Updated 08.02.2021
The rapid erosion of the sandy North African coast, which is disappearing almost in front of our eyes, is very alarming. Scientists are working to measure the effects, evaluate the impacts and understand the causes in order to propose possible solutions.
Will it come to a choice between running water and white sandy beaches? In other words, arbitrating between hydraulic systems and preservation of the environment, natural resources and the living conditions and subsistence of populations on the Mediterranean shores of Africa... “Our work shows that pressure on water resources contributes directly to the vulnerability of the arid coasts between the Gulf of Tunis and the mouth of the Nile,” explains Abderraouf Hzami, a geologist specialised in Geographic Information Systems (GIS) and a PhD student at the University of Carthage in Tunisia. “IndicatorsCoastal Vulnerability Index (CVI) including geomorphology, shoreline erosion/accretion, coastal slope, relative sea level rise, mean wave height and mean tidal range.1 reveal that 70% of the sandy coasts and deltas are already endangered or very endangered, compared with 47% for the southern Mediterranean coastline as a whole”. The problem is characterised by rapid beach erosion.
The sandy beaches are not the only visible victims of the rapid erosion of the Mediterranean coasts; coastal infrastructures are also paying a high price.
© IRD - Makrem Mandhouj
Twenty metres per year
“Judging by the changes in the Gulf of Tunis, where the coastline is retreating at a rate of over twenty metres per year, everything suggests that the sandy beaches will completely disappear in the near future”, explains Oula Amrouni, a geologist specialised in coastal sediments at the National Institute of Marine Science and Technology at the University of Carthage. “You can count the number of rows of beach umbrellas lost over time and the coastal infrastructures that are gradually being washed away by the waves". With the scientific support of geophysicist Essam Heggy,University of Southern California and Nasa Jet Propulsion Laboratory1 the work of this multinational teamWork financed by the Hubert Curien Partnership (PHC) programme, Utique RYSCMED, LMI COSYSMED, the National Institute of Marine Science and Technology of the University of Carthage, the University of South California and UMR HydroSciences Montpellier.1 is showing that this erosion and the coastal flooding that accompanies it, which is particularly marked on the 4,600 km of arid coastline from the Gulf of Tunis to the shores of Tripoli and the Nile Delta, are not an isolated case, but are also occurring along almost all of the Mediterranean coasts at varying levels of intensity – from a few centimetres to over a dozen metres a year.
“Contrary to what you might expect and to what is often suggested, this phenomenon has little to do with climate change or rising sea levels” says the scientist. We must instead look inland for its origins.
The construction of dams and water reservoirs in the Mediterranean basin to supply growing anthropogenic needs prevents the flow of essential silt to compensate for coastal erosion caused by coastal currents.
© IRD - François Molle
Alluvial deficit
It is not the few millimetres of rise in sea levels - on average three millimetres per year worldwide - that could cause such submersion. “In reality, the very geomorphological functioning of the coasts is being affected,” she explains. “They are eroded naturally by marine currents, but were constantly replenished by river alluvium until a few decades ago. Sand and sediments drained from inland made up for the losses.” But that was before... Before water began to be captured from rivers by building dams to meet anthropogenic needs. In addition to retaining water, the dams trap the solid elements it transports and deprive the coasts and beaches of the supply they need to maintain them.
The intense pressure on water resources for anthropic uses can completely dry up the riverbeds in the Maghreb.
© IRD - Gil Mahé
Extreme pressure on water resources
In the past few decades, dams throughout the Maghreb have increased in size and number in line with the dramatic rise in water requirements to meet domestic demand, which has soared with growth in the population and improvements in the standard of living, to water crops and to satisfy the needs of multiple other uses.
Increasing urbanisation, demographic growth and the rise in living standards are dramatically increasing the pressure on water resources throughout the Mediterranean basin.
© IRD - Pierre Treissac
“There is extreme pressure on water resources,” says Gil Mahé, a hydrologist and climatologist at IRD. “Dams store up huge quantities of water in relation to the amounts available: water is thus held back several times between the source and downstream, and in many cases nothing reaches the dry mouth of the river, neither water nor silt”. The impact of periodically organised water release operations remains limited because they only drain the lightest silt particles, leaving the sand at the bottom of the reservoir lakes. For example, the Medjerda, the largest river in Tunisia, has not transported any sand to the coast of the Gulf of Tunis for 40 years, and the amount of much finer clay materials has been halved, leading to the effects that we see on the beaches. The consequences of this damaging process go far beyond the simple question of beach holidays.
A socio-economic time bomb
“The vast majority of the population lives in the coastal zone, so the retreat of the coastline has a profound impact on the whole of the society,” explains Abderraouf Hzami. “It contributes directly to the loss of infrastructures, land and arable land and, by bringing the ocean closer to the water tables, to their salinisation as well as that of the soils”.
The marine environment, deprived of organic alluvium by the construction of dams, is poorer and fishing suffers as a result.
© IRD - Sylvie Bredeloup
Similarly, fishing is significantly affected by the impoverishment of marine environments which are deprived of organic alluvial deposits. In concrete terms, the economic activities of part of the population - often the most socially vulnerable - and food security are now under serious threat. “In this coastal erosion hotspot stretching from Tunisia to Egypt, our work has also established a link between the coastal vulnerability index and the socio-economic vulnerability index,”Socio-Economic Vulnerability Index (SVI) including social factors, population density, infrastructure, urbanisation rate. It is defined as the degree of adaptation of a given society to suffering damage from natural events.1 explains the researcher.
“The decision-makers of the region must become aware urgently of the anthropic and local origin of the problem, as well as of the need to modify the hydraulic works and their management to re-establish sufficient transfer of sand to the coast to keep the beaches in equilibrium,” says Oula Amrouni. “It is all the more important as the effects of climate change will increase the strain on water resources further”.
© IRD - Daina Rechner
After the beach, from which the sand has been swept away, the steps and the promenade are in turn carried off by the waves.
A global scientific debate
Rapid beach erosion is a reality on all the arid and semi-arid coasts of the planet, from California to Australia, Languedoc and the Red Sea. Some European researchers believe, following work by modelling specialists in Delft in the Netherlands, that half of the beaches will have disappeared by 2100. Others, however, led by Australian, American and New Zealand geographers, claim that specific measures such as concrete groynes, coastal protection and other types of dykes and breakwaters, will save the most endangered coastlines. To this, the former group replies that, contrary to the desired effect, these infrastructures may above all accelerate the process by blocking the natural movement of the beaches and contributing to their total loss in doing so. The two sides presented their hypotheses and counter-arguments in several articles published last month in the journal Nature Climate Change.
