Norway: Flood frequency trends in the past

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 (33). Overview studies covering many Scandinavian rivers show no statistically significant trend in river peak discharge over the last century (39). 

Currently, about 30% of the annual precipitation in Norway falls as snow. It is expected that climate change will change the contribution of snow and rainfall to total precipitation in different ways for different parts of the country. As a result, the magnitude and frequency of rainfall versus snowmelt driven floods in Norway will also change. The western part of Norway and the coastal zone experience high flows particularly during autumn and winter as a result of heavy rainfall. The inland and northernmost parts of Norway experience high flows particularly during spring and early summer due to snowmelt. There is increasing evidence for recent changes in the intensity and frequency of heavy precipitation and in the number of days with snow cover in many parts of Norway. The question arises as to whether these changes are also discernable with respect to their impacts on the magnitude and frequency of flooding (34).

Over the last century, and particularly since the end of the 1970s, mean precipitation has increased in the whole of Norway by about 18% (35). Also the intensity and frequency of heavy precipitation events has increased in most parts of the country, particularly in southern and western Norway and in some coastal areas in northern Norway (36). The total amount of snow in the winter (expressed as snow depth (SD) and snow water equivalent (SWE)) significantly increased in the colder inland and northernmost regions of Norway (37). This agrees with the expected impact of global warming: it is likely that some inland and high-altitude areas will still accumulate more snow during winter until the end of the 2050s (35). However, other parts of the country, characterized by comparatively warmer winter climates, already show negative long-term trends in observed SD and SWE. The projections also indicate that such trends will affect larger regions and higher altitudes towards the end of the 21st century.

Peak flow discharge series for 211 catchments spread all over Norway show that flood frequencies have been increasing since 1962 in southern and western Norway, mainly due to increase in the frequency of rainfall dominated events. In the same time flood frequencies have been decreasing in northern Norway, mainly due to decrease in the frequency of snowmelt dominated floods. In Norway, rainfall has become more important as a driver of floods, while the role of snowmelt has become less important. Besides, the timing of snowmelt-dominated floods has become earlier, by on average 5 days per 10 years over the period 1962–2012 (34). These results agree with future projections of the impact of climate change (38): there is strong evidence that rainfall will continue to gain an increasing importance in flooding in Norway, and it will replace snowmelt as the dominant driver of floods in most catchments where snowmelt is the most important driver under current conditions. Only in high-altitude catchments, where precipitation will continue falling as snow, is snowmelt likely to remain the most dominant driver of floods in the future.

Trends in the frequency of peak flow events appear to be more systematic and more pronounced than trends in the magnitude of peak flow events. Particularly for rainfall generated floods, changes in the frequency of extreme precipitation are stronger than changes in the precipitation intensity in Norway (36). 

Norway: Flood frequency trends in the future

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%) (32).

Annual average river runoff is projected to increase in northern Europe by approximately 5 to 15% up to the 2020s and 9 to 22% up to the 2070s, for the SRES A2 and B2 scenarios and climate scenarios from two different climate models (21). The risk of floods increases in northern, central and eastern Europe. Increase in intense short-duration precipitation in most of Europe is likely to lead to increased risk of flash floods (22).

In Norway, the typical hydrological regime is winter low flow, a clearly defined snowmelt flood in spring, summer low flow and the occasional autumn flood due to precipitation as rain (23). The characteristic spring flood of today will be more irregular and less intense, on average. This is because the snow period will be shorter and the snow depth less due to the warming effect. However, the water supply is expected to increase in winter and also in autumn as a result of heavier precipitation. Accordingly, the risk of flooding will diminish in spring but increase in late summer and autumn, particularly in the north (24). The runoff is projected to be reduced during summer; in regions with glaciers an increase is estimated, however, even for the summer season (25).

Recent results (2011)

Ensemble modelling based on locally-adjusted precipitation and temperature data from 13 regional climate scenarios has been used to assess likely changes in hydrological floods between a reference period (1961-1990) and two future periods (2021-2050) and (2071-2100), for 115 catchments distributed throughout Norway. These projections show that western region of Norway and Nordland are associated with the largest percentage increases in the magnitude of the mean annual flood (> 20%) and floods of longer return periods. In addition, catchments located near the coast tend to have an increased flood magnitude in all regions, with the exception of Finnmark. Large catchments with source areas located in inland regions, such as Oppland and Hedmark and parts of Trøndelag, and in the northernmost region, Finnmark and Trøms, are generally expected to have reduced flood magnitudes under a future climate (30).

Differences between regions largely reflect the relative roles of snowmelt versus rainfall in the flood regimes in the catchments. Warmer winter and spring temperatures leading to reduced snow storage and earlier snowmelt will most likely bring about a reduction in the magnitude of floods derived primarily from snowmelt in the future. Simultaneously, increases in autumn and winter rainfall throughout Norway will increase the magnitude of peak flows during these seasons. In areas already dominated by autumn and winter floods, the projected increases in flood magnitude are large. There are also areas currently dominated by snowmelt floods in which autumn and winter rainfall floods are expected to become increasingly important in the future. In some cases, this change in seasonality will also lead to an overall increase in the magnitude of the mean annual flood and of floods of longer return periods. ... A change in seasonality can also have significance for floods associated with other processes such as river ice breakup and jamming (30).

The projected changes in the mean annual flood, the 200-year flood and the 1000-year flood all exhibit similar spatial patterns in which moderate (15 – 30%) to large increases (> 30%) in the magnitude of the maximum daily averaged discharge are expected in western and south-western Norway, in south-eastern Norway in catchments located near the coast, and in Nordland. Projected increases for the 200-year flood exceed 40% for some of the catchments in western Norway and in Nordland. On the other hand, large decreases are projected for inland regions such as Hedmark and in Finnmark. The regions between the zones of large increases versus large decreases generally have transitional values of projected change (i.e. small increases or decreases of < 15%). Catchments with source areas in the mountainous regions of southern Norway (east of the north-south water divide), and in the Trøndelag, and Troms regions tend towards small to moderate changes, although there are local exceptions (30).

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 (31).

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

IPCC conclusions

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 (31).

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 (27).

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 (28).

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 (28).

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 (29).

Adaptation strategies - Norway

The process of climate change adaptation has received little attention in Norway in the past (26). In fact, most impact studies undertaken assume that adaptation will take place automatically, once the sectoral impacts are known. There are several examples in Norway of situations where the capacity to adapt to current climate variability is flawed or may be narrowing.

A flood in eastern Norway in 1995 caused massive damages and was in some areas the largest in 200 years. Despite considerable efforts to improve flood management after 1995, there are cases where municipalities have allowed new construction in affected areas without special measures to prevent future flood damages. A recent study of institutional responses to the 1995 floods concluded that current institutional frameworks provide weak incentives for proactive flood management at the municipal level. Pressures from powerful interest groups to develop business and residential housing often lead to construction in areas that are known to be exposed to floods, avalanches, or mud slides. Such findings support the need for more comprehensive studies of institutional factors determining sensitivity and mediating adaptation at the local level (26).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Norway.

1. Hoyois and Guha-Sapir (2003), In: Anderson (ed.) (2007)
2. Anderson (ed.) (2007)
3. Mitchell (2003)
4. Barredo (2009)
5. Höppe and Pielke Jr. (2006); Schiermeier (2006), both in: Barredo (2009)
6. Höppe and Pielke Jr. (2006), in: Barredo (2009)
7. Becker and Grunewald (2003); Glaser and Stangl (2003); Mudelsee et al.(2003); Kundzewicz et al.(2005); Pinter et al.(2006); Hisdal et al.(2007); Macklin and Rumsby (2007), all in: EEA, JRC and WHO (2008)
8. EEA, JRC and WHO (2008)
9. Wang et al.(2005), in: EEA, JRC and WHO (2008)
10. Milly et al. (2005), in: EEA, JRC and WHO (2008)
11. Hisdal et al. (2007), in: EEA, JRC and WHO (2008)
12. Ramos and Reis (2002), in: EEA, JRC and WHO (2008)
13. Barnolas and Llasat (2007), in: EEA, JRC and WHO (2008)
14. Lehner et al.(2006); Dankers and Feyen (2008b), both in: EEA, JRC and WHO (2008)
15. Christensen and Christensen (2003); Kundzewicz et al.(2006), both in: EEA, JRC and WHO (2008)
16. Palmer and Räisänen (2002), in: EEA, JRC and WHO (2008)
17. Kay et al. (2006); Dankers and Feyen (2008), in: EEA, JRC and WHO (2008)
18. Andréasson, et al. (2004); Jasper et al.(2004); Barnett et al.(2005), all in: EEA (2009)
19. Arnell (2004); Milly et al. (2005); Alcamo et al. (2007); Environment Agency (2008a), all in: EEA (2009)
20. Dankers and Feyen (2008), in: EEA (2009)
21. Alcamo et al. (2007)
22. EEA (2004b), in: Alcamo et al. (2007)
23. Gottschalk (1987), in: Bache Stranden and Skaugen (2009)
24. Eisenreich (2005)
25. Ministry of the Environment of Norway (2009)
26. O’Brien (2006)
27. Ciscar et al. (2009), in: Behrens et al. (2010)
28. Kundzewicz (2006)
29. Kundzewicz (2002)
30. Lawrence and Hisdal (2011)
31. IPCC (2012)
32. Heinrich and Gobiet (2012)
33. Wilson et al. (2010), in: Mediero et al. (2014)
34. Vormoor et al. (2016)
35. Hanssen-Bauer et al. (2015), in: Vormoor et al. (2016)
36. Dyrrdal et al. (2012), in: Vormoor et al. (2016)
37. Dyrrdal et al. (2013); Skaugen et al. (2012), both in: Vormoor et al. (2016)
38. Vormoor et al. (2015), in: Vormoor et al. (2016)
39. Bering Ovesen et al. (2000); Forland et al. (2000); Hyvarinen (2003); Lindström and Bergström (2004); Thodsen (2007), all in: Kwadijk et al. (2016)