• The question of the reinforcement of the dykes arises in the face of the intensification of extreme storms accompanying climate change, according to our partner The Conversation.

  • Thanks to the development of adequate physical and numerical models, future coastal developments will be able to be subjected to virtually all imaginable climatic events.

  • This analysis was conducted by Julien Réveillon, teacher-researcher in energy and fluid mechanics at the University of Rouen Normandy.

During a storm, a levee can experience highly variable pressure peaks, especially when air pockets are trapped between the water and the concrete.

For example, pressures close to five bars leading to forces of 400,000 newtons per meter – the equivalent of the crash of an SUV going at 80 km/h – have already been measured on the Artha dike in Saint-Jean-de -Luz.

The current dykes are sized for this type of impact, but the question arises of their reinforcement in the face of the intensification in strength and frequency of extreme storms accompanying climate change.

Numerical simulations aim to best adapt coastal facilities to the transformations to come.

Defense measures against rising waters

Sea level rise is a direct result of the melting of glaciers and ice caps, as well as the thermal expansion of the oceans.

It is a major threat to coastal regions, as it causes the submersion and erosion of the coastline (the land/sea boundary), the flooding and destruction of dwellings and the contamination of agricultural land.

The implementation of defense measures is already at the heart of the concerns of decision-makers in charge of land use planning and builders of maritime structures.

New strategies are being explored based on the state of the sea off our coasts in several decades in order to estimate the surges caused by storms – extremely difficult predictions.

The most common (so-called “hard”) defense solution consists in trying to fix the coastline and build or reinforce the defense works.

Another solution (known as "flexible") consists in preparing the territory for the arrival of the sea. For example, the municipality of Quiberville in Normandy - one of the 126 French municipalities cited in the official journal in April 2022 as having to adapt priority – took the decision to let the sea enter the low areas of the municipality and to reorganize its layout.

How strong will the waves be in the future?

Marine flooding phenomena generally occur during extreme climatic events.

It is very difficult to predict their intensity because in addition to the average level of the water level and the effects of the tide, they depend on local parameters such as the intensity of the wind pressing on the water, the drop in local pressure or frequency of breaking wave succession.

All of these phenomena raise the water near the protective structures.

To date, to estimate their future impact on coastal structures, they are experimentally reproduced on a small scale in a wave channel, or they are modeled using numerical simulation, in a succession of impacts representing three hours of waves of a “centennial” storm (that is to say whose probability of occurrence is one year in a hundred).

In modelling, the effects of certain parameters are still very difficult to characterize, such as the presence of air in the water, in the form of bubbles, resulting from the foam of previous waves or from pockets and tubes of captured air by the surge or even air cushions forming between the water and the wall of the dikes.

Depending on the circumstances, this air will dampen the shock or even amplify it.

The worst scenario is to trap a small pocket of air between the water and the wall: the mass of the breaking wave brutally crushes the pocket and greatly increases the air pressure and therefore the forces that will be the work.

OUR “COASTAL” FILE

Thanks to the development of adequate physical and digital models, the future coastal developments envisaged can be subjected to virtually all imaginable climatic events in order to assess their effectiveness and resistance.

But these numerical simulations must go beyond the strict framework of physics because they will remain useless if they do not integrate a global vision, including in particular dimensions of sustainable construction and the consent of the populations concerned.

This analysis was written by Julien Réveillon, teacher-researcher in energy and fluid mechanics at the University of Rouen Normandy.



The original article was published on The Conversation website.

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