I grew up when dams were things of beauty and importance. We used to visit the Cotter Dam outside Canberra for picnics, and marvel at such a wall holding back such a lot of water. That dam wall has since disappeared behind a much larger one built immediately in front of it. In my teens I began to hear of proposed dams in New South Wales that would prevent floods, like the great and most destructive floods of 1955 in the Hunter and of 1956 in the Murray-Darling system. Some of the Snowy dams came to completion in the same decade and later. Bert Kelly was able to say, a decade later still, that ‘people are talking about building dams again — I feel an election coming on!’
In time universities and colleges of advanced education replaced dams as the great local electoral offering, and when they had passed their use-by date it became fashionable to argue that dams were bad, destructive of the environment, all the birds and animals would lose their habitat or lives, floods are part of Nature’s plan, and so on.
I’m still rather partial to them, nonetheless. And, knowing that 1 mm of rain over 1 square metre produces 1 litre of water, I am fascinated by the way in which they fill after drought. The woman on duty at Wyangala Dam, near Cowra, told us that her dam would fill in a week with decent rain. At the time it was virtually empty, with only about 5 per cent, and you could see an astonishing difference between the low- and high-water levels. Mind you, the catchment area of Wyangala is 8,300 square kilometres.
By and large, we in Australia have stopped building dams either for hydro power, for irrigation or for flood prevention. In China, however, things are very different. Judith Curry’s Climate etc website had a fascinating piece a month ago about what is happening in the wider world, especially China. I keep having to remember that in population terms China is around 50 times larger than Australia, and that scale difference allows for aspirations and visions that are really beyond our experience. And China is growing rapidly wealthier.
It appears that since the 1950s the Chinese have built around 22,000 dams that are larger in height than 15 metres; that’s about half the world’s total. Now they are planning or building around 100 mega-dams just on the Yangtze and its tributaries, plus a couple of dozen on what the rest of the world calls the Mekong River, and a few more on other rivers in the same southwest region. The government declares that these dams are safe, avoid pollution, guard against climate change and enhance human life.
According to the article from which these figures come, these claims all need assessment. Setting aside the blocking of the flow of rivers (this really is one for cost-benefit analysis, since one reason for building dams is exactly to be able to block the flow) the really interesting attack for me is in the notion that the sheer size and mass of these dams has the capacity to disturb the surrounding rock.
An earthquake occurred in Wenchuan in May 2008 that killed 80,000 people; the earthquake was a little way downstream from a new dam, and it is now argued that the building and completion of the reservoir triggered small quakes through the fault system, which culminated in the large earthquake. The phenomenon is called ‘reservoir-induced seismicity’. The earthquake caused cracks in a few hundred other reservoir walls, and each of them had to be examined carefully to ensure that there was no further danger.
Isn’t hydro power good, though? We are always being told that it is renewable (= good). Well, maybe in part. But The rotting of inundated trees and vegetation in reservoirs emits the greenhouse gasses, carbon dioxide and methane, that rise from reservoir surfaces. Over a projected lifetime of a dam in temperate regions, emissions could be from roughly one-third to nearly two-thirds that of a natural gas plant.
And the article goes on: Nor do big dams protect from floods and droughts. They store water during the wet season and release it during the dry season, thus reversing the natural flow of rivers. Deprived of their annual inundations, downstream marshes, lakes, and wetlands dry out and can no longer absorb floodwaters. During the record-breaking summer flood of 2010, the Three Gorges reservoir rose to 12 meters above “alarm level.” To protect the dam, its operators opened the floodgates to the maximum. Downstream some 968 people were killed, 507 more were missing and economic losses totaled $26 billion.
All in all, at least according to this article, dams are a mixed blessing for China. In fact, the author hasn’t much to say in their favour at all. But I would like an open mind about their value in Australia. We do have destructive floods, but we haven’t had anything like the 1956 one in the Murray-Darling basin since, and one reason should be that we have a series of flood-prevention dams along the Great Dividing Range, and a lot of storage basins along the Darling, such as Menindee Lakes, which I have seen both bone-dry and full to the brim.
Memory tells me that NSW Governments have given up on damming the Shoalhaven, which would provide a decent extra water-supply for Sydney and Wollongong, because the Greens would oppose it tooth and nail. I wonder whether the Greens put dams or nuclear power closer to the bottom of their priorities.