AVALANCHE DISASTERS IN NORWAY

The first overview of all types of avalanche and landslide, including information about more than 3000 avalanche and landslide disasters in Norway over the past 400 years, has been assembled in collaboration with researches at the Norwegian Geological Survey (NGU). Relevant written sources are found from up to 700 years ago, but best documentation is from the 17th. Century, in parish registers and legal examinations pertaining to tax reduction. The oldest written sources are the medieval documents (Diplomatatrium Norvegicum) and the Icelandic annals that mention landslides at Stjørdal ca. 100 and at Støren in 1345, when 500 died. Many legends seem to be based on actual occurrences.

The history of avalanches and landslides partly coincides with that of climatic changes, with avalanches for example around 1680 and landslides in 1743 and 1789. The 19th. Century is characterised by many, fatal avalanches. The 20th. Century on the other hand witnessed a decline in avalanche fatalities. Research shows that the rare giant landslides occur with at frequency of 2 to 5 per century. Thus three major landslides have occurred during the past century; Tafjord 1934, Loen 1905 and 1936, with in all 175 fatalities.

In addition many avalanches and landslides have resulted in more limited damage, often because generations of experience has provided knowledge about local danger. The historical overview is included in a data base of avalanches and landslides held by NGU and may be found at www.skrednett.no


Rockfall and tsunami in Tafjord 1934

THE TIMES, Monday April 9, 1934

Norwegian fiord disaster
Flood caused by fall of rock - 40 lives lost

The fall of a huge mass of rock into Korsnaes Fiord, near Aalesund, in Saturday morning caused the waters of the fiord suddenly to rise in great waves, with the result that the villages of Tafjord and Fjoeraa at least 40 lives were lost.

At 3 a. m. on Saturday a great rock, about 1,200ft high and 600ft, broad, loosened in the mountains, fell into the narrow fiord, and immediately afterwards a flood of waves swept over the country for over half a mile inland. In Tafjord village, the inmost in the fiord, the people were awakened by the thunderous crash of rock falling, and very soon afterwards a great wave washed against the shore, to be followed by a second and a third gigantic wave. The waters spread 700 inland, demolishing everything in their way.

Dreadful scenes followed. In the darkness was heard the noise of the roaring waters, the cries of injured and drowning people, and the crash of falling houses. It was nearly half an hour before the waters retired. The fiord was calm again when daylight revealed the desolate scene to the aw-stricken survivors.


Slides and avalanches

Different types of slides and avalanches are common in Norway, mainly due to the rough topography with steep valleys and deep fjords. Avalanches and slides cause loss of life and severe damage to buildings and the transport system. More than 2000 human lives have been lost in avalanches and landslide disasters during the last 150 years.

Quick-clay slides represent a particularly high risk in eastern and central Norway. Snow avalanches affect large parts of western and northern Norway. Large rock avalanches, which can also generate tsunamis, are most common in northern West Norway and in the northern part of Troms in northern Norway.


Snow avalanches

Snow avalanches include several different types from dry powder avalanches to wet-snow avalanches and slushflows. Dry powder avalanches often occur during periods of heavy snowfall and strong winds, in situations where large areas of wind-drifted snow can fail and cause destructive, high-velocity, powder avalanches. Wet-snow avalanches occur during milder periods or in combination with high snowfall rates.

Powder snow avalanches may also generate exceptionally strong winds which can cause significant damages. At some distances from the avalanche itself, trees can be uprooted and houses demolished by the gale-force wind, even on the opposite side of the valley. During recent years, many fatal avalanches have been triggered by people doing downhill-skiing or snow-boarding in steep terrain; and snow-scooters have also caused several destructive avalanches.

Snow avalanches have caused numerous tragedies in Norway, but the most disastrous single event occurred in Sunndalen, central Norway, in February 1868. There, 32 people lost their lives in a snow avalanche at Gråura. 


Rockfall and rock avalanches

Rockfalls of small volume are common in Norway and occur on most mountain slopes steeper than 45º, and are often triggered during the spring and autumn by ice melting and extreme rainfall episodes. The triggering mechanisms for large-scale rock avalanches are not fully understood. Large rock avalanches are not very common, but historical documents indicate a frequency of between 2 and 4 events every one hundred years in western Norway. However, such events cause major tragedies when large volumes of rock plunge into a lake or a fjord and generate tsunamis. Such disasters cause a greater loss of life and more material damage than all other types of slides and avalanches in Norway. 

One of the largest rock avalanches known occurred in Tafjord  approximately 3000 years ago, when more than 100 mill. m3 of  rock failed on the mountain Veslejordet. The rock avalanche deposited most of the debris where Tafjord village is situated today. The Tafjord disaster of 1934 occurred in almost the same area. The most severe disasters in historical time occurred at Loen (Sogn og Fjordane) when the mountain side of Ramnefjell failed in 1905 and 1936. These two rock avalanches caused destructive tsunamis with heights of up to 74 metres above the lake level, and 175 people lost their lives.


Quick-clay sides and debris flows

Large parts of lowland areas of Norway, especially in eastern and central Norway, are covered by marine clay. The clay was formed when the sea level was much higher than today, up to as much as 220 m above present-day sea level, shortly after the last ice age. The leaching out of salt water from the clay pores and its replacement by freshwater has strongly increased the sensitivity and the instability of such clays. A decrease in shear strength of the clay and the effect of fluvial erosion have resulted in a large number of quick-clay slides in Norway. Intense rainfall and human activity often triggers the slides. The worst natural disaster known in Norway is the Gauldal slide in central Norway in 1345, which took about 500 lives and destroyed more than 50 farms. The quick-clay slide in Verdal in central Norway in 1893 is also one of the largest, when about 3 km2 of clay failed, and 118 people lost their lives.

Major disasters also occur when glacial tills fail on steep mountain slopes and cause debris flows. Such events often occur during periods of intense rainfall or in combination with snowmelt. A well-known debris-flow event, known as 'Storofsen', occurred in eastern Norway in 1789. In this disaster, large areas of eastern Norway were affected by flooding and debris flows, and this caused severe damage and loss of life.