I am looking to write a story where part of the Atlantropa project goes through. Now I know about all the issues with it: the soil is covered in salt and unusable, it raises sea level in other places, makes existing coastal cities landlocked…

But my question is different. I am wondering if it would be possible to achieve some of the effects of the planned project (i.e. lower the Mediterranean by several meters) by placing a dam between the Mediterranean and the Black Sea, but not at Gibraltar (on account that it is too big of a dam to build — in my scenario they begin building it but never manage to finish it).

I guess it's a difficult question to answer, but some people might know more about the hydrology of the Mediterranean than I do.

Thanks a lot for the help!


According to the information I can find, the net contribution of the Black Sea to the Mediterranean sea is about 5000 cubic meter per second

The Black Sea experiences water transfer only with the Mediterranean Sea, so all inflow and outflow occurs in the Bosporus and Dardanelles. Inflow from the Mediterranean has a higher salinity and density than the outflow, creating the classic estuarine circulation. This means that the inflow of dense water from the Mediterranean occurs at the bottom of the basin while the outflow of fresher Black Sea surface-water into the Marmara Sea occurs near the surface. The outflow is 16,000 m3/s (570,000 cu ft/s) (around 500 km3/a or 120 cubic miles per year) and the inflow is 11,000 m3/s (390,000 cu ft/s) (around 350 km3/a or 84 cubic miles per year), according to Gregg (2002).

On the other hand, the net flow through Gibraltar strait amount to 0.038 Sv, with 1 Sv being 1 million cubic meter per second, thus 38000 m3/s.

The net flow through the strait, estimated from the Mediterranean water budget, and the Mediterranean outflow, estimated from currentmeter observations in Espartel sill (western Strait of Gibraltar) from October 2004 to January 2009, made it possible to estimate the Atlantic inflow as the sum of both of them. The obtained mean net flow is 0.038 ± 0.007 Sv, with a seasonal cycle of 0.042 ± 0.018 Sv annual amplitude and maximum in September. The Mediterranean outflow shows a seasonal signal with annual amplitude of 0.027 ± 0.015 Sv peaking in April (in absolute value), and a mean value of −0.78 ± 0.05 Sv. The resulting Atlantic inflow has a mean value of 0.81 ± 0.06 Sv and a seasonal cycle with annual amplitude of 0.034 ± 0.011 Sv, peaking in September, and high interannual variability.

I don't think that damming the Black sea alone will make a dent in the Mediterranean sea level: the same paper I cited above mentions that the average Atlantic inflow is 0.81 Sv, and that means that the flow through Gibraltar can easily accommodate the missing inflow from the Black Sea, simply by reducing or nullifying the Mediterranean outflow.

  • 2
    $\begingroup$ It's as I feared then. Thanks a lot for the explanation of the data. $\endgroup$
    – Nierninwa
    Nov 15 '21 at 15:06
  • 1
    $\begingroup$ If I read correctly it would - in the long run - decrease the salinity of the Black Sea however $\endgroup$
    – Hobbamok
    Nov 16 '21 at 9:56

North Anatolian Fault passes straight under the ... umm... strait of Bosphorus.

And the fault is quite active based on the list of earthquakes listed by Wikipedia for the last 81 years. And it looks like the Anatolian plate is drifting relative to the Eurasian plate at a rate of about 30 mm/year

Now, a dam over Bosphorus will slowly raise the level of the Black Sea and cause just a wee more water to infiltrate and lubricate the fault. Me thinks that is bound to accelerate a bit the fault movement. 50 years at, say, 50mm/y spells real trouble for that dam.

I wouldn't want to be around when a reverse Black Sea deluge happens. because Bosphorus will get deeper and the outburst flood will get to take some of the choked full of hydrogen sulfide mud in the Black Sea.

(image from Wikipedia, By Mikenorton - Own work, CC BY-SA 3.0)

enter image description here

  • $\begingroup$ Oh the plan is to have the dam break after barely 40 years anyways. I'm thinking a well-placed nuke can do the trick. $\endgroup$
    – Nierninwa
    Nov 15 '21 at 15:05
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    $\begingroup$ Mmmm... I see. So will Russians,who will be unhappy to lose access to the Mediterranean Sea and have their fertile plains flooded with sea water. Methinks the nuke will happen earlier and the missile caring it will be marked in Cyrillic alphabet. $\endgroup$ Nov 15 '21 at 15:17

It is very obviously impossible to lower the Mediterranean to any significant degree as long as it is connected to the ocean. Communicating vessels etc.

Yes, stopping the inflow of water from the Black Sea will lower the eastern Mediterranean a little. (The Mediterranean has a water deficit, and average level of the sea decreases from west to east.) As it is, the eastern Med is about 20 cm lower than the Red Sea on the average; stopping the flow from the Black Sea will make it maybe 25 cm lower than the Red Sea on the average.

Fun fact: one century ago the eastern Med was about 50 cm below the Red Sea on the average, and two centuries ago it was 70 cm below the Red Sea. Nowadays, from July to December the eastern Mediterranean is actually some 10 cm above the Red Sea! (Warning! The link goes to ScienceDirect. Better open it in a private browsing window.)


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