[LUM#15] A Dam on the Mediterranean
It’s a tangible reality: beaches are disappearing right before our eyes. What is causing this rapid erosion, and what are its consequences? A team of specialists is studying the coastlines of the southern Mediterranean to better understand and anticipate the phenomenon. Hydroclimatologist Gil Mahé explains.
Did that buoy out at sea seem closer last year? And that beach you always go to—did it seem to have more rows of umbrellas last summer? It’s not your imagination: beaches are disappearing, right before our eyes in some places. “Unless you live by the sea, you don’t realize how fast this phenomenon is happening, ” notes Gil Mahé, a hydroclimatologist at the Hydrosciences Montpellier laboratory*.
While this erosion affects nearly all of the Mediterranean coastline, it is particularly pronounced along the 4,600 kilometers of arid coastline stretching from the Bay of Tunis to the Nile Delta, as evidenced by satellite images analyzed by scientists. “In the Bay of Tunis, the coastline is receding by more than twenty meters per year, ” explains Gil Mahé. To highlight the accelerated erosion of North Africa’s sandy coasts, he teamed up with an international team of researchers fromthe National Institute of Marine Sciences and Technologies 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 southern Mediterranean coastlines. Experts are working to better assess the impacts of this phenomenon and to better understand its causes.
From the dam to the beach
In fact, while rising sea levels are a factor, they are not solely responsible for the disappearance of beaches. “The phenomenon also originates inland, ” explains this specialist in the hydrology of Africa’s major rivers. And it is indeed to the rivers that we must look for an explanation—and especially to the structures that have dotted their banks for decades: hydroelectric dams. The link between dams and beaches? It’s obvious. “Rivers carry sand and sediments from the interior of the continent that constantly replenish beaches naturally eroded by ocean currents; this is the normal geomorphological process of coastlines, ” explains Gil Mahé.
A balance that is literally disrupted by the construction of dams. For while these concrete giants can allow water to flow through when necessary, they block the passage of sand and deprive the coastline of it.
“The finest particles, such as suspended silt, can pass through the dam, but sand, being heavier, settles very quickly and gets trapped at the bottom, sometimes eventually silting up the dam completely,” explains the hydroclimatologist. The result: beaches, which rely on sand replenishment, are at risk.
Socioeconomic bomb
This phenomenon is all the more concerning given that dams have sprung up across the Maghreb in just a few decades, keeping pace with population growth and the resulting increase in water demand. “And the situation isn’t going to get any better in the face of water shortages exacerbated by climate change,” predicts Gil Mahé. “With these dams, we solve one problem, but we create another.” We’re actually creating several, if we are to believe the researchers who have studied the consequences of beach erosion. For the disappearance of rows of beach umbrellas is merely the tip of the iceberg, and while the lack of space to lay out a towel does indeed affect the entire tourism sector, beach erosion represents a veritable socio-economic time bomb. “As the coastline recedes, the ocean gets closer to the water table; they say the saltwater intrusion is penetrating further inland.” The result: land that was previously arable is no longer usable, a major 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 measures, lasting twenty years at most.” For Gil Mahé, the most important thing is to realize the inevitability of this phenomenon. “We don’t fully grasp the scale of the problem, or its exponential nature. In reality, it’s already too late—we can no longer stop the sea from rising, even if we don’t know exactly by how much. What we need to do now is figure out how to adapt in the short term, and where the risk of flooding is too high, start planning for evacuation!” A colossal task when you consider that in coastal countries, proportionally more people live in coastal areas. Millions of people to relocate—climate refugees of the near future. “What should be done with the people and property affected? Everything must be done in a spirit of solidarity; we cannot let people living by the sea lose everything they have.” To “properly leave the coastlines, ” the specialist calls for taking climate projections seriously and planning ahead by starting this process now. “Some elected officials are fully aware of this and are already moving in this direction; what is needed now is a strong commitment to it.”
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*HydroSciences Montpellier – HSM – (University of Montpellier – CNRS – IRD)