Sunday 20 October 2013

Vajont October 1963

You know those Mensa IQ puzzles where they ask for the next couple of terms in a series: 1, 4, 9, 25, 49, __, __, or L, M, M, J, V, __, __, ?  Well what about Botox, Sarin, propylene, beer, __. ?

You will have noticed a tendency to track anniversaries on The Blob.  It's a way of limiting the choice of things to think/write about from the infinite to the dayinit. I'm interested in events at the intersection between the natural world and the folly, hubris or even malice of people. There were good reasons why Italians wanted to develop hydro-electric power in their Alps as their economy started to show green shoots after the debacle of WWII. There was pressure (political, economic, social) behind a plan to build one of the tallest dams in the world at the head of a modest valley carved by a tributary of the Fiume Piave.  The CEO of the local electricity monopoly had been Mussolini's Minister of Finance, so he was well-connected, and interested in money.

The dam was first considered in the 1920s, authorized in 1943, but construction didn't start until 1956.  They had, of course, done a geological survey to determine whether the walls of the valley were strong enough to hold this huge 260m high wall of concrete upright and some, more cursory, investigation had revealed some evidence of ancient land-slips further up the valley.  Construction was authorized and pouring concrete started.  Several outsiders sounded off about the instability of Monte Toc which loomed over the valley that was to be filled with water.  After the dam was built, the sluices were shut and the valley started to fill with water.  Some time later a farmer noticed that a 1700m long crevasse had opened up roughly parallel to the valley on a shoulder of Monte Toc.  The ever-increasing width of this crack suggested that a very large chunk of the local world was thinking about taking a plunge in the lake.  This was worrying but the company had invested a lot in the project and accordingly cast about for more data and solutions to probable future scenarios. With 20/20 hindsight, two significant scientific gaffes were committed.

The first error was to treat anecdote as data. They noted (N=1) that when the water level increased the crack had started to open. When they pulled the plug (N= -1?) and let out some water then the creep stopped. They therefore believed that the water level in the lake was driving any local land-slippage. The second error was to assume that one anomalous value was wrong rather than that it was the key to a new and much better paradigm of what was happening under-ground.  They had drilled four boreholes deep beneath the limestone surface to see if the solid rock went all-the-way-down or was resting on something far less stable.  Three of the holes' piezometers said "solid" and the fourth, which they put down to faulty instrumentation, said "wobbly".

Regardless of these data, the company was determined to filler 'er up and start to generate some return on their investment.  Any rock slide would just have to be managed: if and when it happened enough redundant safety features would have been installed, so that nobody would be hurt.  The company's engineers built a scale model of the valley with a mini-dam and dumped loads of gravel over the edge and measured what the resulting mini-wave got up to.  They concluded that biggest possible landslip would result in a tsunami 20m high.  So they drew a Plimsoll line 25m (25% extra to be sure to be sure) below the top the dam for the maximum safe height.  They then filled up the lake again and waited for what would happen.

The anomalous piezometer (20/20 hindsight here) showed that the whole huge slab of limestone and its attendant topsoil, trees and foxes was separated from the mountain by a layer of clay. When this failed it allowed a near frictionless slide downhill, so the mass traveled far faster and was far more massive than the simulations had considered possible.  Indeed there was more stuff (260 million cu.m) going downhill at 100km/h than there was water in the reservoir.  It was like dropping a walrus from a great height into a bath-tub. Instead of a wave 25m high, the mountainside slopped 50 million tons of water over the edge of the dam in a wall 250m high which wiped 4 villages and 2000 people off the face of the earth in a couple of minutes.  It's reminiscent of Lake Nyos.  The dam survived with only superficial damage and is still there to this day but they never filled the reservoir after the 9th of October.  It's all rather well covered in a National Geographic documentary.

One set of answers to the questions posed at the head of the post:
1,4, 9.25,49,121,169 (squares of primes)
Lundi, Mardi, Mercredi, Jeudi, Vendredi, Samedi, Dimanche (French weekdays)
150ng, 150mg, 150kg, 150t, water=25,000t (quantities that can kill a person)

1 comment:

  1. ahh experts. I remember the engineers, politicians and business men sneering at the anecdotal warnings of the older fishermen in our local in the 1990's who warned that the construction groynes (walls) of thousands of tones of rock would cause huge siltation at Cheekpoint and further downriver. Nonsense says they, the works will remove the massive sand bar at Cheekpoint. There by make it possible to get the size of bulk carrier required of modern ports into berth and hence to make a profit. After all, the models of the work, done by the most capable of dutch marine experts, showed that it would work...did it hell...mind you no loss of life, just livlihood