Pontresina becomes village of the dammed
Defending their village against avalanches or rockfalls has become second nature to the inhabitants of Pontresina in canton Graubünden.
Since the 19th century, they have erected dry stonewalls and an elaborate network of fences to protect their homes and their lives.
Anchored to the slopes above the village are 16 kilometres of steel fences costing at least SFr1,500 ($1,102) per metre.
On August 13, when the community officially takes possession of an SFr7 million dam, it will mark another step in their attempts to avoid a major natural catastrophe.
The dam dispenses with the need to build a further ten kilometres of avalanche defence.
The project was spurred by the realisation that parts of the Schafberg mountain behind the village are permafrost – ground that remains frozen throughout the year.
The trouble is that temperatures in this so-called rock glacier are just above melting point.
If, as feared, the permafrost thaws over the coming years, the threat of landslides could rise dramatically.
Landslide
Marcia Phillips, who works at the Federal Institute for Snow and Avalanche Research in Davos, has been measuring temperatures down to a depth of 25 metres in the rock glacier above Pontresina.
Over the past seven years, the temperature has actually dropped slightly.
“If you’ve got a thin snow cover at the beginning of the winter, it allows the heat to escape from the ground,” Phillips told swissinfo.
“Also a big snowfall at the end of the winter which lasts into the summer protects the ground from direct solar radiation.
“We think that the borehole temperatures have been going down very slightly due to this snow cover phenomenon.”
Defensive measure
Phillips has also spent the last seven years investigating whether the steel fences which hold snow back at the top of mountains to prevent avalanches, can be anchored in frozen ground.
Ideally, they would anchor on bedrock but in locations like Pontresina, engineers have no choice but to build on steep icebound slopes covered in a thick layer of scree.
“The structures are creeping at quite an alarming rate between five and 20 centimetres a year but they seem to be holding up,” said Phillips.
“The problem is that they are designed to last for 100 years and we don’t know how long they’re really going to last because they’re probably going to break or get damaged.”
Drilling into permafrost is a laborious task. If the drill does not get stuck and the hole does not fill with water, the grout to hold the anchor can freeze before it sets.
“We warm the grout to 20 degrees and then put it in the ground so it can set before it freezes,” explained Phillips.
“Otherwise it won’t be strong enough to hold the anchor in place and you can just pull out the anchor by hand.”
Avalanche deaths
Computer models and past experience help the team work out where to place their fences
“Any slope that’s steeper than 30 degrees and has been known to avalanche in the past needs to have fences built to retain the snow if there’s a village or a road underneath it.”
Some 500 kilometres of protective fences are strung across Switzerland’s mountains – representing an investment of more than SFr2 billion. The construction work appears to have paid off.
In the winter of 1999 which saw heavy snowfalls, 17 people lost their lives – a considerable reduction from the 98 deaths in a similar winter back in 1951.
“The structures proved very useful and prevented many destructive avalanches from coming down,” said Phillips.
Building pressure
The old village of Pontresina was built in two separate sections so that avalanches, which tend to funnel through a gully on the mountainside, simply passed through the middle and caused limited damage.
As the village has grown, the two halves have joined up and that is why the centre is threatened by natural hazards.
Felix Keller of the Engadine Academy began investigating the permafrost above Pontresina in 1989 and was one of the first to be convinced of the need for a dam.
The construction is designed to withstand 280,000 cubic metres of avalanche and 100,000 cubic metres of debris flow.
The government met 60 per cent of the SFr7 million costs, with the canton and the community sharing the rest.
Keller said it was worth every Franc – and not simply because it avoided the need to build new fences or strengthen existing ones.
“The damage when a large debris flow occurs is about SFr300 million and even if there are no large debris flows in the next 50 years, this dam is still very cheap.”
swissinfo, Vincent Landon
Switzerland boasts 500 kilometres of avalanche fences.
Engineers build about ten kilometres more every year.
At a cost of between SFr1,500 and SFr2,500 a metre, this represents a total investment of over SFr2 billion.
Some 16 kilometres of fences are anchored to the slope above Pontresina.
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