the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Oct 2025
Flood Volume Allocation Method for Flood Hazard Mapping Using River Model with Levee Scheme
Abstract. A realistic flood risk assessment is important for rivers where the flood protection infrastructures are dictated by varying return periods. For rivers in Japan, design return periods for flood protection infrastructures range up to 200 years. Large-scale flood hazard mapping increasingly relies on global river models, but these models often lack explicit representation of flood protection levees. In this study, we extend the Global River Model (CaMa-Flood) by integrating levee parameters and applying frequency analysis to simulated flood volumes (the cumulative amount of water exceeding channel storage) and downscaling them to high resolution while explicitly accounting for topographic variability and levee protection.
Levees are represented through heights and fractions, with fractions derived from distance to the river centreline and heights refined by simulations. The method applies both to current simulations, using modelled flood volumes directly, and to future hazard assessment, where frequency analysis of annual maxima provides return-period volumes. These volumes are redistributed to high-resolution unit catchments using terrain data and physically constrained by storage availability.
The results show that integrating levee protection reduces simulated flood volumes, with 10–15 % reductions across most return periods in grids containing levees. This reduction reflects the confinement of floodwaters within levee-protected channels, which limits floodplain storage and lowers overbank volumes. At the unit catchment scale, flood extents are also reduced depending on levee fraction and topography. Levees effectively confined floodwaters during moderate to high events, while their influence diminished at extremes where overtopping or volume overestimation became prominent. Findings demonstrate that the levee-integrated downscaling approach captures spatial variability in protection effectiveness, offering a more realistic representation of flood hazard across diverse conditions. By combining hydraulic modelling, frequency analysis, and levee integration, this study provides a comprehensive framework for flood depth mapping, supporting improved resilience planning and basin management.
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Status: open (until 05 Dec 2025)
- RC1: 'Comment on egusphere-2025-4358', Anonymous Referee #1, 27 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-4358', Jonathan Remo, 01 Nov 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4358/egusphere-2025-4358-RC2-supplement.pdf
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RC3: 'Comment on egusphere-2025-4358', Jacob Schewe, 17 Nov 2025
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Review of “Flood Volume Allocation Method for Flood Hazard Mapping Using River Model with Levee Scheme”
General comments
This is an interesting and well-structured paper, presenting a flood downscaling method for the CaMa-Flood river model with a representation of levees. The paper builds strongly on the paper by Zhao et al. (2025) who presented the levee scheme for CaMa-Flood, including identification of levee data and estimation of the associated model parameters, levee fraction and height. The contribution of the present paper lies in developing a downscaling approach (from the 0.25 degree resolution of the CaMa-Flood hydrodynamic model to finer resolution) that respects the presence and characteristics of levees. It is therefore a useful complementary contribution to enhance the utility of the CaMa-Flood model with levee representation, and potentially more generally, since the volumetric downscaling is a fundamentally different approach than the depth-based downscaling method available before. The paper appears generally sound and worth of publication, but some details should be improved, especially regarding methodological explanations and figures.
Detailed comments
Introduction, Methods:
- lines 67ff: It is very useful to list the study’s main goals here. For goals a) and b), these seem very similar to the goals of the related paper by Zhao et al. (2025). Consider clarifying a bit more how the two papers relate, and what are their respective unique contributions.
- line 120: The term “levee distance” is used here (and a few lines below) but has not been introduced yet, therefore it is unclear what it exactly means at this point.
- line 126ff: Here it would be useful to show a schematic illustrating the different parameters. Consider using Fig. 3 for this purpose (parameter names would have to be added there), or adding a figure. Referring (in addition) to relevant figures from Zhao et al. (2025) may also be useful.
- (1): rivlen is not defined. A question is, is this equation valid/acceptable if the river, levees, and/or unit catchments have a complex shape?
- line 133: the reference to “our group conducted…” is a bit unspecific. Is this literature review published, or are its findings discussed in more detail elsewhere? Then please indicate the reference.
- line 136: How was the representative return period determined?
- line 137: Frequent should be Frequency.
- line 137f: I have a (perhaps naïve) question: If I understand right, river water depth is the water depth above the bottom of the river channel, including any overbank water. This means the relationship between total water volume and river water depth will be quite non-linear, with water depth rising quickly as long as it’s below bank height, and more slowly as soon as there is overflow. Annual maximum river depth statistics can contain values from either “sides” of the storage-depth curve (i.e., flooding and non-flooding cases). What are the implications of this for the frequency analysis, using a Gumbel distribution? Would it not be more stable to perform frequency analysis on total water storage, or discharge, for instance?
- line 144: What is meant by “spatial changes”, and by the “localized peak”?
- line 145f: How where levee fractions estimated? The text up to here is only about levee heights.
- line 69: What does low-lying mean here? Anything below the levee crest?
- line 175ff: Can it be said whether the volumetric downscaling has advantages over a depth-based downscaling approach also in the absence of levees? I presume that other, perhaps natural topographic features can also be better represented with the volumetric downscaling, but can this be shown? Perhaps it’s outside the scope of the present paper.
- line 189: While Fig. 2 does usefully illustrate the high-resolution topography data, it does not actually show any areas.
- (3): k is not defined. This equation basically adds up horizontal slices. One could also imagine adding up vertical columns, one for each grid cell. Would that make any sense, or why is this solution here preferable? Just out of curiosity.
- (4): First, eq. (3) provides the finite difference approximation of the integral in eq. (4); then, the curve derived from eq. (4) (or from eq. (3)?) is again interpolated (between which knots?). Consider clarifying which equation is actually used in the algorithm.
- line 209ff: In this case, would overflow to neighboring catchments occur? Is this accounted for in the model?
- line 219: Please explain catmz100.
- (6): The symbol ~ is unclear/ambiguous. Here it shall probably mean the complement, i.e. the cells that are not inside cells. Consider using more conventional notation e.g., from set theory, or explaining in the text.
Results:
- The first paragraph in section 3.1.1, esp. lines 230-236, summarize the achieved improvements; consider moving this text to the end of the results section, or the Conclusions section.
- line 239: Introduce/define metrics such as IoU and FBI (also, accuracy, in line 241), using equations if necessary.
- line 242: “upper half” = northern? Or upstream?
- line 268: Could you briefly explain how these one-in-1000-year rainfall events have been derived?
- line 272: In this design level of around 100yr uniform across Japan?
- line 283: Is unit catchment A different from region [a]? Are they related? Also, can the levee fraction of 0.39 somehow be recognized in Fig. 7?
- line 288ff: Would it be possible and meaningful to validate these results using observational data, analogous to Fig. 5 and 6?
- Line 295ff: In addition to comparing flood areas, would it be relevant to look at the reduction in population exposure (or building/infrastructure exposure) due to levees? Perhaps the gains are even larger there, given levees are may usually be designed to protect a maximum of populations/settlements.
- lines 298-302: These two sentences seem to contradict each other: Either “levee protection continues to limit flood spread” even at higher return periods, or “the levee effect for floods with a return period higher than the design return period will be insignificant”. Please clarify.
- line 320: The broadening into urbanized areas is not visible in Fig. 9. Perhaps urban areas can be overlaid in the plots? Also, the nonlinear increase in flood magnitude and extent, and the increases beyond RP100, mentioned in the following sentences, are not visible from Fig. 9 either.
Conclusions, Limitations:
- line 354f: “Beyond RP 100, the growth in flood extent becomes nonlinear” – again, while this is really interesting, it is not shown.
- line 366: “bias at higher return periods” – perhaps a reference can be provided for this statement.
Figures:
- 1: Explain what the white/blank areas are. Is the levee fraction zero there, or are these cells masked because they belong to a different river that is not considered here?
- Area shown in Fig. 1 seems to only partially overlap with key map in Fig. 5, and no overlap with insets a and b in Fig. 5. Might be useful to see the levee structure in a and b.
- 2 needs a bit more explanation in the caption. Is this for an (illustrative) catchment with six high-resolution grid cells? What is the dashed line? What does flddif stand for? Also I think the figure is not exactly “illustrating the number of grids considered for inundation area calculation at a specific depth increment”, but simply showing the elevation of each grid cell. Consider rephrasing.
- 3, line graph: This is slightly confusing because it is not clear whether the vertical axis refers to the water surface level inside or outside the unprotected area. I think the confusion arises because the outside storage is included within the same graph as the inside and total storage. An inside water level of 8.5m corresponds to any outside water level between 7m (approx.; bottom of the orange line) and 8.5 m, because the outside storage starts filling only once the inside water level has reached the levee height. While the graph seems to suggest that an inside water level of 7m corresponds to an outside water level of 7m. I’m not sure how to best solve this, just pointing out this ambiguity in the figure.
- 5: In the caption, “neutral” should be “natural”, I assume. In the [a] panels, overestimation appears along a narrow strip corresponding to a secondary river (Asa river). Should/could this river be masked? Or simply explain that this is a permanent water body?
- 6: If “observed inundations” refers to city hazard maps, then do these really represent observations?
- 7: The order of natural and levee simulations is different here than in Fig. 5 and 6. Consider swapping either here or there, for consistency.
- 9: Subpanels are missing labels for the two regions a and b.
Citations/references:
- On line 45, the reference to Kimura et al. (2022) should probably be corrected to 2023, as there is no 2022 paper by this author in the reference list.
- The reference list includes Mester et al. (2023) which however is not cited in the text. Either include citation if relevant, or remove.
Reference
Zhao, G., Yamazaki, D., Tanaka, Y., Zhou, X., Li, S., Hu, Y., Hirabayashi, Y., Neal, J., & Bates, P. (2025). Developing a Levee Module for Global Flood Modeling With a Reach-Level Parameterization Approach. Water Resources Research, 61(8), e2024WR039790. https://doi.org/10.1029/2024WR039790
Citation: https://doi.org/10.5194/egusphere-2025-4358-RC3
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- 1
This is a useful study on an important topic. The inclusion of levees in flood extent calculations is a substantial contribution to the flood modelling scientific literature. Overall I find the manuscript well written, the method clearly explained and the test cases useful.
My biggest concern is that there is not much discussion of the uncertainties coming from the various components of the modelling chain and the potential impacts on the test results presented, as well as the future projections and wider applicability globally. I think this needs some further thought and discussion. Ideally some further sensitivity studies would also be included to demonstrate the impacts of the data/model methodology uncertainties.
The literature review of current global flood models is very thin and a paragraph reviewing this model and methodology in the context of the wider scientific contributions to global flood modelling is necessary, both at the beginning and in the conclusions - detailing how this contribution has wider significance.
On line 133 there is mention of a group literature review. What is this? Please provide many more details and make clearer the findings and how they fit in this with work.
What type of river is the Chikuma River? What are its characteristics and the driving processes of floods and the catchment/floodplain characteristics? Please provide enough information to aid the unfamiliar reader.
Some further information on what type of model CamaFlood is, and its major components and applications would be useful in helping this manuscript be standalone. The same goes for the Khanh et al dataset.
Please check all references appear in the reference list - e.g. Tellman is missing
In section 3.1.1 you provide only the improved IoU and FBI. These are not much higher than the natural values and a more honest discussion would be valuable here making clearer comparison, and also explaining more clearly to the reader how they can interpret these numbers. This is also the case for the following sections.
Section 3.4 is very brief. What are all the major rivers in Japan? What are their characteristics? - can you provide the reader with some details? Again there must be a number of key uncertainties in this approach - these should be shown and discussed here.
More should be included on how this method can be applied globally, particularly on larger rivers that do not occur in Japan. What is the range of applicability?
The limitations section would be better integrated into the relevant parts of the discussion and expanded upon more thoroughly.