27 Oct 2021
27 Oct 2021
Status: a revised version of this preprint is currently under review for the journal NHESS.

Partitioning the uncertainty contributions of dependent offshore forcing conditions in the probabilistic assessment of future coastal flooding at a macrotidal site

Jeremy Rohmer1, Deborah Idier1, Remi Thieblemont1, Goneri Le Cozannet1, and François Bachoc2 Jeremy Rohmer et al.
  • 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)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on nhess-2021-271', Anonymous Referee #1, 10 Jan 2022
  • RC2: 'Comment on nhess-2021-271', Anonymous Referee #2, 16 Feb 2022
  • RC3: 'Comment on nhess-2021-271', Anonymous Referee #3, 21 Feb 2022

Jeremy Rohmer et al.

Jeremy Rohmer et al.


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Short summary
We quantify the influence of wave/wind characteristics, offshore water level and sea level rise (projected up to 2200) on the occurrence of flooding events at Gâvres town-French Atlantic coast. Our results outline the overwhelming influence of sea level rise over time compared to the others. By showing the robustness of our conclusions to the errors in the estimation procedure, our approach proves to be a valuable tool to explore and characterize uncertainties in assessments of future flooding.