A dam against the Mediterranean

It's a tangible reality: beaches are disappearing before our very eyes. What is causing this galloping erosion, and what are the consequences? A team of specialists is scrutinizing the coastlines of the southern Mediterranean to better understand and anticipate the phenomenon. Gil Mahé, hydroclimatologist, explains.

Did that buoy off the coast seem less far away last year? And that beach where you've always been, did it seem to have more rows of umbrellas last summer? It's no illusion: the beaches are disappearing in some places. " Unless you live by the sea, you don't realize how fast this phenomenon is taking place," points out Gil Mahé, hydroclimatologist at the Hydrosciences Montpellier laboratory*.

While this erosion affects almost all Mediterranean coastlines, it is particularly marked on the 4600 kilometers of arid coastline stretching from the Bay of Tunis to the Nile Delta, as shown by the satellite images studied by the scientists. " In the Bay of Tunis, the coastline is retreating at a rate of over twenty meters per year," explains Gil Mahé. To highlight the accelerated erosion of North Africa's sandy coasts, he joined forces with a team of international researchers from theInstitut National des Sciences et Technologies de la Mer in Carthage, Tunisia, and theUniversity of Southern California in the USA. Their study shows that 70% of sandy and deltaic coasts are already threatened, compared with 47% of all southern Mediterranean coastlines. Specialists are seeking to better assess the impact of this phenomenon, but also to better understand its causes.

From dam to beach

Although the sea is rising, it is not the only factor responsible for the disappearance of beaches. " The phenomenon also originates inland," explains this specialist in the hydrology of Africa's great rivers. And it's to the rivers that we must turn for an explanation. And especially to those structures that have lined the rivers for decades: hydraulic dams. The link between dams and beaches? It's obvious. " Rivers carry sand and sediment drained from the interior of the continent, which constantly replenish the beaches naturally eroded by marine currents. This is the normal geomorphological functioning of coastlines," explains Gil Mahé.

A balance that is literally hampered by the construction of dams. While these concrete giants can let water flow through, they block the passage of sand, depriving the coastline of this resource.
" The finest particles, such as suspended alluvium, can pass through the dam, but the heavier sand settles very quickly and remains blocked at the bottom, sometimes ending up by completely silting up the dam", explains the hydroclimatologist. As a result, beaches, which are more heavily recharged with sand, are at risk.

Socio-economic bomb

This phenomenon is all the more worrying given that dams have sprung up all over the Maghreb in the space of a few decades to keep pace with population growth and growing water requirements. And it's not going to get any easier in a context of water scarcity aggravated by climate change," predicts Gil Mahé. With these dams, we're solving one problem, but creating another". If we are to believe the researchers who have studied the consequences of beach erosion, we are even creating several of them. The disappearance of rows of beach umbrellas is just the tip of the iceberg, and while the lack of space to spread your towel does indeed affect the entire tourism sector, beach erosion represents a veritable socio-economic bombshell. "As the coastline retreats, the ocean moves closer to the water table, and the saltwater system penetrates further inland. As a result, land that was previously arable is no longer so, a major blow to agriculture. "And what about all the infrastructure built along the coast? Ports? Nuclear power plants? The coastal problem affects millions of people," insists Gil Mahé.

Early retirement

Any solutions? Local and temporary. We can, for example, rethink the construction of dams so that they allow sand to pass through, which would help slow down coastal erosion. " It's possible, but these are only short-term mitigation solutions - twenty years at most. For Gil Mahé, the most important thing is to be aware of the inevitability of this phenomenon. "We don't appreciate the scale of the problem and its exponential nature. In reality, it's already too late, we can't stop the sea from rising, even if we don't know exactly by how much, and what we need to do now is determine how to adapt in the first instance, and where the risk of submersion is too high, think about organizing retirement! A colossal task when you consider that in seaside countries, proportionally more people live in coastal areas. Millions of people to be displaced, climate refugees of the near future. "What can we do with the people and property concerned? We can't let people who live by the sea lose everything they've got. To "leave the coasts properly", the specialist calls on us to give credence to climate projections, and to anticipate by starting this process now. "Some elected politicians are fully aware of this and are already engaged in the process, but what's needed now is a strong commitment".

*HydroSciences Montpellier - HSM - (UM - CNRS - IRD)