[LUM#15] A dam against the Mediterranean
It is a tangible reality: beaches are disappearing before our very eyes. What is causing this rapid erosion and what are the consequences? A team of specialists is studying the coastlines of the southern Mediterranean to better understand and anticipate the phenomenon. Gil Mahé, hydroclimatologist, explains.
Did that buoy offshore seem closer last year? And did that beach you frequent seem to have more rows of umbrellas last summer? It's not an illusion: beaches are disappearing, visibly in some places. "Unless you live by the sea, you don't realize how quickly this phenomenon is happening, " says Gil Mahé, a hydroclimatologist at the Hydrosciences Montpellier* laboratory.
While this erosion affects almost all Mediterranean coastlines, it is particularly pronounced along the 4,600 kilometers of arid coastline stretching from the Bay of Tunis to the Nile Delta, as shown by satellite images studied by scientists. "In the Bay of Tunis, the coastline is receding at a rate of more than 20 meters per year, " explains Gil Mahé. To highlight the accelerated erosion of North Africa's sandy coasts, he teamed up with a group of international researchers fromthe National Institute of Marine Science and Technology in Carthage, Tunisia, andthe University of Southern California in the United States. Their study shows that 70% of sandy and deltaic coasts are already threatened, compared to 47% for all of the southern Mediterranean coastline. Specialists are seeking to better assess the impacts of this phenomenon, but also to better understand its causes.
From the dam to the beach
Although rising sea levels are a factor, they are not solely responsible for the disappearance of beaches. "The phenomenon also has its origins inland, " explains this specialist in the hydrology of Africa's major rivers. And it is indeed to the rivers that we must turn to find an explanation. In particular, to the structures that have dotted 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 beaches that are naturally eroded by sea currents. This is the normal geomorphological functioning of coastlines," explains Gil Mahé.
This balance is literally disrupted by the construction of dams. Although these concrete giants can release water when necessary, they block the passage of sand and deprive the coastline of it.
"The finest particles, such as suspended alluvium, can pass through the dam, but sand, which is heavier, settles very quickly and remains blocked at the bottom, sometimes ending up completely silting up the dam," explains the hydroclimatologist. As a result, beaches, which are replenished with sand, are at risk.
Socio-economic bomb
This phenomenon is all the more worrying given that dams have sprung up across the Maghreb region over the past few decades, in line with population growth and increasing water needs. "And this situation is not going to improve in a context of water scarcity exacerbated by climate change," predicts Gil Mahé. "These dams solve one problem, but they create another." In fact, we are creating several problems, according to researchers who have studied the consequences of beach erosion. The disappearance of rows of beach umbrellas is only the tip of the iceberg, and while the lack of space to lay out towels is affecting the entire tourism sector, beach erosion represents a real socio-economic time bomb. "As the coastline recedes, the ocean moves closer to the water table, and it is said that the salt wedge penetrates further inland." As a result, land that was previously arable is no longer so, which is a serious blow to agriculture. "And what about all the infrastructure built along the coast? The ports? The nuclear power plants? The coastal problem affects millions of people!" insists Gil Mahé.
Early retirement
Solutions? Local, temporary ones. For example, we could redesign dams so that they allow sand to pass through, which would help slow coastal erosion. "It's possible, but these are only short-term mitigation solutions, lasting twenty years at most." For Gil Mahé, it is essential to recognize the inevitability of this phenomenon. "We don't fully appreciate the scale of the problem and its exponential nature. In reality, it's already too late. We can no longer prevent the sea from rising, even if we don't know exactly by how much. What we need to do now is determine how to adapt in the first instance, and where the risk of flooding is too high, think about organizing a retreat!" This is a colossal task when we consider that, in coastal countries, proportionally more people live in coastal areas. Millions of people will have to be relocated, becoming climate refugees in the near future. "What should be done with the people and property affected? Everything must be done in a spirit of solidarity; we cannot allow people who live by the sea to lose everything they have." In order to "leave the coastlines properly, " the specialist calls for giving credence to climate projections and anticipating the process by starting it now. "Some elected officials are fully aware of this and are already working on it. What is needed now is a strong commitment."
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*HydroSciences Montpellier – HSM – (University – CNRS – IRD)