Landslide: what really caused the Vajont dam disaster?

On the 9th of October 1963, just over 50 years ago, a huge landslide caused tonnes of rock and debris to fall into the Vajont reservoir creating a tsunami 200 meters tall which destroyed many villages in the Piave valley. the initial cause of this disaster, which was responsible for the deaths of 2000 people, seems to be a landslide, a natural disaster that mankind could not to anything to prevent, however, there are many reasons to think that this landslide was due to human error more than it was to nature.

The Vajont Dam as seen from Longarone today, s...

The Vajont Dam as seen from Longarone today, showing approximately the top 60-70 metres of concrete. (Photo credit: Wikipedia)

One reason for this is the major error in assuming that no landslides large enough to cause a tsunami would occur in the area. during the dam’s construction there were concerns that the left bank was unstable, so this was investigated and it was found that there had been large landslides in the past on that slope before. however, this idea was dismissed as further investigations including 3 bore hole drilling’s and seismic investigations didn’t find any signs of weakness and apparently proved that the slope was made up of  firm, stable rock that was unlikely to cause a major landslide. Ignoring even the smallest possibility that a landslide could be possible was a major flaw in the construction of the dam, and therefore could have contributed to disaster.

However, a few years later in 1960, a landslide that deposited 700,000 cubic meters of material into the reservoir, made the dam’s engineers realise that the banks were in fact unstable and that the previous suggestion of this was therefore correct. however, the banks were far too large for any artificial reinforcement to occur so engineers decided they would try to stabilise the landslide by lowering it into the lake gradually, thus reducing the size of the possible tsunami. they tried this by repeatedly raising and lowering the water levels in the reservoir, as they had previously found that when the water levels were higher the rate of the landslide movement  increased. this idea did seem to work for a while, and the landslide became more stable, but during one of the last sessions of raising and lowering the water levels the rate of the landslides movement sped up from 3 cm a day to 30 cm a day. It has recently been found that a thin layer of clay exists underneath the limestone slope. this was not discovered in the investigations before the dam was constructed as the bore holes didn’t drill deep enough. a fourth bore hole at the time did seem to show the result of the presence of a lay layer but because none of the other 3 bore holes found this, the result was written off as a technical error. therefore the engineers had no idea that raising and lowering the water levels wouldn’t make any difference, in fact it could have increased the rate of movement, as clay, which is capable of holding large amounts of water, meant that the friction between the limestone layer and the rest of the slope, decreased; allowing the limestone layer to move a lot faster than expected. If the engineers had discovered the presence of the clay, and not ignored the so called outlying result that stated the presence of a clay layer, then the outcome may have been a lot different. the landslide was not expected to increase its rate of movement and fall at such speed. If it was then many lives could have been saved, even if the landslide itself could not have been prevented.

Some aspects of human error that may have contributed to the landslide could have been due to the time in which this construction took place, as levels of technology were not well enough advanced to show accurate findings. when engineers knew that a tsunami could have been a possibility, they conducted a scale simulation to find out if the dam could hold back a tsunami wave if one did occur. they investigated this by changing the speeds at which a possible landslide could occur and observing whether the dam could hold back the height of the wave that these speeds could produce. however as this was not a computer simulation they could only estimate how long the landslide would take to fall, which they estimated as one minute, when in reality the landslide fell in 45 seconds which was enough to cause a much larger wave than predicted, and one that the dam couldn’t hold back. It was thought that if a computer simulation was possible at the time then a correct estimation of the size of wave would have been produced, allowing the engineers to lower the reservoir levels accordingly.

The Vajont dam disaster therefore was largely due to human errors, despite having an apparent natural cause, and if some of these errors could have been prevented, then although the landslide would have been likely to still occur, preventions could have been put in place such as evacuation, that would have saved the lives of those 2000 villagers below the dam. The volume of debris from the landslide was enough to destroy the reservoir, although ironically the dam itself still survives, with little damage.

Sources

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One comment on “Landslide: what really caused the Vajont dam disaster?

  1. Pingback: Are the causes of natural disasters always natural? | Astronomy, geography and more

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