How long are dams like Hoover Dam engineered to last? What's the largest dam ever to fail?

August 11, 2006

Dear Cecil:

How long are these huge dams, like the Hoover Dam, engineered to last? How would we go about replacing it if it began to weaken or became obsolete? What's the largest dam ever to fail?

Don't sweat the big dams, chum. It's the little ones you should worry about. Most of the small and medium-sized dams in this country were built to last just 50 years. (Nowadays, from what structural engineers tell me, the typical design life is more like 100 years.) According to the U.S. Army Corps of Engineers, about 30 percent of the more than 76,000 dams in the United States are older than 50 years--and by 2020, that number will increase to more than 80 percent. That's a lot of old dams, some of which hold back not just water but toxic sediments from early industrial operations. Once they start to go, a lot of people are going to be in deep . . . OK, maybe not raw sewage. But some mighty unpleasant stuff.

You asked about huge dams, though. Just so we can put this on a scientific basis, the International Commission on Large Dams (ICOLD) defines a large dam as one more than 15 meters tall or impounding (i.e., holding back) more than a million cubic meters of water; there are more than 39,000 such dams worldwide. Large dams for obvious reasons are designed to last a lot longer than the small-to-medium kind. According to all published sources I could find and the engineers I spoke to, megastructures such as the Hoover Dam are designed to last indefinitely provided they're properly maintained. The chief construction supervisor of the giant Three Gorges Dam on the Yangtze River in central China--though not yet operational, its recently completed main wall is five times the size of the Hoover's--reportedly said in 1999 that it was "difficult to predict" how long the structure might last, but his team was "endeavouring to assure" that it would hold forever.

You're thinking: Even God doesn't build things to last forever. Notwithstanding press-release claims that the Three Gorges engineers learned "all they could about concrete" in their effort to make the dam extraordinarily durable (one would certainly hope so), let's not expect miracles. Of the large dams tracked by ICOLD, at least 300 have experienced accidents. These weren't all catastrophic failures, but if you're an engineer in charge of building a big dam, you definitely want to give some thought to what can go wrong.

Plenty of bad things can happen to a dam, not all of which involve a collapse. The spillways and pipes may erode, the sluice gates and valves may fail, or, if it's a hydroelectric dam, the powerhouse equipment may (excuse me, will) wear out. The reservoir may silt up--some predict the Three Gorges Dam will become choked with silt in just 50 years. (Lake Mead, formed by the Hoover Dam, lost about 15 percent of its capacity between 1936 and 1964 due to silting--more than five million acre-feet.)

I'm guessing, however, that you're more worried about spectacular disasters than too much mud. Suppose you stroll out on the Hoover Dam and notice--goodness, is that a crack? Repairing a dam wall is tricky. With a small dam the water can sometimes be drained, allowing repairs to be done safely. But imagine trying to drain the Hoover Dam to repair cracks at the bottom--it can't be done. Larger dams are usually drained part way to make repairs easier, but with the really big ones even that's a major undertaking. Divers are sometimes used to inspect dams, but it's not practical or safe to have them attempt major structural repairs. If a smaller dam can't be repaired and poses a risk of failure, it's torn down as soon as funding and resources are available. When the Hoover or Three Gorges gets to be a menace . . . well, we'll cross that dam when we get to it.

Disasters can happen even if the dam doesn't break. The towering 860-foot Vajont Dam in Italy was responsible for approximately 2,000 dead in a 1963 accident--but the structure itself suffered only minor damage. A huge rockfall into the reservoir sent an enormous splash of water over the top in a wave more than 80 stories high, sweeping downstream and wiping out several villages. Still, the mother of all accidents in terms of scale and destruction was surely the 1975 failure of the Banqiao Dam in China (capacity 492 million cubic meters), which was just one of 62 dams destroyed (some intentionally) in a concatenating horror show, the result of record-breaking rains, that left millions homeless and more than 25,000 dead.

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