The Gibraltar barrage
Controlling the sea level of the Mediterranean would be an engineering feat and may be both possible and necessary, writes Mohamed El-Kassas*
In its Autumn 2007 issue, Beyond focused on the issue of climate change and its expected impact on Egypt. Several prominent scientists, including Mostafa K. Tolba, Mohamed El Raey, Samia El-Marsafawy and others, contributed valuable articles pointing the way forward in dealing with this critical global threat. As a follow up, we present today a valuable article by the renowned Egyptian scientist Professor Mohamed Kassas, in which he explores the possibility of addressing the global climate threat to the Mediterranean region by the construction of a barrage across the Strait of Gibraltar to control the inflow of water from the Atlantic and hence the level of the Mediterranean's surface water.
The Mediterranean is a word that has three connotations: a body of saltwater (sea), a basin comprising the sea and its adjoining coastal lands, and a region with a climate type (hot, dry summers and mild, wet winters) that prevails in an area broader than the basin and in homologues encountered far beyond the basin: southern Africa, southern Australia, western South America and western North America.
The Mediterranean Sea is a body of water that extends for around 4,000 kilometres from Gibraltar to the Syrian coast. It is the largest inland sea (an area of approximately 2.5 million square kilometres), and it lies between Asia, Africa and Europe. Its maximum depth is around 4,400 metres (average depth is around 1,500 metres), with a total water volume of around 3.7 million cubic kilometres. It is connected to the Atlantic Ocean through the Strait of Gibraltar (15 kilometres wide, 290 metres deep), with the Sea of Marmara and the Black Sea through the Dardannelles (four kilometres wide, 55 metres deep), and with the Red Sea through the Suez Canal since 1869.
The Sicilian Channel (110 kilometres wide, 350 metres deep) divides the Mediterranean Sea into a western basin (around 850,000 square kilometres) and an eastern basin (approximately 1,650,000 square kilometres). The Mediterranean Sea comprises a number of sub-basins: the Ligurian, Adriatic, Aegean, Ionian and Tyrrhenian seas.
The hydrological processes of the Mediterranean Sea set the framework of its ecology, resources, environment and likely impacts of future global warmth. The water body comprises three layers that differ in their salinity and temperature profiles: a surface layer (depth 75-300 metres), an intermediate layer (depth 400-600 metres, warmer and more saline), and a deep layer (below 600 metres). The eastern basin is warmer than the western basin and hence evaporation rates are higher and the level of water surface lower. The sea level of the Mediterranean is lower than that of the Atlantic (lowest around 80 centimetres in the eastern basin).
The deficit is compensated by an influx of Atlantic water through the Strait of Gibraltar (approximately 40,000 cubic meters per second, or around 2,760 cubic kilometres per year). This figure is the sum of Atlantic-Mediterranean exchanges through surface and subsurface currents. Estimated turnover time for Mediterranean waters is 80-100 years.
The water deficit of the Mediterranean Sea and the narrow inlet of the Strait of Gibraltar suggested to the German engineer Herman Söergel in 1926 the idea of a barrage across the strait that could be used to control the inflow of water from the Atlantic and hence control the level of Mediterranean surface water. Söergel had a dream project that would provide a mechanism for lowering the Mediterranean for a few hundred metres, to: expose wider shoreline stretches that would provide space for coastal resorts, etc (a 100-metre drop would result in the exposure of 150,000 square kilometres of Mediterranean littoral); a Gibraltar hydraulic power plant would be an added asset; expose a land bridge between Africa and Europe (the Sicilian divide); the Sicilian bridge and the Gibraltar cross-over would provide two highway connections between Europe and Africa (forming a united continent: Atlantiropa).
This was earlier than the concern about global warmth and its likely impact on sea levels. The Söergel scheme included complementary projects such as locks across the Suez Canal.
Herman Söergel first reported his project in papers (March 1928), then a book, Lowering the Mediterranean, published in 1929 in German, French, English and Spanish.
The Strait of Gibraltar
The Mediterranean is what remained of its giant ancestor: the Tethys Ocean. It has a very long geological history with major tectonic movements including continental drifts. During the Tertiary Age (65 million years ago), the Mediterranean Sea was progressively reduced, the gap between Europe and Africa was closed and the connection between the Mediterranean and the Atlantic was cut off. But some 5.3 million years before present a new series of westerly oriented tectonic shudders shook the region, breaking open the land bridge between lberia and North Africa, and allowing the Atlantic waters to once again surge through the Strait of Gibraltar into the dried basin. The present day size, shape and features of the Mediterranean were finally set around five million years before present. But during the Quaternary Age (one million years before present) events of the major glacials and interglacials caused the lowering (regression) and the rising (transgression) of the Mediterranean.
"The whole Mediterranean Sea has the shape of a bottle of which the nine-mile (14.4 kilometre) Strait of Gibraltar is the neck", wrote E C Simple in The Geography of the Mediterranean Region in Relation to Ancient History, published in 1932. The relative narrowness and shallowness (290 metres) of this Atlantic- Mediterranean gateway made it seemingly a practicable site for a barrage that would control the inflow of water from the Atlantic Ocean to the Sea. Without this inflow the "level of the Mediterranean would tend to subside", to quote André Siegfried.
So‘rgel's project revisited
So‘rgel's scheme (lowering the Mediterranean Sea) was based on the balance sheet of the Mediterranean waters. It showed that around 70 per cent of its water deficit was compensated by inflow from the Atlantic. A dam across Gibraltar would initiate a process of Mediterranean dry up. So‘rgel calculated that an annual drop in water level would be "in excess of one metre". The Gibraltar dam, with navigation sluices, would be a major engineering structure.
Very likely global warmth would cause global water bodies to expand by an increase of temperature (1-3¼C), water levels rising 20-80 centimetres. Melting ice formations may increase the volume of marine waters causing a substantial rise (several metres). Coastal countries of the two connected basins, the Mediterranean and the Black Sea, face the threat of sea level rises. Approximately 30 countries have developed coastal lands that would be threatened by sea level rises. Building protective walls may be considered in very limited sites, but not for the thousands of kilometres of shoreline. Revisiting Soergel's scheme deserves consideration. The objective is not to lower the sea level, but to provide a mechanism for controlling this level, and preventing the inundation of otherwise threatened coastal lands.
Four studies need to be carried out preparatory to the project: engineering design, including costs; formulating the basis of an international treaty on maritime traffic; formulating the basis of a regional convention including the 30 or so countries directly concerned and deciding ownership and mechanisms for management; environmental impact assessments.
The engineering study would require background survey data on the Gibraltar site, particularly water depths, bottom configuration (including the sill that characterises the bottom), and the hydrographies of Atlantic-Mediterranean exchanges.
The international treaty may require the UN General Assembly to establish an intergovernmental negotiation group to draft the treaty (comparable to treaties set for the Suez Canal, etc).
The regional convention -- under the aegis of the Union for the Mediterranean -- would set the arrangements for ownership and management.
The environmental impact studies may be carried out with the help of the institutions of the Convention for the Protection of the Mediterranean Sea Against Pollution, Barcelona, 1976 (United Nations Environment Programme).
* The writer is a Professor Emeritus, Environmental Sciencein, Cairo University