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Dam Failures

Dam failures are of particular concern because the failure of a large dam has the potential to cause more death and destruction than the failure of any other man-made structure. This is because of the destructive power of the flood wave that would be released by the sudden collapse of a large dam. Tailing dams, which sometimes store toxic materials, may pose additional dangers eg Omai Tailings Dam, Guyana failed in 1995 releasing cyanide slurries and Stava Tailings Dam, Italy failed in 1985 killing 268 people. Many dams, both large and small, have failed but only a few have had a significant impact on the practice of dam design and engineering geology. The most common causes of dam failures are:

Overtopping of embankment dams due to inadequate spillway discharge capacity to pass flood waters. This is one of the most common causes of dam failures and has nothing to do with the geology of the dam site. Any embankment dam will fail if the spillway is too small and flood waters rise high enough to flow over the top of the dam wall. The estimation of the size of the maximum flood a dam will have to survive during its life is a science which has undergone continuing evolution over the last century with the result that many dams built decades ago may now be judged to have inadequate spillways even though the spillways were designed to standards of safety which were accepted as adequate at the time of construction of the dam. Many millions of dollars has been spent upgrading the flood handling capacities of many existing dams, both embankment and concrete dams, as a result.

The Burrinjuck Dam Flood Security Upgrading project is a recent example.

Faults in construction methods (eg inadequate compaction of fill) or use of the wrong type of construction materials (eg silt) may lead to internal erosion or piping failures of embankment dams. An example is the failure of the Teton Dam in Idaho, USA in 1976.

Geological problems with the dam foundation. The failure of the St. Francis Dam falls into this category. After the failure it was found that some of the foundation rock, a conglomerate, disintegrated when the rock was immersed in water so that the rock lost all its strength when saturated. This is exactly what happened as the newly completed dam filled with water for the first time and the dam failed shortly afterwards. Another example of a dam break due to foundation failure is the Malpasset Dam in France which failed in 1959. This was the first collapse of a modern, thin concrete arch dam.

Landslides which fall into the storage reservoir, sending a wave of water over the top of the dam may cause a dam to fail, or the dam may survive if made of concrete but a destructive flood may still devastate the river valley downstream as happened at the Vaiont Dam in Italy in 1963 when over 1900 people were killed.

Earthquakes can certainly cause damage to dams but complete failure of a large dam due to earthquake damage appears to be very rare. The Lower San Fernando Dam in California, USA did fail during an earthquake in 1971 which caused the fill in the dam wall to liquefy resulting in the collapse of the upstream part of the dam. A disastrous flood was only prevented because the reservoir level happened to be low at the time of the earthquake and no water escaped downstream.

Dams are likely to exist, perhaps for hundreds of years, even after they are no longer required for their original purpose. During these years, dangerous alterations to the operation of the dam and/or its structure may lead to failure eg South Fork Dam (Johnstown) which failed in 1889. Incorrect operation of a dam at any time can result in overtopping and failure eg Euclides da Cunha Dam, Brazil which failed in 1977.

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