River floods Sweden
Sweden: Vulnerabilities – Floods in the past
In the 1990s, floods have occurred almost regularly: in 1993, 1995, 1998 and 2000, to mention the most notable ones. With the exception of the 1995 flood, most recent floods have been induced by rainfall in summer or autumn, whereas the snowmelt-induced spring flood has traditionally been the largest flood of the year. The year 2000 was quite extraordinary, and contained two distinct floods. Not surprisingly, 2000 emerged as the year with the highest average precipitation over all of Sweden, since measurements began in 1860 (21).
An increasing trend in streamflow magnitude and a trend in the timing of floods have been detected in annual and seasonal flows in near-natural catchments in Denmark, Finland, Norway and Sweden, given that they tend to arrive earlier in spring (35). According to a previous study (22), both runoff volumes and flood magnitude increased substantially between 1970 and 2002, but similar conditions were experienced in the 1920s. The linear regression line for the average runoff from all of Sweden increased by 5% over the past century, but the trend was not statistically significant. An analysis of floods in Sweden has shown that neither the results from statistical trend tests nor visual inspection of the data suggest any change in flood frequency (22,36).
The conditions in Sweden, with slightly increasing runoff, are consistent with results reported from nearby countries (23). In general, it has been difficult to show any convincing evidence of an increasing magnitude of floods in the near region (24).
A Nordic analysis of runoff (25) showed that river flow regime changes may differ considerably in different parts of Nordic countries. It is quite natural: the physico-geographical conditions in the study area - extending from Iceland over Faroe Islands, Norway, Denmark, Sweden and Finland to Estonia - vary considerably. Nine types describing the different runoff regime were classified in the area. The most remarkable trends were increasing annual, autumn and winter runoff in south-western Norway and increasing winter runoff in Estonia and southern Finland during the latter part of the 20th century.
Sweden: Vulnerabilities – No impact climate change observed
Changes in river flow may appear from shifts in land cover, constructions in the river channel and climatic change, but currently there is a lack of understanding of the relative importance of these drivers. One might expect changes in land cover to affect transpiration by trees and the retention of water in the soil, and thus the volumes of (ground)water flowing into the rivers and streams. Constructions directly change river flow, and climate change does so indirectly through changes in temperature, humidity and precipitation.
Scientists in Sweden collected gauged river flow time series from 1961 to 2018 from across Sweden for 34 disturbed catchments to quantify how the various types of disturbances have affected river flow. Differences in trends from observations versus hydrological modeling were explored to quantify these effects. The time series were selected carefully, making sure that they represent several distinctly disturbed catchments for each type of change affecting river flow. At least four catchments with similar disturbance for each type of change were studied (37). The results are:
- No impact climate change: The catchments studied for climate change were otherwise undisturbed and not exposed to any major changes than climate. There were no systematic trends in the observed river flow from climate change, even though clear trends were found in precipitation and temperature. Only in one catchment a climate trend was detected beyond natural variability between years and decades (37).
- Minor impact land cover changes: The studied land cover changes are those from wildfires, storms and urbanization. No drastic shift in river flow volume could be related to these changes in the studied catchments. The impacts of tree removal by wildfires and storms are minor. This may be somewhat surprising, since other studies reported extensive effects on river flow due to changes in land cover. These studies, however, are from warm and dry climates, and cannot be compared with northern countries where snow is one of the dominant drivers of river flow (38). There was an effect of urbanization: locally, peak flows have increased in the order of 20% due to urbanization (37).
- Major impact constructions: With respect to constructions in river basins, the study focused on dams. Most rivers in Sweden are regulated with dams for hydropower production to store the snow melt during spring and release water throughout the year when electricity is needed (39). Today, hydropower amounts to half of the electricity supply for the country. As a consequence, the rivers are fragmented by in total some 1,800 hydropower plants across the country. River regulation by dams has the largest impact on flow regime, with almost 100% reduction during spring flood near the dams and roughly half of that at the river outlets to the sea. Winter flow was increased by orders of magnitude (37).
Sweden: Vulnerabilities – Future flood probability
Changes in dry and wet spell characteristics in Europe have been projected for 2021–2050 compared with 1961–1990, based on Regional Climate Model simulations under the A1B emission scenario. From the results it can be concluded that significant changes in dry and wet event characteristics are expected with high confidence in the southernmost (mainly France, Italy, and Spain) and northernmost (mainly Iceland and Scandinavia) regions of Europe, respectively. Southern Europe is most probably facing an increased risk of longer, more frequent, severe, and widespread droughts, while northern Europe is facing increased risk of intensified wet events. For precipitation, the most pronounced changes are found for the Iberian Peninsula in summer (−17.2%) and for Scandinavia in winter (+14.6%) (34).
According to the various climate scenarios the local runoff in Sweden will change in 2071-2100 relative to 1961−1990. In a normal year a 10% to over 40% runoff increase in the northern 2/3 of Sweden is projected. For the southern 1/3 of Sweden the projection varies between a 25% decrease or a 25% increase (26).
The country's westerns and southwestern parts are expected to experience flooding along watercourses more often or much more often due to climate change. The increasing 100-year flows in the mountain regions may also spread along watercourses with flooding as a consequence, but there is some uncertainty for regulated watercourses. High flows with a return period averaging 100 years will increase sharply, particularly in western Götaland, south-western Svealand and north-western Norrland.This shorter return period means they can occur several times a century.
In the eastern parts of the countrythese high flows will decrease as warmer winters will result in less remaining snow cover, which will to a smaller spring flood. The return period will increase instead (26,27).
Sweden: Vulnerabilities – Flood damage
Several major floods have affected Sweden in recent years. The floods around Lake Vänern in 2000/2001 and Arvika in 2000 are two examples.
The cost up until 2100 for the flooding of buildings around the major lakes – Vänern, Mälaren and Halmaren – was estimated at a total of SEK 7.9 billion at today’s hundred-year flood (28). Damage costs for shipping, roads, railways, agriculture, forestry, water treatment works, sewage system, power station and industries totaled an additional SEK 3.2 billion.
Today’s hundred-year flood, as well as smaller floods with shorter return frequencies, will have a reduced return frequency in some parts of the country. In the area around Lake Vänern, for example, it is estimated that the hundred-year floods will have a return frequency of 20 years. The hundred-year floods in a changed climate will therefore be higher than at present in these areas, which means that larger areas will be flooded. The return frequency will be longer in other parts of the country.
Europe: casualties in the past
The annual number of reported flood disasters in Europe increased considerably in 1973-2002 (1). A disaster was defined here as causing the death of at least ten people, or affecting seriously at least 100 people, or requiring immediate emergency assistance. The total number of reported victims was 2626 during the whole period, the most deadly floods occurred in Spain in 1973 (272 victims), in Italy in 1998 (147 victims) and in Russia in 1993 (125 victims) (2).
Throughout the 20th century as a whole flood-related deaths have been either stable or decreasing while economic burdens of flooding and related societal disruptions have become decidedly worse. 20th century flood disaster death tolls have been typically averaging fewer than 250 per year (3).
Europe: flood losses in the past
The reported damages also increased. Three countries had damages in excess of €10 billion (Italy, Spain, Germany), three in excess of 5 billion (United Kingdom, Poland, France) (2).
Expressed in 2006 US$ normalised values, total flood losses over the 1970–2006 period amounted to 140 billion, with an average annual flood loss of 3.8 billion (4). Results show no detectable sign of human-induced climate change in normalised flood losses in Europe. There is evidence that societal change and economic development are the principal factors responsible for the increasing losses from natural disasters to date (5).
Policy makers should not expect an unequivocal answer to questions concerning the linkage between flood-disaster losses and anthropogenic climate change, as this field will very likely remain an important area of research for years to come. Longer time-series of losses are necessary for more conclusive results (6).
Europe: flood frequency trends in the past
In 2012 the IPCC concluded that there is limited to medium evidence available to assess climate-driven observed changes in the magnitude and frequency of floods at a regional scale because the available instrumental records of floods at gauge stations are limited in space and time, and because of confounding effects of changes in land use and engineering. Furthermore, there is low agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of these changes. There is low confidence (due to limited evidence) that anthropogenic climate change has affected the magnitude or frequency of floods, though it has detectably influenced several components of the hydrological cycle such as precipitation and snowmelt (medium confidence to high confidence), which may impact flood trends (32).
Despite the considerable rise in the number of reported major flood events and economic losses caused by floods in Europe over recent decades, no significant general climate‑related trend in extreme high river flows that induce floods has yet been detected (7).
Hydrological data series do not indicate clear upward trends in the frequency and magnitude of floods in Europe. The direct anthropogenic causes include land use change, river channel modifications and increased activities in areas vulnerable to floods. Thousands of square kilometres of impermeable surfaces have been created, coastal urbanization has been extensive. The overall impact of these changes probably exceeds the impact of trends in meteorological variables in today's Europe (8).
In western and central Europe, annual and monthly mean river flow series appear to have been stationary over the 20th century (9). In mountainous regions of central Europe, however, the main identified trends are an increase in annual river flow due to increases in winter, spring and autumn river flow. In southern parts of Europe, a slightly decreasing trend in annual river flow has been observed (10).
In the Nordic countries, snowmelt floods have occurred earlier because of warmer winters (11). In Portugal, changed precipitation patterns have resulted in larger and more frequent floods during autumn but a decline in the number of floods in winter and spring (12). Comparisons of historic climate variability with flood records suggest, however, that many of the changes observed in recent decades could have resulted from natural climatic variation. Changes in the terrestrial system, such as urbanisation, deforestation, loss of natural floodplain storage, as well as river and flood management have also strongly affected flood occurrence (13).
Europe: projections for the future
In 2012 the IPCC concluded that considerable uncertainty remains in the projections of flood changes, especially regarding their magnitude and frequency. They concluded, therefore, that there is low confidence (due to limited evidence) in future changes in flood magnitude and frequency derived from river discharge simulations. Projected precipitation and temperature changes imply possible changes in floods, although overall there is low confidence in projections of changes in fluvial floods. Confidence is low due to limited evidence and because the causes of regional changes are complex, although there are exceptions to this statement. There is medium confidence (based on physical reasoning) that projected increases in heavy rainfall would contribute to increases in rain-generated local flooding, in some catchments or regions. Earlier spring peak flows in snowmelt- and glacier-fed rivers are very likely, but there is low confidence in their projected magnitude (32).
More frequent flash floods
Although there is as yet no proof that the extreme flood events of recent years are a direct consequence of climate change, they may give an indication of what can be expected: the frequency and intensity of floods in large parts of Europe is projected to increase (14). In particular, flash and urban floods, triggered by local intense precipitation events, are likely to be more frequent throughout Europe (15).
More frequent floods in the winter
Flood hazard will also probably increase during wetter and warmer winters, with more frequent rain and less frequent snow (16). Even in regions where mean river flows will drop significantly, as in the Iberian Peninsula, the projected increase in precipitation intensity and variability may cause more floods.
Reduction spring snowmelt floods
In snow‑dominated regions such as the Alps, the Carpathian Mountains and northern parts of Europe, spring snowmelt floods are projected to decrease due to a shorter snow season and less snow accumulation in warmer winters (17). Earlier snowmelt and reduced summer precipitation will reduce river flows in summer (18), when demand is typically highest.
For the period 2071-2100 the general feature is a decrease of extreme flows in areas where snowmelt floods are dominating in the present climate. The hundred year floods will attenuate by 10-50% in northern Russia, Finland and most mountainous catchments throughout Europe. An increase by similar amount is projected in large areas elsewhere, whereas a mixed pattern is likely in Sweden, Germany and the Iberian Peninsula (2).
Increase flood losses
Losses from river flood disasters in Europe have worsened in recent years and climate change is expected to exacerbate this trend. The PESETA study, for example, estimates that by the 2080s, some 250-400 million Europeans could be affected each year (compared with 200 million in the period between 1961 and 1990). At the same time, annual losses due to river flooding in Europe could rise to €8-15 billion by the end of the century compared with an average of €6 billion today (29).
From an assessment of the implications of climate change for future flood damage and people exposed by floods in Europe it was concluded that the expected annual damages (EAD) and expected annual population exposed (EAP) will see an increase in several countries in Europe in the coming century (33). Most notable increases in flood losses across the different climate futures are projected for countries in Western Europe (Belgium, Denmark, France, Germany, Ireland, Luxembourg, the Netherlands and the United Kingdom), as well as for Hungary and Slovakia. A consistent decrease across the scenarios is projected for northern countries (Estonia, Finland, Latvia, Lithuania and Sweden). For EU27 as a whole, current EAD of approximately €6.4 billion is projected to at least double or triple by the end of this century (in today’s prices), depending on the scenario. Changes in EAP reflect well the changes in EAD, and for EU27 an additional 250,000 to nearly 400,000 people are expected to be affected by flooding yearly, depending on the scenario. The authors stress that the monetary estimates of flood damage are uncertain because of several assumptions underlying the calculations (only two emission scenarios, only two regional climate models driven by two general circulation models, no discounting of inflation to future damages, no growth in exposed values and population or adjustments, estimates of flood protection standards); the results are indicative of changes in flood damage due to climate change, however, rather than estimates of absolute values of flood damage (33).
Large differences across Europe
Annual river flow is projected to decrease in southern and south-eastern Europe and increase in northern and north-eastern Europe (19).
Strong changes are also projected in the seasonality of river flows, with large differences across Europe. Winter and spring river flows are projected to increase in most parts of Europe, except for the most southern and south-eastern regions. In summer and autumn, river flows are projected to decrease in most of Europe, except for northern and north-eastern regions where autumn flows are projected to increase (20). Predicted reductions in summer flow are greatest for southern and south-eastern Europe, in line with the predicted increase in the frequency and severity of drought in this region.
Climate-related changes in flood frequency are complex and dependent on the flood generating mechanism (e.g. heavy rainfall vs spring snowmelt), affected in different ways by climate change. Hence, in the regions where floods can be caused by several possible mechanisms, the net effect of climate change on flood risk is not trivial and a general and ubiquitously valid, flat-rate statement on change in flood risk cannot be made (30).
Flood risk tends to increase over many areas owing to a range of climatic and non-climatic impacts, whose relative importance is site-specific. Flood risk is controlled by a number of non-climatic factors, such as changes in economic and social systems, and in terrestrial systems (hydrological systems and ecosystems). Land-use changes, which induce land-cover changes, control the rainfall-runoff relations in the drainage basin. Deforestation, urbanization and reduction of wetlands diminish the available water-storage capacity and increase the runoff coefficient, leading to growth in the flow amplitude and reduction of the time-to-peak. Furthermore, in many regions, people have been encroaching into, and developing, flood-prone areas, thereby increasing the damage potential. Important factors of relevance to flood risk are population and economy growth, flood protection strategy, flood risk awareness (or flood risk ignorance) behaviour and a compensation culture (30).
Adaptation strategies - General
Non-structural measures are in better agreement with the spirit of sustainable development than structural measures, being more reversible, commonly acceptable, and environment-friendly. Among such measures are source control (watershed/landscape structure management), laws and regulations (including zoning), economic instruments, an efficient flood forecast-warning system, a system of flood risk assessment, awareness raising, flood-related data bases, etc. As flood safety cannot be reached in most vulnerable areas with the help of structural means only, further flood risk reduction via non-structural measures is usually indispensable, and a site-specific mix of structural and non-structural measures seems to be a proper solution. As uncertainty in the assessment of climate change impacts is high, flexibility of adaptation strategies is particularly advantageous (31).
EU Directive on flood risk management
The new EU Directive on flood risk management, which entered into force in November 2006, introduces new instruments to manage risks from flooding, and is thus highly relevant in the context of adaptation to climate change impacts. The Directive introduces a three-step approach (2):
- Member States have to undertake a preliminary assessment of flood risk in river basins and coastal zones.
- Where significant risk is identified, flood hazard maps and flood risk maps have to be developed.
- Flood risk management plans must be developed for these zones. These plans have to include measures that will reduce the potential adverse consequences of flooding for human health, the environment cultural heritage and economic activity, and they should focus on prevention, protection and preparedness.
Adaptation strategies - Sweden
In the event of flooding of Lake Vänern and Lake Mälaren, the most cost-effective measure is judged to be increasing the drainage potential. The cost of this amounts to approximately SEK 650 million for Lake Mälaren. For Lake Vänern, the cost has been specified as being in the range SEK 1−6 billion. A more accurate cost estimate for Lake Vänern requires landslide mapping for the Göta Älv river valley (26).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Sweden.
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- Dankers and Feyen (2008), in: EEA (2009)
- Alexandersson (2002), in: Lindström and Bergström (2004)
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- Lindström and Bergström (2004)
- Hisdal et al. (1995), in: Hyvärinen (2003)
- Swedish Commission on Climate and Vulnerability (2007)
- Kundzewicz (2009)
- SOU (2006), in: Swedish Commission on Climate and Vulnerability (2007)
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- Kundzewicz (2006)
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- IPCC (2012)
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- Heinrich and Gobiet (2012)
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- Arheimer and Lindström (2019)
- Kuentz et al., (2017), in: Arheimer and Lindström (2019)
- Arheimer et al. (2017), in: Arheimer and Lindström (2019)