the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2025
The impact of climate change on dam overtopping flood risk
Abstract. There is unequivocal evidence that climate change will change the risk profile of dams, which are critical pieces of infrastructure that safeguard water supply and provide flood mitigation for populated areas. However, the challenges involved in estimating the probability of extreme floods under climate change have meant that few studies have estimated the plausible changes in the risk of extreme floods that have the potential to overtop dams. A recent examination of contemporary scientific findings pertinent to climate change impacts on flood risk has informed the projection of extreme flood risk and dam overtopping risk estimates made here. We project changes in the exceedance probabilities of overtopping risk for 18 large dams in Australia under a range of global warming assumptions, where consideration is given to the impacts of climate change on rainfall depth, rainfall temporal pattern, and rainfall losses resulting from changes in antecedent catchment wetness. We used event-based flood modelling and Monte Carlo sampling to appropriately represent the range of uncertainties associated with projecting estimates of extreme flood risk. Our results are presented in terms of changes per degree of global warming, which facilitates their interpretation in terms of different greenhouse gas emission scenarios and future time horizons. We found that increases in rainfall depth had the largest impact on increasing dam overtopping flood risk for all 18 dams under climate change. Under 4 °C of global warming, which approximates conditions towards the end of this century under a high emissions scenario, the risk of overtopping floods was between 2.4–17 times that of historical conditions for the 18 dams investigated. We also found that the risk of overtopping has more than doubled compared to the historical baseline for four of the dams investigated here as a result of global warming that has already occurred.
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Status: open (until 22 Mar 2025)
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RC1: 'Comment on hess-2024-403', Anonymous Referee #1, 27 Jan 2025
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General comments
This paper addresses a critical and underexplored issue—the implications of climate change on dam hydrological safety. The topic is highly relevant given the increasing frequency and severity of extreme weather events, making the findings applicable to dam owners, policymakers, and climate adaptation professionals globally. The study offers a robust method for addressing uncertainties, and its analysis of 18 large dams across varied Australian climate regions is particularly noteworthy.
However, while the study’s objectives are clear and the methodology is sound, there are areas for improvement:
- First of all, the title of the article clearly refers to the impact of climate change on dam overtopping. However, most of the paper focuses on the impact on the hydrological loads to dams, neglecting the specific aspects related to dam safety. Furthermore, the only considerations regarding dams are taken into account in a very simplified manner (e.g., the assumption of the reservoirs being at a full supply level prior to the storms). The article title does not embody the methodology and results presented, and cannot be accepted as a valid title.
- Throughout the entire manuscript, the term “risk” is misused (even in the title). In the dam safety context (and in any context related to natural hazards), the risk is defined as the combination of a potential hazard and its consequences. However, in the manuscript only the occurrence of the hazard and its probability are studied and quantified. Therefore, the manuscript should be reviewed and the term “risk” should be adequately replaced by “probability” or “hazard”, depending on the case.
- The methodology is quite dependent on the Australian context. The introduction and discussion sections could better frame the global implications of the findings, as they currently focus primarily on Australia. Specify what assumptions and methods can be replicated to other regions or contexts. In line 342, the authors say that the approach “could be translated globally for estimating flood risk under climate change elsewhere in the world”, which is not clear.
- The graphical abstract includes a graph representing the dam crest flood level vs the exceedance probability:
- This graph is not represented in the article.
- The y-axis does not correspond to the exceedance probability.
Specific comments
Introduction
- The introduction provides a strong rationale for the study. However, it lacks a succinct statement of the research gap. Explicitly contrast the current study with previous works on flood risk and dam safety.
- The global context is underdeveloped. Adding examples from other regions (e.g., Europe or Asia) could broaden the impact.
Materials and methods
- Line 121: the description of the R2ORB emulator needs more detail. A brief explanation of how it works and its advantages compared to other tools would be helpful.
- Line 129: “we assumed that the reservoir was at a full supply level prior to the storm”: this is a strong simplification of the methodology. Please justify:
- The reasons (lack of data, lack of time…)
- The potential impact (have you done an example calculation?)
In terms of dam safety, this is equivalent to not considering antecedent catchment wetness when calculating floods. I strongly recommend the authors to at least perform one example analysis of this effect. This is important in an article that focuses on dam safety (it’s in the title).
- Line 134: “outflow hydrograph”: do you mean the catchment’s outflow hydrograph, or the reservoir’s outflow hydrograph? Please harmonize the vocabulary throughout the paper.
- More information on the Monte Carlo simulation framework is needed in Appendix A and in the description of the methodology.
- Line 162: impacts of climate change on what?
- No mention to dam operations is presented here. What are the assumptions? How dams are considered operated in the baseline period and in the future periods? Just a short indication is given in line 138.
Results
- The results are presented effectively, but their practical implications could be expanded to aid decision-makers.
- Line 263: specify the three flood drivers.
- Line 272: “the reservoir outflow flood frequency curve” should be “the reservoir inflow flood frequency curve” because it refers to the DCF, which is the flood entering the dam’s reservoir.
- The RS factor does not illustrate the importance of the underlying AEP (i.e., how unsafe is a dam under historical conditions). When presented, results should simultaneously show the base AEP and the RS factor. Moreover, given that the results are anonymized and Figure 5 is just a summary of the results for the 18 dams analyzed, maybe an anonymized figure (similar to Figure 4, but only for the combined effect) in an Appendix could help shedding light on this issue.
- Synthesize somewhere the simplifications assumed in the methodology and the potential improvements.
- The Results section is somehow succinct and could benefit from more detailed analysis. For instance, the authors haven’t studied in more detail the hydrological response of each catchment to the changes in the rainfall parameters (rainfall losses, storm temporal patterns…). Figure 5 is presented as a black box without details and hardly exploitable.
Discussion
- Line 367: the assumption made by the authors that climate change will lead to increasing the potential for dams to attenuate floods is not backed by the findings presented in this paper. I recommend to replace “will result” by “could result”.
- Paragraph lines 384 to 399: authors justify the use of outputs from global climate models instead of regional ones, while in reality none of these outputs have been used here. The authors have simply applied a series of temperature increases to the hydrological drivers, without relying on any climate model. This could lead to misunderstanding the process followed. I recommend to replace this paragraph by a justification of the simplified methodology proposed.
Conclusion
- Line 429: it is the first time that this result appears in the text. It should be mentioned before.
- Line 440: replace “practical approach” by “simplified approach”.
Technical corrections
- Line 42: rephrase “moisture delivered”.
- Lines 56-57: consider revising the citation format.
- Line 59: repetition “that that”.
- Line 89: reference for the Australian Rainfall and Runoff.
- Table 1: instead of indicating dam owners, indicate dam type.
- Line 169: replace “The rates of changed” by “The rates of change”.
- Figure 4:
- replace “notional overtopping” in legend by “DCF”
- replace “Shift in overtopping risk” by “Shift in overtopping AEP”
- y-axis does not represent the AEP, but the return period.
- Figure 5: legend indicates that outliers are not shown, but it is the case in plots (b), (c) and (d)
- Line 325: change “appear” to “appears”
Citation: https://doi.org/10.5194/hess-2024-403-RC1
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