the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A systemic and comprehensive assessment of coastal hazard changes: method and application to France and its overseas territories
Marc Igigabel
Marissa Yates
Michalis Vousdoukas
Youssef Diab
Abstract. In the context of climate change, height and frequency variations of extreme sea levels (ESL) are studied using deterministic and probabilistic approaches. However, this type of approach does not highlight the dynamic effects (waves, currents) generated by meteocean events, beyond their effects on sea-levels. In particular, ESL estimates are certainly calculated by considering the main determining physical factors but cannot report on all the effects of these factors. Ultimately, this can lead to confusion between ESL and hazard. This article proposes a systemic assessment method to analyze coastal hazard changes at regional scales, integrating parameters influencing sea-levels, as well as factors describing the geomorphological context (length and shape of the coast, width of the continental shelf), meteocean events (storms, cyclones and tsunamis), and the marine environment (e.g., coral reef state and sea ice extent). French mainland and overseas territories were selected to apply the method. We have thus highlighted the need to study not only the sea-level variability, but also the current and future characteristics of meteocean events. The long, concave coasts bordered by a wide continental shelf appear particularly sensitive to variations in the intensity or trajectory of meteocean events. Coral reef degradation in the tropics and the decrease in seasonal sea ice extent in the polar regions can also significantly change the nearshore hydrodynamics and impacts on the shoreline. These results help to predict the types of hazard (shoreline erosion, rapid submersion and/or permanent flooding) that will increase the most in different coastal zones.
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Marc Igigabel et al.
Status: open (until 11 Oct 2023)
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RC1: 'Comment on nhess-2023-154', Anonymous Referee #1, 07 Sep 2023
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Interesting work.
1. How the author correlates the hazard with climate change?
2. How the frequency variations of extreme sea levels (ESL) was studied by the author?
3. Why author have selected France for study purpose?
4. Have author have compared their work with the other researchers?
5. Through some light regarding need of the study?
6. Steps are explained in a detailed manner. It is requested to minimize the same.
7. What is the importance of geomorphology in your study?
8. Give citations wherever required.
9. Add below mentioned papers and cite them in the text:-
-- Verma, S., Verma, M. K., Prasad, A. D., Mehta, D., Azamathulla, H. M., Muttil, N., & Rathnayake, U. (2023). Simulating the Hydrological Processes under Multiple Land Use/Land Cover and Climate Change Scenarios in the Mahanadi Reservoir Complex, Chhattisgarh, India. Water, 15(17), 3068.
-- Kumar, V., Kedam, N., Sharma, K. V., Mehta, D. J., & Caloiero, T. (2023). Advanced Machine Learning Techniques to Improve Hydrological Prediction: A Comparative Analysis of Streamflow Prediction Models. Water, 15(14), 2572.
-- Mehta, D., Hadvani, J., Kanthariya, D., & Sonawala, P. (2023). Effect of land use land cover change on runoff characteristics using curve number: A GIS and remote sensing approach. International Journal of Hydrology Science and Technology, 16(1), 1-16.
-- Shaikh, M. M., Lodha, P., Lalwani, P., & Mehta, D. (2022). Climatic projections of Western India using global and regional climate models. Water Practice & Technology, 17(9), 1818-1825.
Citation: https://doi.org/10.5194/nhess-2023-154-RC1 -
AC1: 'Reply on RC1', Marc Igigabel, 14 Sep 2023
reply
Thank you for your comments and questions. Here are our answers and ways to improve the article.
- How the author correlates the hazard with climate change?
The impact of climate change on coastal hazards is the result of the interaction between climate and ocean variables (which may be highly correlated with one another). For this reason, we considered that « given the multiple effects of climate change on sea levels (GMSL and RSL), atmospheric conditions, and wave conditions, the evolution of the factors and parameters can only be assessed using global development hypotheses » (line 136).
- How the frequency variations of extreme sea levels (ESL) was studied by the author?
General principles are presented in the introduction (line 73-89) and a more detailed presentation is given in the method (line 159 – 176). The method focuses on changes in the magnitude and frequency of occurrence of the present 100-year ESL (ESL100), following Vousdoukas et al. (2017).
- Why author have selected France for study purpose?
They are two main reasons.
The first reason is explained at the end of the introduction (line 95-99): “France and its overseas territories are located in different latitudes (equator, tropics and temperate zones), exposed to different climates, and characterized by different geomorphological configurations (continental or island).” Indeed, the application of this method to selected territories allowed to highlight the expected differences in the future evolution of coastal hazards.
The second reason is that the authors had a good access to French territory data:
- bathymetric maps (provided by the SHOM)
- analysis of storm surges computed from the national REFMAR database (provided by the SHOM)
- Continuous water height and wave measurements from the French national observation services ReefTEMPS and DYNALIT (provided by the LIttoral ENvironment et Sociétés (LIENSs) laboratory)
- Scientific reports relative to extreme sea levels and morphodynamics along French coasts (provided by Cerema).
- Have authors compared their work with the other researchers?
First, we have considered the work of other researchers on physical and biological changes due to climate change on the ocean and the coast (sea level, wave, geomorphology, coral reefs, sea ice extent, etc.). The main references cited in this field are :
- the Concepts and Terminology for Sea Level (e.g. Gregory et al., 2019)
- the mechanisms generating sea-level rise (e.g. Cazenave and Le Cozannet, 2013 ; Frederikse et al., 2020 ; Haigh et al., 2020; Talke et al., 2020) and storm surges (e.g. Bertin et al., 2012; Dodet et al., 2019; Calafat et al., 2022)
- sea level rise projections (e.g. Lowe et al., 2010 ; Church et al., 2013; Slangen et al., 2017; Bamber et al., 2019; Dayan et al., 2021 )
- Height and frequency variations of ESL (Hunter, 2012; Buchanan et al., 2016; Vitousek et al., 2017; Vousdoukas et al., 2017; Vousdoukas et al., 2018)
- Changes in the wave directional frequencies (Casas-Prat and Sierra, 2010) and shoreline changes (Ruggiero et al., 2010; Forbes, 2010 ; Lantuit et a., 2011 ; Lamoureux et al., 2015, Masselink et al., 2016 ; Ranasinghe, 2016; Lavaud et al., 2022 ; Martins et al., 2022)
- Coral Reefs changes (Hoegh-Guldberg, 2007 ; Albright et al., 2018)
- How to translate hazards into Impacts or Risks.
Second, we have studied how these biophysical changes were integrated in coastal hazard and risk analysis. It appears that some authors adopted an analytical approach that is convenient for the issue they are addressing, for example the implications of extreme coastal water levels for potential coastal overtopping (Almar et al., 2021). But in many cases, as indicated in the introduction (line 40-44), the analytical approach should be completed with a more systemic approach. For example, to assess future flood damage in the major coastal cities (see e.g. Nicholls et al., 2008; Hallegatte et al., 2013; Rassmussen et al., 2022) or on the coast worldwide (Tiggeloven et al., 2020), the hazard should not be represented only by ESLs. In particular, when the objective is to assess risk in a comprehensive way, further reflection is needed on the definition of hazard in the context of climate change.
Therefore, we considered that a shortcoming existed in the scientific litterature and we proposed a systemic assessment method, as indicated in the abstract, “to analyze coastal hazard changes at regional scales, integrating parameters influencing sea-levels, as well as factors describing the geomorphological context (length and shape of the coast, width of the continental shelf), meteocean events (storms, cyclones and tsunamis), and the marine environment (e.g., coral reef state and sea ice extent)”.
- Through some light regarding need of the study?
In addition to justifying our proposal scientifically (as indicated above, filling a gap in the state of the art), we could actually add in the text an explanation of the need to disseminate our method in response to operational needs. We propose to add the following sentence, at the end of the introduction (after line 95):
The proposed systemic method emphasizes the need to focus on the analysis and interpretation of the modelling results, by putting them into perspective with respect to the biophysical conditions (both current and forecasted).
- Steps are explained in a detailed manner. It is requested to minimize the same.
For step 1, we propose to shorten three paragraphs (line 178-192). They would be replaced by :
In general, tidal simulations show no significant impacts of RSL rise on tidal amplitude during the 21st century at regional scales, although this does not exclude potential local effects (Haigh et al., 2020 ; Idier et al., 2017). Given the strong uncertainties on RSL rise, we will assume here that the tides are in a steady state and, consequently, that the tidal range does not change the allowance.
For step 2, we propose to shorten one paragraph relative to meteocean event types (line 204-209). It would be replaced by :
The increase in wave damage could also be assessed, considering local changes in significant wave height (average height of the highest one-third of the waves in a given sea state). However, trends in coastal wave climates are reported, with a low confidence level, in the IPCC (2019) report. Therefore, these trends will not be explored in detail: the focus will be on the strong differences that already exist between the wave climates of the maritime facades.
For step 3, we propose to delete one paragraph (261-267).
It will avoid redundancy and information that are not essential for the implementation of the method.
Paragraphs justifying the choice of qualitative parameters would be kept unchanged.
- What is the importance of geomorphology in your study?
The main interest of our study is to consider geomorphology in conjunction with the other factors determining coastal hazards, which appears in our conclusions (lines 640 to 663).
- Give citations wherever required.
It is interesting, as you suggest, to refer to work on water resource management and flood risk in the continental domain. This enriches the reflection on the methods, even if it goes a bit beyond the scope of the current work. Additional quotations have been added (those you mentioned and two others relating to regional sea‐level change and the use of artificial intelligence in the field of coastal risks). On the other points, we made sure to systematically cite the authors we referred to.
- Add below mentioned papers and cite them in the text:
The first, third and fourth references can be cited line 44, by indicating :
In comparison, on land, systemic approaches are used in studying surface runoff and defining strategies in water resource and flood risk management (Shaikh et al. 2022; Verma et al., 2023; Mehta et al., 2023).
The second reference could be mentioned at the very end of the conclusion :
As a follow-up of this study, our method may be improved in the future, by exploring the capabilities of artificial intelligence (AI). In addition to large-scale hydrodynamic model outputs and other environmental data, analyses may integrate deep learning method outputs. AI has already provided interesting results in the field of hydrology (Kumar et al., 2023) and for the prediction of storm surges (Tiggeloven et al., 2021).
Finally, the references we propose to add are as follows:
Hamlington, B. D., Gardner, A. S., Ivins, E., Lenaerts, J. T. M., Reager, J. T., Trossman, D. S., et al. (2020). Understanding of contemporary regional sea‐level change and the implications for the future. Reviews of Geophysics, 58, e2019RG000672. https://doi.org/10.1029/2019RG000672
Kumar, V., Kedam, N., Sharma, K. V., Mehta, D. J., & Caloiero, T. : Advanced Machine Learning Techniques to Improve Hydrological Prediction: A Comparative Analysis of Streamflow Prediction Models. Water, 15(14), 2572, 2023
Mehta, D., Hadvani, J., Kanthariya, D., and Sonawala, P. : Effect of land use land cover change on runoff characteristics using curve number: A GIS and remote sensing approach. International Journal of Hydrology Science and Technology, 16(1), 1-16, 2023.
Shaikh, M. M., Lodha, P., Lalwani, P. and Mehta, D.: Climatic projections of Western India using global and regional climate models. Water Practice & Technology, 17(9), 1818-1825, 2022.
Tiggeloven, T., Couasnon, A., van Straaten, C. et al. : Exploring deep learning capabilities for surge predictions in coastal areas. Sci Rep 11, 17224. https://doi.org/10.1038/s41598-021-96674-0, 2021.
Verma, S., Verma, M. K., Prasad, A. D., Mehta, D., Azamathulla, H. M., Muttil, N., and Rathnayake, U.: Simulating the Hydrological Processes under Multiple Land Use/Land Cover and Climate Change Scenarios in the Mahanadi Reservoir Complex, Chhattisgarh, India. Water, 15(17), 3068, 2023.
Citation: https://doi.org/10.5194/nhess-2023-154-AC1
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AC1: 'Reply on RC1', Marc Igigabel, 14 Sep 2023
reply
Marc Igigabel et al.
Marc Igigabel et al.
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