| 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.
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.