Wednesday, September 24, 2008
BIG ENGINEERING 9 - GREENING THE SAHARA
The Lualaba River is the major tributary to the Congo. It runs from the African Rift Valley lakes around Zaire's border, indeed forming the border with the Central African Republic. I have not been able to find figures for how much water it carries but I would be surprised if it was less than 10% of the Congo's total of the 41,800 m3/s of the whole.
Separated from it by about 300km of mountains is the drainage basin of Lake Chad which has been drying up since explorers first got there over a century ago, indeed a lot longer.
I propose a tunnel from the point where it forms the border with the CAR to that catchment area. If the Norwegians have been cutting tunnels at £7 million a km, including actually putting a road in, then the theoretical cost, assuming 2 tunnels connecting or one twice the size of any road tunnel would be about £4 billion. It might also be possible to produce hydro-electricity but if so this would be a bonus. If all but a token amount of the water were diverted this way it would still be more than 3 times the 2,830 m3/sec of the Nile. Incidentally 4,100 m3/sec would cover a square kilometre by one meter in 4 minutes.
Chad was not always the parched place it is now, nor was the Sahara. In Roman times North Africa was the breadbasket of Rome & Emperors were much more worried about losing the lifeline to what is now the Sahara than they were about Britain.
This is the Sahara during the last interglacial between about 5,000 & 10,000 BC with a covering article.
Lake Chad is at an elevation of 280m. The smaller lake in what is now Algeria is at a much lower level & though it would require tunnels & canals of about 1,000km but this is certainly feasible, comparable with Australia's Snowy Mountain Scheme started in 1949. I don't know the exact elevation of the Libyan lake but it is closer to that of Chad & might be more difficult. To the north of the main Algerian lake the area of the lake adjoining the Mediterranean is now a basin below sea level which would be filled before the Lualaba reached the Mediterranean.
Even at the rate of a km to the depth of 1m every 4 minutes (131,000 a year or a circle of 400km diameter) I think it would take at least several decades to fill all these lakes (the Algerian one might be quite deep) & with the amount that would be absorbed by soil & evaporation I would not even hazard a guess when it would actually flow into the Mediterranean. However any water that is lost to soil will still help plants & almost all lost to evaporation will return as dew & eventually rain within the Sahara & often within the basin feeding the lake it came from. In prehistory the Sahara contained all sorts of wildlife, including Hippopotamuses. We have cave paintings from areas that are now desert showing it. Putting that much water into the Sahara, with virtually none leaving it, could hardly fail to bring life.
On comparable costs it should all be under £10 billion. A lesser scheme taking water from higher up the river & moving it by canal has been proposed & seems to have had environmentalist approval.. It may well be that the present situation of the Sahara is partly because of Man, or more directly of the sheep & goats we introduced. That being so we should expect genuine environmentalists who claim to believe mankind has made a mess of the Earth to press for this enthusiastically. Those who are Luddites under false colours will oppose it or nominally support it while inventing endless difficulties. And it will be difficult - things worth doing usually are.
Following a comment on a later proposal I have realised that the amount of water coming through the Chad Mountains would require about 6 or more such tunnels running in parallel or perhaps 2 with double the diameter (ie 4 times the cross section). Even with economies of scale that increases the cost, however my estimate of Norwegian tunnelling costs at the time is about twice what they have managed quite often. So probably a bit more expensive than the £4 bn estimated but still cost is not a significant problem - politics is.
I suppose accepting you can't do things just because they are, probably, physically possible is one of the signs that you are entering adulthood.
After all it took Roman engineers to build aqueducts acros France, the natives being too uncivilised to manage it themselves.