Jostedalsbreen Nasjonalparksenter

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Videos and photos from event

Her you can see videos and photos from the flood which hit the National Park Centre. Further down on this page you can also read about extreme weather and why this happened.

 

The Fosdøla river flows towards the National Park Centre. Photo: Ann-Helen Blakset.

 

Flood on the parking lot and at the entrance area. Photo: Ann-Helen Blakset.

 

 

Video outside day 1. Photo: Ann-Helen Blakset

 

 

What is extreme weather?

Very briefly defined, extreme weather is weather that endangers life and property over an area of a certain size. The term extreme weather is also used to refer to a single storm, such as ‘Extreme Weather Jacob’. Since 1995, the Norwegian Meteorological Institute has given names to extreme weather – both male and female names each time. The names follow the alphabet, and ‘Agnar’ was the first named extreme weather, in 1995. The naming is intended to make communication easier by ensuring that everyone involved clearly knows what kind of weather event it is.

Since June 2018, the Norwegian Meteorological Institute has issued hazardous weather warnings using colour codes. Named extreme weather warnings are always sent out in a red colour. The weather phenomena for which extreme weather warnings are issued are wind, precipitation (rain, showers, snow) and water levels (storm surge). All of these weather phenomena can lead to damage to buildings and other infrastructure. In addition, large amounts of precipitation such as rain will often lead to flash floods and landslides where the terrain is steep enough for this, while large amounts of snow and sleet, as well as rain on snow, can lead to snow or mudslides.

The aim of the alert is to warn society and its inhabitants so that they can implement measures to safeguard assets and reduce the risk of loss of life. A warning plan has therefore been drawn up to ensure that information from the Norwegian Meteorological Institute about particularly dangerous weather, ‘extreme weather’, reaches the responsible authorities such as the main rescue centres, county emergency response offices and police authorities.

Extreme weather warnings are consequence-based. This means that the criteria for issuing warnings are based on the consequences the storm may have for individuals and society. For example, it usually takes more precipitation in western Norway than in southern Norway before an extreme weather warning is issued. This is related to how nature, infrastructure and housing are dimensioned and adapted to the precipitation amounts that are common for the different parts of the country.

 

The extreme weather ‘Jakob’ and why it happened

Extreme weather ‘Jakob’ hit Western Norway on Thursday 31^st of October and Friday 1^st of November 2024. In addition to a large part of Western Norway, smaller parts of Telemark, Buskerud and Innlandet were also affected. On Tuesday 29^th of October, the first warning of an event involving very heavy rain was issued by the Norwegian Meteorological Institute. For the same event, NVE had published a hazard warning at a high level, for the risk of flooding, as well as landslides and mudslides (Figure 1).

 

‘Jakob’ occurred as a result of an elongated frontal zone affecting the same geographical area over a longer period of time (~24 hours). This type of rain event is referred to in Norway as an ‘atmospheric river’ (Figure 2, Figure 3 and Figure 4), and this phenomenon has led to several extreme weather events in Norway in recent years. An atmospheric river is a relatively long and narrow zone of water vapour transport, where the water vapour originates in tropical areas. The tropical air masses are transported to our latitudes with the help of strong winds in the atmosphere, and when the air masses arrive in Norway, the water vapour is converted into rain. What makes atmospheric rivers so scary is that they often move little when they hit land, leading to enormous amounts of rain over a long period of time in a small, confined area. Typical consequences of this phenomenon are flash floods, landslides and mudslides. Atmospheric rivers are reinforced by mountains, making Norway particularly vulnerable to this meteorological phenomenon.

Atmospheric rivers are often associated with strong winds at higher altitudes, pulling precipitation deep into fjords and mountains (Figure 4 and Figure 5). This was also the case with ‘Jakob’ and contributed to the effect being so strong in Oppstryn.

 

 

 

Weather phenomena are classified, like precipitation, by stating an annual probability, which in turn is based on statistics from historical data. For example, we can talk about a 100-year event, which indicates that the event statistically occurs once every 100 years. Another way of putting it is that such an event, which for example could be a certain amount of rainfall in the course of 24 hours, has a 1 per cent chance of occurring annually. This does not rule out the possibility of a ‘100-year event’ happening two years in a row, or even in the same year.

The best-known major flood disaster in Norway is Storofsen, which particularly affected large parts of the interior of Austlandet in July 1789. It is estimated to have been slightly below a 500-year rainfall event, but in this case it coincided with late snowmelt in the mountains, which greatly exacerbated the situation. The extreme weather event ‘Jakob’ is estimated by the Norwegian Meteorological Institute to have been close to a 50-year event (Figure 6).

 

 

 Risk of flooding

Very often, such extreme precipitation events lead to flooding and landslides. Smaller watercourses in particular can react quickly to the precipitation and increase to damaging floods in a short period of time. When the precipitation decreases, the water flow quickly decreases again. Large watercourses, such as Gudbrandsdalslågen, take longer to build up flood flows, but the floods also last much longer, typically several days.

Fosdøla has a relatively small and steep catchment area of 11.5 square kilometres (Figure 7) stretching from 28 to 1836 metres above sea level, with an average gradient of 31 degrees. Annual precipitation is approximately 1,260 mm, and about two-thirds of this is winter precipitation (October-April). The catchment area is also heavily influenced by glaciers, with around 19 per cent width coverage. In addition, as much as 65 per cent of the catchment area is classified as bare rock, which again indicates rapid runoff. Such a small and steep catchment area will react quickly to precipitation, and it is therefore not unnatural that the river was flooded as a result of the extreme weather ‘Jakob’.

The Norwegian Water Resources and Energy Directorate (NVE) publishes alert maps for a number of natural hazards, including flooding, for the whole country. These maps show areas that can potentially be prone to flooding. The maps are based on terrain models and data from over 300 hydrological stations. They are based on the premise that flood water levels are a function of the catchment area, percentage covered by lakes and runoff. They are not based on detailed hydrological or hydraulic calculations, and they say nothing about probabilities. The assessment map for the lower part of Fosdøla is shown in Figure 8. The maps are generally quite rough.

Flood zone maps have been prepared for some watercourses. These are far more accurate and are based on detailed calculations. These maps show flood zones for 20-, 200- and 1,000-year floods in the watercourse, i.e. floods that can be expected once every 20, 200 or 1,000 years, or that can also be said to have an annual probability of 5 per cent, 0.5 per cent and 1 per thousand. Unfortunately, flood zone maps are only available for selected stretches of watercourses where the damage potential is high, and mainly only for the larger watercourses. In the region around the National Park Centre, such flood zone maps are available for the lower part of Hjelledøla, down to the outlet in Oppstrynsvatnet, and in Stryneelva, from Oppstrynsvatnet to the outlet in the fjord in Stryn.

 

Landslides as a result of precipitation

Heavy and/or prolonged precipitation can trigger all types of landslides, but the most common when the precipitation comes in the form of rain are mudslides (Figure 9) and landslides (Figure 10). Both of these types of landslides are debris avalanches, and since moraine from the last ice age is the most common type of debris in Norwegian valley sides, the avalanche material is most often moraine, although debris avalanches also occur in other types of debris, such as river deposits, clay and previously deposited avalanche material. While a landslide can occur on any valley side that is steep enough, mudslides generally follow streams or other depressions in the terrain, such as ravines. A typical feature of both is that the release area can be small, but that the landslide develops downstream and carries more and more material with it, both loose masses and forest and other flotsam. A debris avalanche can also develop when an otherwise small stream becomes flooded and begins to erode in width, thus developing into a debris avalanche.

Common to both types of avalanches is that they occur in relatively steep terrain, usually steeper than 30 degrees, but they can also be triggered in terrain with a slope of up to 25 degrees. The most important differences between landslides and debris floods are linked to the shape of the terrain and the water content of the avalanche, but there are fairly smooth transitions between the types, and they can be difficult to distinguish. Quick clay landslides are a specific type of loose mass landslide that often occur in much flatter terrain, but are not a problem in the area around the National Park Centre.

 

Water is generally very important in connection with avalanche release. The water pressure in the ground affects the stability of the loose masses. If the ground is saturated with water and the water pressure increases to what we call overpressure, the water pressure will help to ‘lift’ the overlying material (comparable to driving a car over wet asphalt), and thus help to destabilise the conditions over a potential landslide surface. In Norwegian valley sides, we typically have a fairly thin layer of moraine (typically 0.5-5.0 m) over solid and often quite dense rock. Overpressure can occur at this interface, and we often see landslides clearing away loose masses all the way down to solid rock (Figure 10).

In the context of landslides, we often talk about a safety factor. The safety factor is the ratio between the forces that drive the landslide and the forces that hold it back. If the safety factor is 1.0, conditions are barely stable. If the safety factor is lower than 1.0, this indicates that the driving forces are greatest and the avalanche is moving. Conversely, if the safety factor is greater than 1.0. An attempt has been made to illustrate this in Figure 11. If water pressure is applied to the slide surface, this will act as lubrication on the slide surface and reduce the stabilising force, thereby reducing the safety factor. The effect will be similar if the landslide material is solid rock and the sliding surface is, for example, a rock fissure or a weak zone between two rock layers.

 

Anything that can store water in the terrain is negative in terms of avalanche risk. Human intervention that impedes natural drainage is unfortunate, and forestry roads or tractor roads constructed for forestry purposes are a fairly common ‘culprit’. Such roads usually run across the valley sides and can carry water to undesirable places, where it can trigger avalanches, as shown in the example from Nesbyen in Hallingdal (Figure 12).

 

Mapping of landslide risk

In the same way as for floods, NVE publishes alert maps for landslides. These cover the whole country, but are only based on terrain and mathematical models for avalanche triggering. The map does not say anything about probability and is only intended as an initial assessment of whether landslides can potentially occur in an area. The map also forms part of the basis for more detailed hazard zone mapping. This is typically done in areas where there are settlements or other infrastructure that can be damaged by landslides.

Hazard zone mapping is also carried out using digital terrain models and mathematical models, but detailed visual inspection of the mapping areas is also carried out. Hazard zone maps are very important tools for municipalities when planning the development of their areas. The map is also the basis for planning safety measures. The hazard zone map shows hazard zones for all types of avalanches together and indicates recurrence intervals (Figure 13). Most often, the caution maps are conservative and show hazard areas that are larger than those indicated by the hazard zone mapping. The case of Oppstryn is one of very few cases where the caution area for avalanches is smaller than the hazard zone (Figure 13).

 

What can be done?

Extreme weather seems to be happening more and more often, so what can we do to protect ourselves against damage from such weather? The most important measure that can be taken to reduce the consequences of floods and landslides is good land use planning. On the basis of maps of flood and landslide risk (discussed above), we must avoid building or laying infrastructure in flood and landslide-prone areas. If this cannot be avoided, or if existing buildings are located in exposed areas, safety measures must be implemented.

 

Flood protection

To protect against flooding, the most common measure is to establish flood defences. This can be in the form of embankments (Figure 14 and Figure 15) or walls. The measures are dimensioned according to hydraulic calculations of the dimensioning flood water level. They must also be protected against erosion, so that a heavy flood does not erode and possibly break through the defence. In large watercourses, where the flooding can last for a long time, probably several days, the groundwater level can be affected, and it is often necessary to have a pumping system behind the protection.

In recent times, it has become increasingly popular to consider so-called ‘nature-based solutions’ (NbS), where measures are taken that seek to mimic nature’s own processes as far as possible. In the case of flooding, this may involve preserving or establishing edge vegetation and preserving wetland areas. Another effective measure is to ‘give the river room’ to flow. Flood plains that are flooded will slow down the river and reduce the risk of flooding further downstream. Floodplains are also a valuable biotope, with a great wealth of species. Nevertheless, rivers are often channelled and have buildings and other infrastructure close to them, so it is not always an option to ‘give the river space’.

 

Dim. Flomvannstand – Dim. Flood water level

Erosjonssikring (ordna steinlag) – Erosion protection (arranged stone layer)

Vekstjord – Top soil

Tetningslag – Sealing layer

Drenerende støttefylling – Draining support fill

Opprinnelig terreng – Original terrain

Drensrør – Drainage pipe

Fotgrøft – Foot trench

Grøft for tetningslag – Trench for sealing layer

Filterduk, klasse 5 – Filter cloth, class 5

Organisk materiale fjernet – Organic material removed

Filterduk, klasse 1 – Filter cloth, class 1

Drensgrøft – Drainage trench

 

For a small and steep watercourse such as Fosdøla, there are few protection options other than flood prevention and erosion protection with large stones, as has been done alongside the National Park Centre. This can be done as gently as possible by using local materials and local stone, thereby minimising transport and CO2 emissions. Furthermore, it is important to carry out care and maintenance along the watercourse. Although riparian vegetation is important, fallen trees and other debris that can be carried by the river in the next flood should be removed. During the extreme weather event ‘Jakob’, trees, roots and other flotsam and jetsam were carried along with the river until it clogged up under the footbridge (which is now gone), causing the river to flow over the fortification and towards the National Park Centre. Trees along the edges, or planted on the flood embankments, should not be too large. Windfalls will easily create wounds, where the river can erode particularly sharply. All bridges and culverts must be dimensioned for large floods, preferably with an extra ‘climate surcharge’.

 

Avalanche protection

In addition to mapping and good land use planning, which is by far the most important measure, it is common to protect buildings and infrastructure using landslide barriers, which stop landslides or direct landslides away from settlements and to areas where the consequences are minor (Figure 18 and Figure 19). For mudslides that feel depressions in the terrain, such as streams and ravines, there are various measures to capture the landslide material and slow down the energy before the mudslide reaches settlements or other infrastructure. Catchment dams can be established in the landslide course (Figure 19), often several in a row, or similarly with catch nets (Figure 16 and Figure 17). What they have in common is that they involve intervention in the terrain, and that they must have access for later maintenance and emptying after events.

 

 

 

 

For landslides, there are also a number of different nature-based solutions, or hybrid solutions, which combine nature-based solutions with traditional structural solutions. The use of local materials, wood or stone, are important elements in this. Forests have an important function in the context of landslides. In particular, forests can be important in preventing the triggering of landslides, or at least reducing the likelihood of it. Forests have several favourable properties, both hydrological and mechanical (Figure 20). Forests absorb a great deal of water through evotranspiration, thereby delaying soil waterproofing, especially during the growing season, and tree crowns slow down and disperse precipitation before it hits the ground. Furthermore, the roots act as a mechanical anchor for the soil. This is particularly effective for trees with deep roots, which can reach below the potential sliding surface for landslides (Figure 20).

 

 

HISTORICAL DEVELOPMENT OF WEATHER

 

Named extreme weather events

Although the Norwegian Meteorological Institute has been naming extreme weather events since 1995, there are no good statistics on how the frequency of such events has changed over the past 30 years. One problem is that the criteria are not exact, and that notification procedures have varied. The time series is also too short to look for trends, because we are talking about incidents that have occurred on average once or twice a year in one area of Norway or another, at the same time as random variation from year to year has varied from 0 to 4 incidents. What can provide more useful information is to look at the time development of measured temperature, precipitation and wind, especially the extreme values.

 

Historical temperature development

The time periods that we define as ‘normal’ are 30 years. Previously, the normal period was 1961-1990, while the normal period has now been moved to 1991-2020. The average temperature in Norway increased by 1°C from the period 1961-1990 to 1991-2020. The temperature has increased particularly in the winter months, while the differences between the two periods are small in May, June and October. In Western Norway, the proximity to the open sea has had a dampening effect on the changes. On an annual basis, they are slightly below the changes on a national basis (0.9°C), and the differences between the seasons are smaller, but the change is also greatest in winter. The average number of days in the year with a mean temperature below 0°C has decreased throughout the country, while the number of days with a mean temperature above 10°C has increased, with the exception of some mountain areas where such days still do not occur. In Western Norway, the changes are generally smallest in the high mountains and greatest in some lowland areas. The areas with the greatest changes tend to be slightly inland from the coast, particularly for the number of days with an average temperature above 10°C. The number of days with a minimum temperature below zero has also decreased the least in the mountains, while the reduction has been greatest in coastal areas. The number of days where the temperature crosses zero has therefore also decreased at the coast. Further inland, there has been little change or a slight increase in the number of such days on an annual basis. The number of such days has increased particularly in winter, while it has decreased in autumn.

 

Precipitation development historically

Precipitation in Norway increased by around 7 per cent between 1961-1990 and 1991-2020. This is partly due to an increase in the number of precipitation days (approx. 3 per cent), but even more to an increase in average precipitation per precipitation day (4.3 per cent). And the increase has been even greater when we look at daily precipitation, which on average is exceeded once a year (5.6 per cent). The same applies to the annual maximum for the largest amount of precipitation on five consecutive days. This means that the proportion of precipitation that falls on days with high intensity has increased. On average for Norway, winter precipitation has increased the most, while autumn precipitation has decreased slightly. This is particularly the case in Western Norway, and while autumn was the season with the most precipitation in 1961-1990, winter took over as the season with the most precipitation in the period 1991-2020.

 

Historical wind development

Data for wind is poorer than for temperature and precipitation, but what we have in terms of data and modelling suggests only small changes in average wind in Norway over the past 30 to 60 years. On average, a 2 per cent increase is calculated from 1961-1990 to 1991-2020, but this increase is not statistically certain, and there are large geographical variations. The only areas with statistically significant changes are in Troms and Finnmark, where we see a trend towards lower average wind speeds.

 

WHAT CAN BE EXPLAINED BY GLOBAL WARMING?

 

Temperature development in Norway and global warming

A warming of 1°C in 30 years from 1961-1990 to 1991-2020 is fully in line with what can be expected in Norway as a result of man-made global warming with medium to high greenhouse gas emissions. In winter, the temperature increase has been even slightly higher than expected, while it has been less in May, June and October. Both are due to variations in typical circulation patterns in the atmosphere. In winter, these variations have amplified the warming, while they have more or less cancelled out the warming in May, June and October. On an annual basis, such variations even out, and we can essentially say that the warming over Norway between the two periods can be explained as a result of global warming.

 

Precipitation trends and global warming

To what extent can historical precipitation increase be explained by global warming? On average for Norway, annual precipitation from 1961-1990 to 1991-2020 has increased somewhat more than the average climate model suggests, although this is within the uncertainty range of the models. This may mean that the climate models have weaknesses when it comes to precipitation, but it may also mean that natural variations have added to the increase due to global warming. It’s particularly in winter that we’ve had a larger precipitation increase than the models indicate, and that’s when the increase in temperature is greatest, and therefore possible water content in the air. Autumn precipitation has decreased slightly, particularly in Western Norway. Both of these deviations can be explained by differences between the two periods in typical circulation patterns in the atmosphere. It is uncertain to what extent the circulation pattern is affected by global warming, but we can nevertheless say that most of the precipitation over Norway between the two periods can be explained as a result of global warming.

Wind development and global warming

Climate models show more variability for wind than they do for temperature and precipitation. One signal we see, and which is supported by historical developments, is a reduction in average wind in the Barents Sea and parts of Northern Norway. This may be due to the sea ice limit moving north.

 

WHAT SHOULD WE PREPARE FOR?

 

Temperature

If greenhouse gas emissions continue to increase, climate models predict an average increase in annual temperature over Norway from 1991-2020 to 2071-2100 of between 2 and 4.5 °C (3.4 °C on average). In Western Norway, a slightly smaller increase is predicted (2.8 °C on average). The largest increase is predicted in winter and the smallest in summer or spring.

 

Precipitation

If greenhouse gas emissions continue to increase, climate models predict an average increase in annual precipitation over Norway from 1991-2020 to 2071-2100 of between 6 and 20%. No increase in the number of days with precipitation is calculated, only in intensity. The daily precipitation that is exceeded on average once per year is calculated to increase by 10-30%. The number of days that exceed the 1991-2020 value for the average annual maximum is calculated to double. In both Norway and Western Norway, the largest increase is calculated in autumn and winter, and the smallest increase in summer. We know that variations in atmospheric circulation patterns are of great importance for how precipitation is distributed locally. There is therefore great uncertainty in local values, but a general feature is that when precipitation first occurs, several days with high intensity must be expected.

 

Wind

Going forward, there is a large discrepancy between climate models when it comes to wind, but the main trend is towards small changes or a reduction in the mean wind over Norway. This does not rule out that the strongest storms may become stronger, and there is a study that shows that the number of low pressure areas that come into Norway will decrease, but that those that hit us may become stronger, so that what is a 100-year event today may become a 50-year event towards the end of the century in some places. The projections for wind are far more uncertain than they are for temperature and precipitation. Locally, it will play a major role if the typical storm tracks change. Many models indicate that they will on average move northwards. This may result in reduced storm activity in some places, but increased activity in other places. It may also result in a change in the typical wind direction during strong winds.

CONCLUSION

The climate change that we are both in the midst of and that we expect will intensify further throughout the rest of the century, suggests that we must expect more “‘Jakob’-like” events, or even more powerful ones. What today may be a 100-year event, may be a 50-year event by the end of the century. It is therefore important that all spatial planning, all buildings and all infrastructure are adapted to this. Good mapping and understanding of the processes that lead to damaging floods and landslides is absolutely essential. Further development should, to the greatest extent possible, play more in harmony with nature. Public bodies, the state, the county and not least the municipalities play important roles and have a great responsibility.