When the sea rises

It's an undeniable fact: the sea is rising. In some places, the beach is losing ground metre by metre, and homes with a view of the sea sometimes find their feet in the water. How does this happen, and how can it be better managed? Explanations from beach physicist Frédéric Bouchette and economist Hélène Rey-Valette.

Today, 6 million people live in coastal communities. 9 million in 2040, according to forecasts by Cerema, the French center for studies and expertise on risks, the environment, mobility and development. More and more people, but less and less space: between 1950 and 2010, 27.7 km² of beaches were lost in mainland France. The cause: erosion and rising sea levels, linked in particular to global warming. " A rise of 3.2 millimeters per year over the last 20 years", explains Frédéric Bouchette, researcher at the
Géosciences Montpellier* laboratory and member of Gladys, the collaborative network dedicated to academic research in coastal dynamics. "A priori, this rise in water levels leads
to a retreat of the shoreline, which is known as the coastline," adds
.

Coastal resilience

However, this shift in the coastline is not a systematic phenomenon. " The coastline shows a certain hydro-sedimentary resilience, i.e. the capacity of beaches to adapt and/or rebuild themselves in such a way that the land is not necessarily condemned to be invaded by sea water", explains Frédéric Bouchette. Except that... " Except that for this coastal hydrosedimentary resilience to work, there has to be enough sand, and today that's no longer the case." The main culprits are coastal developments - dykes, harbours and groins - which block the natural movement of sand, and, to a lesser extent, the dams that
hold it back in rivers. "But also massive unregulated sediment withdrawals,
which take tons of sand from the sea for use in construction, transport and road infrastructure. Not to mention urbanization and infrastructure built too close to the coastline, preventing
beaches from retreating naturally.

As a result, the coastline is no longer able to adapt to rising sea levels
and in the space of 30 years, an average of 100 meters of water has been nibbled away
from the land. " These proportions are abnormally high compared to what has been
observed in the past", explains Frédéric Bouchette. And what about the future? "There's a large degree of uncertainty
in the evolution of the coastline, and it's one of the most complex issues in coastal research," explains the specialist, who stresses the importance of geophysics in developing prediction tools. "We make uncertainty projections using methods similar to those used in finance. The complexity of the dynamics intrinsic to the wave-sediment relationship makes evolutions difficult to predict; but the models we develop enable us either to give trends, or to understand certain singular evolutions such as the creation of sandy spits. This is essential information for decision-makers in the sector," insists Frédéric Bouchette.

Spatial recomposition

Raising awareness among coastal managers to anticipate the consequences of climate change is just one aspect of Hélène Rey-Valette's research. "Implementing policies to adapt to sea-level rise involves assessing damage differentials with and without adaptation," explains the researcher from Montpellier's Centre d'économie de l'environnement(CEE-M)**. Together with economist Cécile Hérivaux from BRGM, the French Geological and Mining Research Bureau, she assessed the damage and benefits for the Occitanie region in the 2,100-year timeframe
for a 1-meter rise in sea level. Without adaptation? 77,000 inhabitants, 34,000 homes, 4,600 businesses affected. 11,500 hectares of farmland and 570 hectares of beach and dunes lost. But also 4 aquifers potentially affected by salinization, i.e. 7.5 million cubic meters of drinking water per year impacted. So what can be done to avoid reaching this point? " Until now, we've talked about retreating, withdrawing, relocating, but the idea today is to move towards spatial recomposition," explains the economist, who sees this as a genuine paradigm shift involving learning to live with risk. "Yet our surveys show that coastal dwellers don't take the measure of it, notably because of what we call the optimism bias, which means that those who benefit from the advantages of living near the beach minimize the risk. We need to study perceptions in order to inform elected representatives and help them better understand
where the population stands."

Opportunity

1 million people now live on the coast, a population directly
concerned by the spatial reorganization that will involve relocating certain neighborhoods and, above all, rethinking territories in the light of all the challenges and consequences of climate change. "Hélène Rey-Valette adds: "For example, with regard to artificialization, waterproofing, soft mobility, energy issues and heat islands. But relocating to where? "We'll need to work with the coastal communities, which will be faced with an influx of population and will have to resize their towns, schools and infrastructures. Those who have to move will also lose their neighborhoods, their friends - it's a complex situation," admits Hélène Rey-Valette. This is where the researcher comes in, proposing solutions to make these policies economically and socially feasible. But also to find the right tempo: "in terms of acceptability, we recommend bringing about a gradual change, but the
IPCC tells us that we don't have the time, so we need to help elected representatives put in place appropriate policies that are also as gradual as possible".

For the economist, it's a real transformation in the offing: "We're going to have to rethink the city as a whole, and consider more reasonable and concerted approaches. From this point of view, climate change can also be seen as an opportunity to rethink our lifestyles, but we mustn't miss the train."

*GM (UM, CNRS, U Antilles)
**CEE-M (UM, CNRS, Inrae, Institut Agro)