dam safety

In general, there are 100 ways dams can fail.

Of the more than 80,000 dams in the U.S., about a third pose a “high” or “significant” hazard to life and property… 27,000 sites in Potential Failure Mode.

A number of the things that we’re finding today are problems that occurred during design and construction 40 and 50 years ago.

I think risk assessments, at least in North America, are moving more and more toward probabilistic approaches to risk assessment. In the earlier days, we began looking at probabilistic hydrology issues. Nowadays, our seismic hazards are determined by probabilistic seismic hazard analyses. One of these days, we may put all of this together in some sort of logic tree and come up with probabilistic failure of the dam by overtopping, by seismic, by liquefaction and a number of other different factors.

The professionals are on it:

Q. What are the chief causes of dam failures in North America?

Paul: Overtopping.

Brian: Overtopping, seepage.

Dan: Statistically, I thought it was seepage.

Brian: I think overtopping is the highest.

Gus: I think overtopping is the high and seepage and piping is probably second.

Paul: Overtopping is something that’s avoidable. You can perform the necessary calculations and develop a “fix” to prevent it.

Gus: Overtopping might occur because the spillway was inoperable. That’s the type of thing that causes dams to overtop in general, other than extreme floods.

Warren: The chief causes are more programmatic in that whatever technically caused it, it probably had something to do with a lack of maintenance or lack of an oversight program. Some of that gets back into funding.

How many industrial sludge ponds might fail?
What are the expected ecological impacts and decontamination strategies?

How does the sludge get produced and how could it escape?

When aluminium is extracted from bauxite via the so-called Bayer process, red sludge forms as a by-product. The sludge is in large reservoirs of mud and water.

What caused the accident is yet unclear, but it is likely that heavy rain has caused the dam containing the reservoir to break.

It is also possible that the reservoir was just not large or strong enough to hold the sludge it was filled with.

What is the chemical composition of the sludge?

It contains mainly fluoride, sulphate and aluminate, but also chrome, nickel, manganese and heavy metals such as lead. Its arsenic concentration is at least a hundred times above the allowed threshold for drinking water.

How might contact with the toxic sludge affect human health?

The most dangerous thing about the stuff is that it is – to put it simply – a dilute solution of sodium hydroxide with an extremely alkaline pH value, between 11 and 12.

This means it cauterizes eyes and skin and attacks the lung when inhaled.