Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site
- 1BRGM, 3 av. C. Guillemin, 45060 Orléans Cedex 2, France
- 2Institut de Mathématiques de Toulouse, 118 Rte de Narbonne, 31400 Toulouse, France
- 1BRGM, 3 av. C. Guillemin, 45060 Orléans Cedex 2, France
- 2Institut de Mathématiques de Toulouse, 118 Rte de Narbonne, 31400 Toulouse, France
Abstract. Getting a deep insight into the role of coastal flooding drivers is of high interest for the planning of adaptation strategies for future climate conditions. Using global sensitivity analysis, we aim to measure the contributions of the offshore forcing conditions (wave/wind characteristics, still water level and sea level rise (SLR) projected up to 2200) to the occurrence of the flooding event (defined when the inland water volume exceeds a given threshold YC) at Gâvres town on the French Atlantic coast in a macrotidal environment. This procedure faces, however, two major difficulties, namely (1) the high computational time costs of the hydrodynamic numerical simulations; (2) the statistical dependence between the forcing conditions. By applying a Monte-Carlo-based approach combined with multivariate extreme value analysis, our study proposes a procedure to overcome both difficulties through the computation of sensitivity measures dedicated to dependent input variables (named Shapley effects) with the help of Gaussian process (GP) metamodels. On this basis, our results outline the key influence of SLR over time. Its contribution rapidly increases over time until 2100 where it almost exceeds the contributions of all other uncertainties (with Shapley effect > 40 % considering the representative concentration pathway RCP4.5 scenario). After 2100, it continues to linearly increase up to > 50 %. The SLR influence depends however on our modelling assumptions. Before 2100, it is strongly influenced by the digital elevation Model (DEM); with a DEM with lower topographic elevation (before the raise of dykes in some sectors), the SLR effect is smaller by ~40 %. This influence reduction goes in parallel with an increase in the importance of wave/wind characteristics, hence indicating how the relative effect of the flooding drivers strongly change when protective measures are adopted. By 2100, the joint role of RCP and of YC impacts the SLR influence, which is reduced by 20–30 % when the mode of the SLR probability distribution is high (for RCP8.5 in particular) and when YC is low (of 50 m3). Finally, by showing that these results are robust to the main uncertainties in the estimation procedure (Monte-Carlo sampling and GP error), the combined GP-Shapley effect approach proves to be a valuable tool to explore and characterize uncertainties related to compound coastal flooding under SLR.
Jeremy Rohmer et al.
Status: final response (author comments only)
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RC1: 'Comment on nhess-2021-271', Anonymous Referee #1, 10 Jan 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-271/nhess-2021-271-RC1-supplement.pdf
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AC1: 'Replies to Reviewer #1’s comments on “Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site ”. (nhess-2021-271)', Jeremy Rohmer, 28 Mar 2022
We would like to thank Reviewer #1 for the constructive comments. We agree with most of the suggestions and, therefore, we have modified the manuscript to take on board their comments. In the attached documment, we recall the reviews (in italics) and we reply to each of the comments in turn (in blue).
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AC1: 'Replies to Reviewer #1’s comments on “Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site ”. (nhess-2021-271)', Jeremy Rohmer, 28 Mar 2022
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RC2: 'Comment on nhess-2021-271', Anonymous Referee #2, 16 Feb 2022
The paper “Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site” by Rohmer et al. presents an analysis of future flooding probability at Gâvres (France).
Lot of work have been put in the paper, which introduces several results that are exhaustively explained and discussed (perhaps even too much! see below). The research topic matches well the scope of the journal and the analysis presented seem to be correct. However, there some flaws that make the manuscript hard to follow and some aspect must be clarified. Therefore, I believe that the paper should undergo a major revision before it is accepted for publication. See below for a list of comments:
- The abstract is too long. If I remember correctly, NHESS recommends that it does not exceed 200 words. Perhaps you could shorten the paragraph starting at line 16; in fact, there is no need to provide so many details on the SLR contribution, given that you also introduce other variables further on the paper. Analogously, you could shorten the title (which is also rather long), removing information that are given in the text, e.g., the terms “uncertainty”, “dependent”, “at a macrotidal site” could be left off (but this is just a suggestion)
- Why did you not account for river discharge? Isn’t it relevant? You should at least provide some information on the mean discharge and explain why it is not considered in the analysis.
- I believe that Sect. 2 and 3 should be reorganized, as right now it is hard to understand what has been done. The data should be introduced in a dedicated section, introducing the study area and describing the hindcast, current sea level, projected sea levels. Next, the workflow should be detailed in a separated “Method” section, presenting first the hydrodynamic model and its validation (at least introduce appropriate references); next, the GP Metamodel should be described along with the selection of the events used to validate it against the hydrodynamic model. Then you can introduce the steps needed to force the validated GP Metamodel and analyze the results, that is Sect. 3.2 and 3.4. Finally, you could wrap up the section with a summary merged with what is now Sect. 3.5. Please make sure that all the different bootstraps are clearly explained in the Methods section, i.e., the N realizations of the forcing conditions, the B repetitions to compute the Shapley effects, the 20 repetitions to mimic the variability of waves. In the current form of the paper, it is quite hard to understand the methodology.
- Please fix the legend in Fig. 3. Lines of the 90% CI should be dashed if I understand correctly. Also, in panel a) it seems that the upper bound lies outside of the realization ensemble (it cannot be).
- Are you sure the offshore variables are dependent? In other words, is there the need to use a different equation rather than Eq. 5? Looking at the scatter plots of Fig. 4 I cannot appreciate any significant correlation pattern. Please provide some measure of the dependence between the time-series of different forcing.
- It is unclear to me why and how SWL and SLR contributions are split. Please clarify how you group the contributions of water levels between the two effects in the Methods section.
- Fourteen plots are a lot, make sure the total size does not exceed the journal recommendations while guaranteeing appropriate resolution of the figures. For example you could put Figures 9, 10, 12, 14 in the Supplement and summarize the related results in a single section. This way results would be more directly interpreted and their importance would be better framed.
- Please review the English grammar. I am not a native speaker but I found quite a few typos here and there.
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AC2: 'Replies to Reviewer #2’s comments on “Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site ”. (nhess-2021-271)', Jeremy Rohmer, 28 Mar 2022
We would like to thank Reviewer #2 for the constructive comments. We agree with most of the suggestions and, therefore, we have modified the manuscript to take on board their comments. In the attached document, we recall the reviews (in italics) and we reply to each of the comments in turn (in blue).
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RC3: 'Comment on nhess-2021-271', Anonymous Referee #3, 21 Feb 2022
The comment was uploaded in the form of a supplement: https://nhess.copernicus.org/preprints/nhess-2021-271/nhess-2021-271-RC3-supplement.pdf
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AC3: 'Replies to Reviewer #3’s comments on “Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site ”. (nhess-2021-271) ', Jeremy Rohmer, 28 Mar 2022
We would like to thank Reviewer #3 for the constructive comments. We agree with most of the suggestions and, therefore, we have modified the manuscript to take on board the comments. In the attached document, we recall the reviews (in italics) and we reply to each of the comments in turn (in blue).
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AC3: 'Replies to Reviewer #3’s comments on “Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site ”. (nhess-2021-271) ', Jeremy Rohmer, 28 Mar 2022
Jeremy Rohmer et al.
Jeremy Rohmer et al.
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