I somehow have managed to live this long without learning about the Atlantropa proposal to dam the Strait of Gibraltar. Every description of the idea starts with the appeal of a tremendous electric power source. As demonstrated by the Lago Mare event, the Mediterranean operates as a net water sink, due to low rainfall and high insolation. Water flowing through the Strait of Gibraltar (and the Bosporus, Nile, etc.) are required in order to prevent the sea level from falling rapidly.
The strait of Gibraltar is only 14 kilometers across at its narrowest point, and about 300 meters deep on average. If the dam were largely built by simply dumping sand from the growing peninsula at its angle of repose, about 1.25 cubic kilometers of fill would be required, or about twice the volume of the largest Syncrude Tailings Dam. It would also be 10 times the material required by the Tarbela Dam in Pakistan, which cost $1.5 billion to build and generates 3.5 GW of electricity.
How much electricity would such a dam generate? I couldn’t find the answer anywhere on the net. I did find this uncited claim that a recent high school physics exam in Germany asked students to calculate the available wattage.
Well, I think I remember high school physics. Let’s give it a try.
According to this study the average net water deficit in the Mediterranean is between 500 and 700 mm per year. The surface area is 2.5 million square kilometers. That’s 1500 cubic kilometers per year, or 48,000 cubic meters per second. As a mass flow, we’re talking about 50,000 tons of seawater per second. Multiply by g=9.8 m/s^2, and you get 490 megawatts per meter. (The electric output is not much lower, because turbines run at >90% efficiency.)
What’s this per-meter business? Well, a hydroelectric dam requires a water level drop from one side to the other. If we want to make power, we can’t just dam the sea … we also have to let the sea level fall. To achieve the same, presumably profitable, ratio of fill volume to power output as the Tarbela dam, we would have to let sea level fall by 72 meters, generating 35 GW of power, or 300 TWh per year … about the total electricity consumption of Italy.
This is a little disappointing, for such an ambitious geoengineering project. The original Atlantropa proposal goes further, lowering sea level by 100 or even 200 meters, thus generating up to 1000 TWh per year … and substantially altering the coastlines. In 1920 I’m sure that was an unfathomable amount of power, but today it’s only a quarter of European electricity consumption … not even enough to entirely displace fossil fuel for electricity generation there.
However, if we cannot eliminate greenhouse gas emissions, there might be one other reason to build such a dam: rising sea levels. The Mediterranean coastline has seen substantial investment in waterfront property over the past, say, 3000 years. Those investments are threatened by the predicted rise of 0.5 to 2.0 meters over the next 100 years. If the construction cost is $15 billion (in proportion to Tarbela), perhaps existing property owners would be well advised to cover the cost of such a dam, and the few hundred megawatts of electric power generated would merely be icing on the cake.