standards Organisation, 2009, 2021) and the UN/IDRR terminology on disaster risk reduction (UNISDR, 2017), risk is defined as “potential loss of life, injury, or destroyed or damaged assets which could occur to a system, society or a community in a specific period of time, determined probabilistically as a function of hazard, exposure, vulnerability and capacity”. As such, exposure (line 201) cannot be defined as “the economic damage caused by an event”, it should be the overall value of exposed elements at risk, reduced by the damage ratio (as a factor coming from the vulnerability part of the risk equation). Having said this it remains unclear what is meant by “social exposure” (line 206), and how social vulnerability can be conceptualised here (see e.g., Sorg et al., 2018; Spielman et al., 2020).

Section 4, lines 2019ff.: The authors argue that exposure and vulnerability will be extremely high in case of “impossible flood events” – which is not a contradiction of the risk concept. It depends on the way risk is expressed, in terms of annual risk as in many cost-benefit analyses (then risk is relatively small) or in terms of probable maximum loss (then the values are relatively high). The authors may wish to expand their vision here.

The same is valid for their argumentation in line 360ff. on levees, increasing exposure and vulnerability. It is not made clear why “vulnerability is small” if exposure increases behind a levee – social vulnerabilities may be the same or even higher, economic vulnerabilities depend on the susceptibility of elements at risk (as physical vulnerability does), and the institutional dimension of vulnerability may even be higher (see e.g. the discussion in Papathoma-Köhle et al. (2021)). The authors are kindly invited to clarify their statement.

References mentioned:

Attems, M.-S., Thaler, T., Genovese, E., and Fuchs, S.: Implementation of property level flood risk adaptation (PLFRA) measures: choices and decisions, WIREs Water, 7, e1404, https://doi.org/10.1002/wat2.1404, 2020.

Blöschl, G., Hall, J., Viglione, A., Perdigão, R. A. P., Parajka, J., Merz, B., Lun, D., Arheimer, B., Aronica, G. T., Bilibashi, A., Boháč, M., Bonacci, O., Borga, M., Čanjevac, I., Castellarin, A., Chirico, G. B., Claps, P., Frolova, N., Ganora, D., Gorbachova, L., Gül, A., Hannaford, J., Harrigan, S., Kireeva, M., Kiss, A., Kjeldsen, T. R., Kohnová, S., Koskela, J. J., Ledvinka, O., Macdonald, N., Mavrova-Guirguinova, M., Mediero, L., Merz, R., Molnar, P., Montanari, A., Murphy, C., Osuch, M., Ovcharuk, V., Radevski, I., Salinas, J. L., Sauquet, E., Šraj, M., Szolgay, J., Volpi, E., Wilson, D., Zaimi, K., and Živković, N.: Changing climate both increases and decreases European river floods, Nature, 573, 108-111, https://doi.org/10.1038/s41586-019-1495-6, 2019.

Blöschl, G.: Three hypotheses on changing river flood hazards, Hydrology and Earth System Sciences, 26, 5015-5033, https://doi.org/10.5194/hess-26-5015-2022, 2022.

Commission of the European Communities: Directive 2007/60/EC of the European Parliament and of the Council of 23 October 2007 on the assessment and management of flood risks, Official Journal of the European Union, L 288, 27-34, 2007.

International Standards Organisation: ISO Guide 73: Risk management – vocabulary, Geneva, 15 pp., 2009.

International Standards Organisation: ISO 14091:2021, Adaptation to climate change – Guidelines on vulnerability, impacts and risk assessment, Geneva, 56 pp., 2021.

Kuhlicke, C., Seebauer, S., Hudson, P., Begg, C., Bubeck, P., Dittmer, C., Grothmann, T., Heidenreich, A., Kreibich, H., Lorenz, D. F., Masson, T., Reiter, J., Thaler, T., Thieken, A. H., and Bamberg, S.: The behavioral turn in flood risk management, its assumptions and potential implications, WIREs Water, 7, e1418, https://doi.org/10.1002/wat2.1418, 2020.

Papathoma-Köhle, M., Thaler, T., and Fuchs, S.: An institutional approach to vulnerability: evidence from natural hazard management in Europe, Environmental Research Letters, 16, 044056, https://doi.org/10.1088/1748-9326/abe88c, 2021.

Sofia, G., Roder, G., Dalla Fontana, G., and Tarolli, P.: Flood dynamics in urbanised landscapes: 100 years of climate and humans’ interaction, Scientific Reports, 7, 40527, https://doi.org/10.1038/srep40527, 2017.

Sorg, L., Medina, N., Feldmeyer, D., Sanchez, A., Vojinovic, Z., Birkmann, J., and Marchese, A.: Capturing the multifaceted phenomena of socioeconomic vulnerability, Natural Hazards, 92, 257-282, https://doi.org/10.1007/s11069-018-3207-1, 2018.

Spielman, S. E., Tuccillo, J., Folch, D. C., Schweikert, A., Davies, R., Wood, N., and Tate, E.: Evaluating social vulnerability indicators: criteria and their application to the Social Vulnerability Index, Natural Hazards, 100, 417-436, https://doi.org/10.1007/s11069-019-03820-z, 2020.

UNISDR: Terminology on disaster risk reduction, United Nations Office for Disaster Risk Reduction, Geneva, 2017.

The authors present in this paper a discussion about the role of impossible floods in risk management. They discuss several reasons why flood events considered "impossible" are usually disregarded in flood risk management, and they give suggestions for improving the assessment of risks associated to these unexpected events.  

General comments

Overall, I found the paper engaging, timely and well written. The Sword of Damocles is a clever metaphor to describe  situations with impending severe flood risk, and I think it is very much needed given the recent catastrophic floods occurred all over the world. Having said so, some aspects of the manuscript could be improved. Some of the ideas presented have already appeared in previous opinion and review papers, in particular in the work by Merz et al (2015); it would be good to include more references where appropriate, in order to highlight the novel concepts presented in the work. 

In addition, a number of statements should be better formalized, because the reasoning is sometimes too loose for a scientific publication, at least in my opinion. Finally, some examples could be better explained by adding more references about real-world events.

Specific  comments

- In the abstract and in other sections, the Authors seem to use as synonyms "impossible floods" and "mega-floods", but I think this can be confusing for the reader. Some countries like India and Bangladesh experience almost every year widespread flooding, and the size of related impacts would easily qualify them as mega-floods in many other countries, yet they far from being considered impossible. Conversely, more localized floods might have not so large impacts but still be regarded as "impossible". 

- Section 3: the discussion could be more compelling by presenting more examples of past floods illustrating the reasons and the points made by the Authors. Ideally, examples should come from different continents (e.g. the 2022 floods in Pakistan might easily qualify as unexpected due to the sheer size and duration of the event; during the 2023 floods in Emilia-Romagna, Italy,  multiple failures of flood defences caught by surprise the population in lowland areas; and the September 2023 floods in Lybia 2023 were a terribly fitting example of the Damocles metaphor).

- Section 3.1. Another important reason that should be discussed here is socio-economic development (urbanization, floodplain development etc) which largely contributes to change exposure and vulnerability.  Hence, past flood events that were moderately harmful might cause much larger damages, should they occur again under present conditions  (see for instance the analysis by Paprotny et al, 2018)

- Section 3.3. The reasoning here makes sense and I mostly agree, could you provide some references for these statements?

- L209-218 and 224-229: the authors might want to enrich their discussion by including the concepts of risk aversion and social vulnerability presented in previous works (e.g. Koks et al, 2015; Mechler, 2016; Kind et al 2017). By the way, it is worth mentioning here that low-probability, high-impact events are crucial also for the insurance sector, and this is reflected by the use of  probable maximum loss (PML) as a standard metric for risk characterization.

L216-218: Also, economic indirect impacts from severe floods (e.g. failures in transport and energy networks, disruption of business and production) might be comparable to direct damages (as in the example of 2011 floods in Thailand described by Merz et al., 2015) 

L240-249: I beg the authors' pardon, but I'd like to play the role of the devil's advocate in the comparison between top-down vs bottom-up approaches. According to the discussion here, my impression is that the two approaches are alternative and the latter is preferable than the former, but is it always the case? Flood risk management is not exclusively a local problem and some sort of integration at (at least) river basin scale is needed, which includes also finding a compromise between contrasting local priorities, which has to be done as some intermediate level between top and bottom. For instance, raising dikes upstream can increase downstream risk, whereas flood detention areas comes at a cost for local communities in terms of restrictions even if this protects downstream communities. For these reasons, people and local stakeholders might be in favour of traditional flood control structures (e.g. dikes, river cleaning) and not see favourably alternative measures (river restoration etc) which are perceived as less effective and/or more costly. The authors might want to enrich the discussion by replying to these considerations.

Section 4: how would you calculate in practice social risk? is there any past study that did something similar?

Section 5: Ideally, all listed actions should be accompanied by references to real-world applications, that would make the discussion more useful for the reader

References

Koks, E. E., Jongman, B., Husby, T. G. & Botzen, W. J. W. Combining hazard, exposure and social vulnerability to provide lessons for flood risk management. Env. Sci. Policy 47, 42–52 (2015).

Kind, J., Wouter Botzen, W. J. W. & Aerts, J. C. J. H. Accounting for risk aversion, income distribution and social welfare in cost–benefit analysis for flood risk management. WIREs Clim. Change 8, e446 (2017).

Mechler, R. Reviewing estimates of the economic efficiency of disaster risk management: opportunities and limitations of using

risk-based cost–benefit analysis. Nat. Hazards 81, 2121–2147 (2016).

Merz, B., S. Vorogushyn, U. Lall, A. Viglione, and G. Bl€oschl (2015), Charting unknown waters—On the role of surprise in flood risk

assessment and management, Water Resour. Res., 51, 6399–6416, doi:10.1002/2015WR017464.

Paprotny, D., Sebastian, A., Morales-Napoles, O. & Jonkman, S. Trends in flood losses in Europe over the past 150 years. Nat. Commun. 9, 1985 (2018).