Landslides and gullies
In Sweden, the most noticeable events in the natural changes to the landscape are falls in rock and Quaternary deposits and – mainly – landslides and gullies. Forces of nature are working to adapt cliffs and slopes to an equilibrium. Land uplift, the climate and anthropological influence alter the stability of the ground.
Most common types of mass movements, such as falls in Quaternary deposits, landslides and gullies, are found in areas below the highest shoreline. They are formed in connection with snowmelt and the thawing of frozen ground, as well as intense or prolonged rainfall, i.e. when the water pressure in the ground is high, or when there are sharp fluctuations in the groundwater level. Large landslides have become increasingly common during the past century, probably as a result of anthropological interference, e.g. by construction activity that undercuts or overloads dangerous slopes.
Most of the Swedish Quaternary deposits and minor terrain configurations are a result of the deglaciation of the latest continental ice sheet. Due to the pressure of the weight of the inland ice, lowlands were submerged during the deglaciation. Marine conditions prevailed in southwestern Sweden while fresh water or water with low salinity occurred in the Baltic basin up to Bothnian bay.
As soon as the pressure of the inland ice started to lighten, the crust began slowly to elevate. The highest situated traces of the shoreline are at different altitudes throughout Sweden, depending on how far the crust had been depressed, how much the local sea surface had transgressed, and the time at which the area had become deglaciated. The highest shoreline is at 286 m above sea level on the coast of central northern Sweden. Furthest to the south, the highest shoreline coincides almost with the present shoreline. The rate of the present land uplift is almost 9 mm per year along the coastland of northern Sweden.
Landslides occur in more or less gentle slopes made up of glacial silt and clay, mainly in quick-clay. Most of the dangerous slopes are bordered by open water – a river or a lake. About four per cent of the land surface consists of clay and silt. Based on known landslide scar frequency, about one per cent of the land surface is of primary interest due to high risk of spontaneous landslides. The latter area is fairly well populated.
In the woodlands of interior Sweden, slide scars in gentle slopes of silty till are known. These landslides have probably been triggered by earthquake vibrations.
Most of the largest and most costly landslides have taken place in the River Göta älv valley and its surroundings between Lake Vänern and Göteborg. The total volume eroded by landslides and gullies in that valley is roughly calculated to ca 500 million cubic metres.
In the Göta älv valley there are several important communication links. A warning system that registers movements in clay has been constructed for certain parts of the railway line east of Göta river.
Gullies have been formed in areas of light clays, silt and sand. In some areas the loss of cultivated land is severe. The areal extent of gullies is several times higher than that of landslides.
Rivers in the northern part of Sweden have cut deep into glaciofluvial and fluvial terraces of sand and silt. The shores are 5–20 m high bluffs and in the valley terraces there are well-developed gully systems. At least some gullies have been caused by poorly planned forest clearing leading to increased rates of surface water run-off.
Earthquakes may cause landslides
Landslide scars developed in glacial till appear frequently in some areas of northernmost Sweden. All facts known indicate that sliding occurred shortly after deglaciation and were triggered by earthquakes. During the last deglaciation, major faulting accompanied by violent earthquakes occurred in the region. Waterlogged soils may temporarily decrease in stability when shaken by an earthquake and are then easily mobilized. Most of the slides developed in gentle slopes with a fairly thick cover of overburden and in many places considerable amounts of soil were displaced, a million cubic metre or more is not uncommon.
Larger landslides in Sweden
The latest large landslide with casualties, occurred at Tuve on November 30, 1977. Between 4.05 and 4.09 p.m., seven electric cables were broken. 65 single-family houses were completely destroyed by the slide.
On October 1, 1918, a small landslide (0.2 ha) with disastrous consequences occurred at Getå on the slope to Bråviken Bay. A train with about 300 passengers crashed in full speed into the slide. Fire broke out in the wrecked wagons. 41 victims were identified but the exact number of casualties is not known.
- ca 1150 Bohus, River Göta älv: 37 hectares
- 1648 Intagan, River Göta älv: 27 hectares, more than 85 casualties
- 1730 Gunnilse, River Lärjeån: 30 hectares
- 1759 Bondeström, River Göta älv: 11 hectares
- 1918 Getå: 0.2 hectares, at least 45 casualties
- 1950 Surte, River Göta älv: 22 hectares, 1 casualty
- 1957 Göta, River Göta älv: 15 hectares, 3 casualties
- 1977 Tuve, Hisingen island, Göteborg: 27 hectares, 9 casualties
- 2006 Småröd, Bohuslän, 10 hectares