How long are dams like Hoover Dam engineered to last?

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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?

— Craig S., Jacksonville, Florida

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.

— Cecil Adams

http://www.straightdope.com/columns/read/2666/how-long-are-dams-like-hoover-dam-engineered-to-last

万得福

How Long Will The Hoover Dam Last?
http://zidbits.com/2013/05/how-long-will-the-hoover-dam-last/

Hoover Dam began operations in 1936. It impounds one of the largest reservoirs (by volume) in the United States. Originally called “Boulder Dam”, it was renamed Hoover Dam after congress voted to make the name official in 1947.

Even today, 75 years after it was built, it remains one of the largest, most reliable hydroelectric generators in the US. Something like that can’t last forever, can it? Does the Hoover Dam have a set lifespan or expiration date?

3 MILLION HORSES

Hoover Dam provides power to Nevada, Arizona and large swathes of California.turbines In 2011, president Obama signed a bill extending the previous supply contracts to the year 2067. He also set aside 5% of its power to be made available for purchase by Native American tribes and electric cooperatives.

53.5% of its total output goes to California, with 15.4% of that number going to Los Angeles alone. Arizona uses 18.9% while 23.3% is allocated to Nevada.

So how much total power does Hoover Dam supply to its customers annually? A whopping 4.2 billion kWh of electricity every year. It achieves this through its efficient turbines which have a rated capacity of 2,998,000 horsepower. There are a total of 17 main turbines which were replaced and upgraded with more efficient and powerful units back in the late 80s. There are also 2 smaller turbines which exist to provide the dam itself with electricity.

MOLLUSKS END THE POWER HOUR

Without continual maintenance or a human presence, the dam would stop producing power after the hydroelectric system and/or turbines stopped functioning. Speculation by several engineers who work at the dam believe that this could take anywhere from a few months to a few years.Hoover Dam

Several things can cause the system to malfunction, with the most likely candidate being the quagga mussel. It has no natural predators in North America and workers must routinely scrape them out of the pipes and grates they like to colonize.

With humans gone, their unchecked growth would cause blockage in the cooling pipes which would lead to overheating of the turbines. When the turbines overheat, the system automatically shuts down the offending turbine to prevent a catastrophic failure of the hydroelectric system. No turbines means no power.

BUILT TO LAST

The dam itself was built to last. It was built to be as tough as the canyon walls which surround it. A total of 3,250,000 cubic yards of concrete was poured to create the dam – it’s enough to build a monument 100 feet square and 2 1/2 miles high.hoover dam construction

One common misconception is that the dam was built with a single, continuous concrete pour. Since concrete contracts & heats up as it cures, if it was all poured at once, it would have taken 125 years for the concrete to cool off. Such a long cure time would have also created a weaker dam, one highly susceptible to stress and microcracks.

Instead, the concrete was poured in separate sections which took only 2 years to complete. Rectangle shaped sections were marked off and concrete was poured into each, creating individual blocks. The separate blocks had their curing process sped up due to ice-cold water that was refrigerated then pumped in through embedded cooling pipes which helped the concrete cure evenly.

ALL GOOD THINGS…

Despite concrete’s tenacity, a lack of maintenance will eventually cause the dam to crumble. However, speculation puts the time at which this happens in the thousands of years, several reports put it at 10,000+ years. dam wallIf humans were to vanish of the face of the planet, the Hoover Dam would be one of the last remaining visible and recognizable monuments of our species on the planet.

With the turbines non-functional, the Colorado River dries up as water no longer passes through the dam. The dam’s reservoir, Lake Mead, slowly rises until the water finds a way over, around, under or through the dam. Wind, water, and possibly earthquakes will take their toll and even the massive wall of concrete will wear down and erode, inevitably succumbing to the elements.

Fun Fact: Many pieces of unique art decorate the dam. One such piece at the base of the dam, set in a terrazzo floor, is an inlaid star chart designed by Oskar J.W. Hansen. The apparent magnitudes of stars on the chart are shown as they would appear to the naked eye at a distance of about 190 trillion miles from earth.

The planetary bodies are placed in a way that someone could calculate the precession of the Pole Star for the next 14,000 years accurately. It’s also accurate enough that future generations could look upon this chart and determine the exact date on which Hoover Dam was completed.


References:
Denton, Sally. “Hoover’s Promise: The Dam That Remade The American West”, American Heritage’s Invention & Technology
Bureau of Reclamation (2006). Reclamation: Managing Water in the West: Hoover Dam. US Department of the Interior.
“Hoover Dam“. National Historic Landmark summary listing. National Park Service.
Rhinehart, Julian (September 10, 2004). “The Grand Dam“. Bureau of Reclamation.