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dam-l The Guardian, London "Villages of the Damned"
The Guardian, London, Thursday April 27, 2000
Villages of the damned
Christopher Kilburn on the
hazards of a world carried away by vast
engineering projects
The 1990s have seen a blooming
of dam controversies. In south east Turkey,
20,000 people are under threat
from reservoir flooding by the Ilisu dam
project. In northwest India,
the Sardar Sarovar dam will displace hundreds
of thousands from their homes
in the Narmada river valley. Through
Vietnam, Laos and Cambodia,
there are plans for 15 dams along the Mekong
that could disrupt several million.
Most staggering of all is
China's Three Gorges dam across the Yangtse River.
Due for completion in 2009, the
dam will stretch for 2.3 kilometres, stand
more than 180 metres high and,
with its 26 turbines, generate the energy
of nearly 20 nuclear power
stations. It will be the world's largest dam and
will hold back a reservoir
600km long and more than a kilometre wide. At a
cost of $30bn, the spur for the
Three Gorges dam is a mixture of community
protection, economic
development and national prestige.
The Yangtse regularly floods
its banks and major overflows, such as in
1954, have killed tens of
thousands and left millions homeless. The
project's ambitions are to
prevent such floods and to provide much-needed
energy to China's rapidly
industrialising south. Creation of the reservoir,
however, will definitely flood
the homes of two million people and disrupt
water supplies to the natural
floodplains on which 75 million others depend
for fishing and farming.
The debate about Three Gorges
is a gargantuan version of events 40 years ago
in north eastern Italy.
Exhausted by the second world war and eager to
develop its only natural energy
resource, the Italian government had
sponsored a massive expansion
of hydroelectric power schemes in the Alpine
foothills north of Venice.
Centrepiece of the programme
was the Vajont dam which, nearly 265m
high, was in 1960 the highest
double-arch dam in the world and a tribute to
advanced engineering design.
The plan was to transform the tiny Vajont
river valley into a reservoir
five kilometres long, at the cost of hundreds of
homes. Reservoir filling began
in February 1960.
Almost immediately small
movements were recorded on Mount Toc, just
upstream from the dam on the
valley's northern flank. Such movements
were at first considered normal
signs of the flanks adjusting to the weight of
water in the reservoir.
In October, with the reservoir
less than half full, a crack two kilometres
long opened high up on the face
of Mount Toc. New geological studies revealed
that the moving slope was not
part of the original mountain, but the unstable
relic of a giant landslide
several thousand years old. They also revealed that
the thickness of the moving
mass was not restricted to surface layers but
extended to more than 50m
beneath. The mass was far too big to be stopped,
but might it be possible to let
it creep gently into the valley floor?
Reservoir-filling was resumed
in late 1961 after a 12-month hiatus.
The Vajont dam was a per fect
showcase for Italian expertise. National pride
and commercial pressure drove
the project onward.
It was never completed. At
10:39 pm on October 9, 1963, 500 million
tonnes of mountainside crashed
into the Vajont reservoir. The collapse took
less than than a minute.
A giant wave surged over the
dam and, crashing to the valley floor, punched
the air from the mouth of the
Vajont gorge with the force of an atomic bomb.
A kilometre downstream, the
bars in Longarone were crowded as people
from nearby villages settled
down to watch Celtic play Real Madrid. Four
minutes later, Longarone and
1,909 lives ceased to exist.
The Vajont tragedy has become
an object lesson for engineers and geologists.
Nobody had imagined that a
mountain slipping at rates of less than 30
centimetres a day could have
suddenly collapsed within seconds.
Forty years later, debate
continues as to what actually took place. The
conventional explanation is
that, after two weeks of heavy rain, percolating
rainwater had become trapped
inside Mount Toc as water in the reservoir
blocked the routes through
which rainwater would normally have escaped.
Acting as a jack, the trapped
water separated adjacent cracks in the old
landslide until, the entire
mass was able to slide into the reservoir. The
"water-jack" explanation
suggests that the total amount of trapped water is
crucial to triggering sudden
collapse, and that rock drainage can restabilise
a slope by allowing the
separated cracks to close as water escapes. Indeed,
since the Vajont catastrophe,
estimating the jacking pressure of trapped
water has become fundamental to
forecasting whether or not reservoir
filling will destabilise a slope.
New studies of the Mount Toc
collapse, however, have prompted an
alternative view that the role
of water is to create, and not just to separate,
cracks within rock. Water
readily corrodes the tips of rock cracks.
Under most circumstances, the
amount of corrosion is insignificant. But
when a rock is under stress, so
that its existing cracks are only just stable,
a small amount of corrosion may
be enough to trigger a cascade of cracking,
in which small cracks grow and
join together to form a giant plane of
failure.
Sudden collapse occurs as soon
as the failure plane has been formed.
Cascade-cracking has been
recognised as a precursor to other abrupt and
violent geological events,
including earthquakes and volcanic eruptions, so it
would be remarkable if the same
process could not also occur in rock slopes.
Unlike crack separation,
however, cascade cracking is unstoppable once it
has accelerated beyond a
critical rate and no amount of sudden drainage will
restabilise the slope. The key
point is that the safety conditions against
cascade cracking are more
stringent than those against crack separation by
water pressure. So if cascade
cracking was responsible for the collapse at
Mount Toc, then current
slope-stability forecasts, based only on water
pressure, may prove to be
inadequate when assessing the impact of new
reservoirs on adjacent valley walls.
Nearly 40 years on, attempts to
understand the Vajont tragedy are still
yielding insights into how
reservoirs can destabilise rock slopes. The new
lessons must be learned
quickly. The 600km of the Three Gorges reservoir
will cut dozens of recent and
ancient giant landslides and the superficial
resemblance with Vajont is overwhelming.
Most clearly, though, Vajont
shows how human pride and pressure can
trigger what is later described
as a natural disaster.
--
Doris Shen
International Rivers Network
1847 Berkeley Way
Berkeley, CA 94703
doris@irn.org
tel: 510.848.1155 ext. 317
fax: 510.848.1008
IRN http://www.irn.org
learn how you can help stop Three Gorges Dam http://www.floodwallstreet.org
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