User:Jim gower/Strait of gibraltar dam

Strait of Gibraltar dam new article content ... A loose rock barrage or dam across the Strait of Gibraltar was proposed at the CIESM meeting in Marseille, France in October 2013 (ref) with the purpose of protecting the coasts of the Mediterranean and Black Seas from global sea level rise, and improving transport links between Europe and Africa. A dam across the Strait of Gibraltar has been proposed at least twice before, once by the German Atlantropa project in the 1930s (ref) as a way of creating new land and generating electricity, and once in 1997 (ref) as a way of averting a possible ice age. About 6 million years ago a natural dam was briefly formed when the Strait was closed by plate tectonic motion (ref).

Global sea-level rise is now seen as one of the most significant impacts of human-induced climate change, and is expected to seriously affect most coastal areas in the next 10 to 100 years. However, for the coasts of the Mediterranean and Black Seas, a dam on the Strait of Gibraltar can provide long-term protection, since evaporation tends to naturally lower Mediterranea Sea level. The dam would be designed to (initially) cause a height difference between its two sides of only a few metres, limiting its impact on shipping, but requiring lock gates at the dam and on the Suez canal. Such a dam would have significant environmental impact, perhaps most importantly causing a long-term rise in Mediterranean salinity. It would also have the advantage of allowing fast, high-volume road and rail links across the Strait.

The dam would be a major project, requiring international agreements and resources. It will probably not be built soon, but discussions need to start to quantify costs, benefits and impacts.

Natural blocking of the Strait of Gibraltar Pliny the Elder in the first century AD recounted a popular story in his Natural History by which the Mediterranean Sea was created when the Atlantic Ocean gained admission through the Strait of Gibraltar. The idea that the Mediterranean was once dry land has been referred to by many writers since, with increasing evidence from geology, botany and evolutionary biology. It was finally confirmed by drilling missions of the Glomar Challenger in 1970 and 1975 (Hsu, 1983). Plate tectonic movement is thought to have closed the Strait about 6 million years ago and most recently reopened it about 5.3 million years ago. This blocking event is referred to as the Messinian Salinity Crisis.

Under present climate conditions, the estimated Mediterranean freshwater deficit (excess of fresh-water removal by evaporation, over input by rain and rivers) is about 50 cm/yr (Mariotti et al., 2002). This is therefore the rate at which the Mediterranean Sea level would sink if the Strait were blocked and no inflow from the Atlantic were allowed. This deficit would empty the Mediterranean in a few thousand years, depositing its salt in beds which are one of the modern signatures of the Messinian Salinity Crisis, and raising global sea-levels by about 10 m.

The fact that Mediterranean sea level has been much lower than at present is confirmed by erosion of deep canyons by many Mediterranean rivers, notably the Rhone and Nile, both near the present coast line and in areas now far from the sea (for example, the Nile River at Aswan). Canyons are also found in the present deep bed of the Mediterranean, showing that large areas must have dried completely (Hsu, 1983).

Dam history A Gibraltar dam was most famously proposed for the German Atlantropa project of the 1930s (see for example Cathcart 2006, and references therein). This was designed to lower Mediterranean Sea level by 200m, provide new land, and allow power generation from the resulting inflow at the Strait. Proponents did not explain how the world would cope with the resulting 1.6 metre global sea-level rise. The project aimed to provide 1930s Germany with new land, assuming a German-led, united Europe. However, a 200 m sea-level drop would leave present coasts high and dry and cause dramatic environmental change. In the present proposal a much more modest sea-level reduction is sufficient to stabilize Mediterranean and Black Sea levels, maintaining coast lines in their present position.

Dams have also been proposed for the Red Sea and the Persian Gulf. In both cases, dams would require rather more rock fill than at Gibraltar, and in both cases the proposals were made with the object of lowering the enclosed sea-level by several tens of metres by evaporation, and then generating electricity from water inflow through the dam, as for Atlantropa. Sea water inflow would then maintain the new, lower sea-level. As with Atlantropa, these projects are unlikely to be popular since they involve a significant decrease in sea-level of large bodies of water, with major impacts on many coastal communities and ecosystems. Other options for carbon-free power generation are available through wind, solar or nuclear generation.

Johnson (1997) urged construction of a dam across the Strait of Gibraltar to prevent what he saw as a coming ice age induced by the building of the Aswan Dam on the Nile. The chain of logic was unverified, and the claimed effects of Mediterranean salt water outflow appeared exaggerated (Rahmstorf, 1998).

Dam construction

Oceanographic implications The net inwards flow of water from the Atlantic at Gibraltar totals about 1,200 km3/year, 0.04 Sverdrups or 40,000 cubic metres per second. The present near-surface flow inwards is about 1 Sverdrup, almost balanced by a flow outwards, below about 50m depth, driven by the higher density of water in the Mediterranean compared to the Atlantic. A dam would block this deep outflow and cut the near-surface inflow from 1 Sverdrup to the flow needed to stabilize Mediterranean sea level, about 1,200 km3/year. This replaces the excess of evaporation over the combination of precipitation and river flow into the Mediterranean and Black Seas. Since evaporation removes only fresh water, the main effect of replacing the 1200 km3/year deficit with salty (37 PSU (Practical Salinity Unit, or parts per thousand) Atlantic water will be that Mediterranean average sea water saltiness, or salinity, will increase at a rate of about 0.012 PSU per year, or 1 PSU every 80 years.

We need to understand and quantify the long term effects of this salinity rise. If a 5 PSU rise in Mediterranean salinity is acceptable, then we have 400 years before this value is reached, and time to plan responses. Salinity could be stabilized by pumping at the dam, but this requires a significant increase in flow rates through the dam and an increase in cost.

There is also concern that ventilation (oxygen inflow to deeper water) would be reduced by higher salinities. At present, oxygen is carried to deep waters when high-salinity surface water cools and sinks. The effect of future climate change on this process has been studied by Hermann et al. (2008). They concluded that increased stratification would reduce deep oxygenation, but it is unclear whether a higher-salinity Mediterranean will lead to increased stratification.

There is also clear evidence from layers of sapropels (from the Greek, sapros pelos, or stinky mud) in Mediterranean sediments that deep oxygenation has been interrupted in the past, making deep waters anoxic, most recently 8,000 to 10,000 years ago. These interruptions are ascribed to heavier rain-fall leading to lower salinity surface water in the Mediterranean, which prevents high-oxygen surface water from mixing to the bottom (Rossignol-Strick et al., 1982, Ruddiman, 2008).

Power generation The potential energy from 40,000 tons of water per second dropping 1 m is about 400 MW, worth about $100 million per year if converted to electricity. This value would increase in future years as global sea level rises above the Mediterranean.

Channel and bridge design If the channel for water flow over the dam is designed to be 20 m deep, then the Froude number is equal to 1 at a flow rate of 14 m/s. The kinetic energy from the potential energy of water dropping 1 m implies movement at 4.4 m/s, so that the flow will be sub-critical for this level difference. The channel width for the flow would therefore need to be a total of about 450m, perhaps consisting of nine 50m wide channels, assuming no impediment to the flow and hence no power generation. These flow channels could easily be bridged by a road or rail link, and could be harnessed later for power generation when height differences are larger. Tides at Tangier just outside the Strait, have a range of about 2 m peak to peak, while tides in the Mediterranean are much smaller. A mean height difference of 1m would therefore be boosted to near 2 m at high tide (giving currents of about 7 m/s) and reduced to nothing at low tide, allowing migrating whales and fish to pass.

The average flow required to maintain a constant Mediterranean level is be expected to vary from year to year, and on the longer term as climate changes. A suitable flow control system needs to be designed, probably using gates that can be raised and lowered. Only the long-term average flow is important for maintaining sea level, so that a rapid response in flow control at the dam is not required. Any system for siphoning or pumping out deep salty water allows for additional control. The height control systems need to be secure enough to prevent disasters due to a breach in the dam. Additional reinforcements may need to be added as the height difference across the dam increases.

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