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Calculating flood waves in Lyngen

12.09.2007
It is hoped that detailed mapping of the fjord floor in Lyngen in Troms, northern Norway, will show how flood waves generated by avalanches crashing into the sea may affect the land. The unstable section of Nordnesfjellet, 600-800 metres above the beautiful fjord, is moving up to three centimetres a year.

Seisma, a research vessel belonging to the Geological Survey of Norway (NGU), is sweeping Storfjord with seismic and state-of-the-art, side-scanning sonar. Metre by metre, scientists are studying the floor of the fjord.

Well prepared

On the basis of these detailed maps, staff from the Norwegian Geotechnical Institute (NGI) will construct a precise terrain model of the sea floor. They can use this to calculate how flood waves will build up and sweep over the shores if Nordnesfjellet should one day crash into the sea.

“It’s vital to get as much information as possible about both the mountain and the floor of the fjord. This will enable us to be prepared in case the worst should happen,” Oddvar Longva, a geologist at NGU, tells me. He is the skipper of the Seisma and has a great deal of experience of surveying conditions on the floor of Norwegian fjords.

Large flood waves

The worst scenario here is that several million cubic metres of mountainside will one day sweep over the E6 trunk road and end up in the fjord. This will generate a flood wave that will hit the boroughs of Lyngen, Kåfjord and Storfjord. As many as 6000 people, along with buildings, industrial plants and farmland, may be inundated.

”It’s not very likely that the mountainside will crash into the fjord in the very near future, but if it does the consequences will be enormous. There is therefore a great deal of risk associated with an unstable section of mountainside,” Terje H. Bargel, a geologist at the Geological Survey of Norway (NGU), stresses.

”When we’ve learnt how an unstable section of mountainside behaves, and mapped the terrain below sea level, it will be easier for local authorities to draw up contingency plans to provide warning and ensure evacuation,” he continues.

The danger

Terje Bargel heads the avalanche group at NGU and is very satisfied that the authorities are now putting priority on investigations of major avalanches.

”The Government has allocated an additional five million NOK this year to study the risk of major avalanches. NGU is concentrating particularly on large avalanches involving more than 100 000 cubic metres of rock that may crash right down into fjords and generate tsunamis. We’ve studied 13 sites in the county of Troms in northern Norway that have the potential to develop major avalanches. Lyngen is by far the most important area,” Terje Bargel says.

Installing equipment

It is the northern part of Nordnesfjellet that is moving. NGU has studied the mountain on several occasions by both mapping the geology and gathering geophysical data.

The hazardous stretch of mountainside is four hundred metres broad, five hundred metres high and between fifty and one hundred metres deep. New instruments are now being installed to monitor the mountainside.

Laser meters will monitor movements in the mountainside and tension rods will measure the widening of the fissures. Meteorological instruments are also being set up to monitor the wind, temperature and precipitation.

By Gudmund Løvø

Terje Bargel | alfa
Further information:
http://www.ngu.no

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