Switzerland: Vulnerabilities – Trends in the past

After a 100 year period between 1875 and 1977 during which only two major floods occurred, Switzerland experienced several catastrophic flood events during the last three decades. This apparent increase in frequency of extreme floods since the 1970s coincided with significant investments that have been made in fundamentals, planning and protective measures in the area of natural hazard prevention. Nevertheless, the total annual loss resulting from floods, landslides and debris flows has increased substantially since 1972 (25).

Since the early 19th century, there have been 16 extensive or very extensive floods necessitating supra-cantonal intervention. Nine of these occurred during the last 30 years. In today’s monetary terms, the damage caused by major floods totals between CHF 500 million and several billions (25). In the 19th century, such events often claimed dozens of lives. Thanks to comprehensive preventive measures and improved emergency response, the number of victims has decreased significantly in the intervening period (29).

The lack of river maintenance, concreting of the river banks, and the waterproofing of urban areas represent aggravating factors for floods. Of all hazards in the Alpine regions, floods are creating the most economic damages, and many of the most densely populated areas of the Alps have been hit severely by floods in recent years (22).

Large floods in mountain basins are now more frequent than in the past. From an analysis of peak discharge time-series recorded in 27 gauging stations in the Swiss Alps a significant increase was found of flood peaks during the last century. This may be an effect of recorded increases of temperature and precipitation in the same period (21). Significant upward trends in flood magnitude were also found in some Swiss alpine catchments by (35).

Switzerland: Vulnerabilities – The 2005 flood

At almost CHF 3 billion, the floods of August 2005 gave rise to the most extensive total financial losses ever caused by a single natural event in recent decades in Switzerland. Six people lost their lives in the floods and landslides. The overall picture of the flood of August 2005 ultimately reflects the pattern that previously characterised such major events (29).

Several floods occurred in the 19th century whose extent of damage actually matches or exceeds that of 2005. Thus, when viewed in the context of this longer period of time, the extent of the damage caused in August 2005 appears less unique than it does from the short-term perspective. Hence, it may be assumed that the flood of August 2005 was not a unique event and that the repeated occurrence of similar events must also be expected in the future (29).

None of the channel and slope processes that occurred in August 2005 arose for the first time. Nevertheless the events of August 2005 had one unique characteristic: first, the floods affected a very wide area, i.e. from the Alpine region to the Alpine foothills and into the Swiss Central Plateau, and, second, in some locations, the damage that arose as a result of the extreme intensity of the channel and slope processes was particularly extensive (29).

Switzerland: Vulnerabilities – Projections for the future

It seems likely that alpine climate change will lead to changes in timing and amount of run-off in European river basins and that floods and droughts will become more frequent. The projected decline in precipitation in the Alps plus rise in temperature could produce a 40-70% reduction in runoff (23). Higher temperatures in winter reduce the amount of spring snowmelt, raise the evaporation, and hence reduce run-off in spring. Temperature increase leads to an expansion of the vegetation period and increases transpiration reducing discharge. On the other hand, glacial melting is enhanced leading to greater discharge during summer (28).

Winter floods will become more frequent. Under the hypothesis of a 2⁰C temperature increase and of 10% increase in the precipitation intensity, the 100-year flood discharge will reduce its return period to about 20 years (21).

In 2003 the knowledge of the relationship between climate change and extreme events in Switzerland was summarized in the following conclusions (27):

• At present, natural catastrophes are observed to occur more frequently. This could either be accidental, the result of natural long-term climate change, or of climate change from anthropogenic causes. It is difficult, or may even be impossible, to identify or exclude a statistically valid trend in the frequency of rare extreme events. Indeed, it may not prove possible to positively identify long-term changes in the frequency of extreme events until their extent has become very considerable and extensive damage has been caused.
• In contrast, statistical predictions are possible for trends in 'intensive' events. It can be shown, for example, that heavy precipitation events (which do not usually lead to damage) have become more frequent since the beginning of the last century. Also, the volume of precipitation in winter has increased substantially in almost all parts of Switzerland since the beginning of the last century.
• Our present knowledge of meteorological processes suggests that the frequency and intensity of certain extreme events (heat waves, heavy precipitation and floods in the lower regions during the winter months, drought to the south of the Alps in summer and in the inner Alpine valleys, and landslides) will increase with the change in climate. This anxiety is corroborated by calculations using climate models. In contrast, the frequency of days with frost and very cold periods will decrease.
• The probability and geographical distribution of extreme events will alter gradually with the change in climate. The extent and character of the changes will differ depending on the location and character of the extreme events. It is not at present possible to give a quantitative assessment of these effects.

An increase in precipitation intensity and the rise in snow line bring about the potential for more frequent floods, landslides and debris flows (24,25). However, the actual incidence of these natural hazards is determined by other factors, such as soil moisture, soil cover, snowmelt, discharge regime, topography, and size of catchment areas (24).

In small catchment areas of the Swiss central plateau and in the Alps, the biggest floods usually occur in summer after short but heavy thunderstorms. It is inconclusive whether a change in frequency will occur and in what direction (25).

A recent expert assessment (26) concludes that to the north of the Alps and at altitudes up to 1500 m a.s.l. the size and frequency of winter and spring floods is likely to increase. In contrast, summer floods on the central plateau are expected to become smaller. To the south of the Alps, floods tend to become more severe in all seasons except for summer. Due to rising precipitation intensities, landslides are expected to become more frequent in winter and spring.

Water tower of Europe

Because the Alps are the primary source for such major rivers as the Rhine, Rhone, Po, and Danube, the impact of reduced mountain precipitation would be felt far beyond the mountainous regions themselves (23).

Within a 30-km radius in the Gotthard region of central Switzerland, surface runoff feeds river systems that flow into the North Sea (the Rhine River basin, representing about two-thirds of total water exported from Switzerland), the Mediterranean (the Rhone River basin, with about 18% of water exports), the Adriatic (the Ticino that converges with the Po River in Italy and represents 10% of runoff from Switzerland) and the Black Sea (via the Inn that converges with the Danube in Germany, and accounts for about 5% of surface runoff) (33). Any change in precipitation regimes, snowpack depth and duration, and the speed of glacier retreat in this very small part of the Alps will have important impacts not only for the mountain valleys but also for the populated lowland regions of Germany, south-eastern France, northern Italy, and central and eastern Europe that depend on alpine water resources for an important part of their water supply (32).

Because of the large interannual variability of runoff, as a result of the sharply curtailed glacier mass in the mountains and possibly long and dry summers, the volume of summer- time glacier melt waters may no longer be sufficient to feed water into river catchments at a time of the year when precipitation amounts are low and the snowpack has already melted earlier in the year (32).

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 (34).

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 (34).

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 (30).

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 (31).

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 (31).

Adaptation strategies - Integrated Flood Protection Strategy

In Switzerland, two thirds of all communes have experienced flooding in the last 30 years. The total loss during this period amounts to CHF 8 billion. In view of the integrated approach to disaster reduction, an integrated flood protection strategy has been developed. The cornerstones of this strategy are (25):

• Analysis and documentation of the existing danger: Hazard maps serve as a basis for prevention measures.
• Safeguard of the required space for flowing water: Sufficient space for extreme quantities of runoff water simultaneously guarantees space for the ecological function of watercourses.
• Integral action planning: It is imperative that the principles of sustainability be taken into account for planning and organisational measures as well as for technical safety constructions.
• Minimisation of damage: Maintenance of watercourses (= maintaining the existing safety conditions) as well as measures for spatial planning (= preventing a rise in the potential for damage by keeping space free or restricting the use of space) are of paramount importance.
• Emergency planning: Good preparation (forecasting, alerting and mobile measures etc.) can minimise the ever present residual risks. In addition, insurances can help make damages bearable.
• Flood protection as a federal task: Interdisciplinary cooperation among experts from all areas and inclusion at a sufficiently early stage of the political authorities as well as the concerned population are a precondition for sustainable protection policies.
• Flood retention in Mattmark reservoir (an example): Whereas most dams and reservoirs were built for hydropower purposes only, there is increasing discussion of their use as multi-purpose facilities. Today, a certain proportion of the water storage capacity in the Mattmark reservoir in the canton of Valais is to be reserved for the retention of water for flood protection. For this purpose, it was necessary to increase by 2 m the height of the crest of the existing side spillway in order to ensure effective flood routing without endangering the Mattmark earthfill dam. For operational purposes, it will be necessary to manage this reserved flood retention volume using adequate and precise inflow forecasts.

The experience gained in the past culminates today in the acknowledgement that a holistic approach must be adopted to the management of flood events: preparedness, response and recovery complement each other and must be even more closely coordinated. This requires comprehensive hazard information which lies at the centre of cycle of risk (29).

Adaptation strategies - Spatial planning and Infrastructure

The expected increase in damage potential as well as the possibility of more frequent floods require higher protection against floods. A possible answer to prevailing uncertainties are so-called no regret measures, such as, for instance, sustainable flood protection: renaturalisation and broadening of rivers will reduce the risk of severe flooding and help minimizing the risks of intensified flood intensities, and even in case of unchanged flood intensity, these measures are beneficial, e.g. for river ecosystems (25).

​Zoning policies can be used to limit the exposure to flooding of people and assets. Zoning regulations entail the determination of areas with a certain flood risk (i.e. the 100-year flood zone) and setting up certain land-use requirements for these zones. Such requirements could constitute, for instance, a complete ban, restricting certain uses, requiring certain building standards, giving recommendations and providing information to inhabitants in certain zones (36). In France and Switzerland, zoning policies include zones where developments are completely prohibited and zones where there are conditional uses or construction requirements (37). 

The flood events of 2005 and 2007 clearly demonstrated that runoff volumes and sediment volumes often far exceeded expectations and the critical loads for some preventive measures were reached or even exceeded. The major challenge consists, therefore, in optimising preventive measures against the background of the uncertainties that always remain in relation to natural hazards (29).

“Safety valves” are needed which can relieve channels that are overloaded, for example through the gradual and deliberate flooding of prepared areas. Such protection concepts have been implemented, for example on the Urner Reuss and Engelberger Aa rivers and have proven successful there (29).

A country-wide hazard assessment is currently being carried out for settlement areas in Switzerland, the results of which are being presented in the form of hazard maps for floods, landslides, rock fall processes and avalanches. Hazard maps provide the scientific basis for the implementation of spatial planning measures. … Hazard maps are expected to be available for all municipalities throughout Switzerland by 2011 (29).

Adaptation strategies - dealing with uncertainties

The shock of major flooding events in the 1990s and early 2000s caused a shift in the framing of flood management policy, to better account for inherent uncertainty and non- stationary conditions. The key issue for policy makers is how this learning process can be replicated so that water managers focus more heavily on adaptive processes in sectors or regions where future stresses may go beyond historical experience. One step towards realizing how to achieve this is to directly link up physical impact information with vulnerability and adaptive capacity assessments at local to regional levels. Another step could be for regional and local policy makers to better share lessons learnt and planning practices across different regions as well as aspects of water management (32).

The increased integration of uncertainty and worst-case scenario planning could also improve the integration of longer term climate related issues into planning at local to regional scales, which may have been resistant or apathetic to contemplating climate impacts in planning decisions (32).

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 Switzerland.

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. Allamano (2009)
22. Agrawala (2007)
23. Bogatai (2007)
24. OcCC (2007), in: Federal Office for the Environment FOEN (Ed.) (2009)
25. Federal Office for the Environment FOEN (Ed.) (2009)
26. KOHS (2007), in: Federal Office for the Environment FOEN (Ed.) (2009)
27. OcCC (2003), in: Federal Office for the Environment FOEN (Ed.) (2009)
28. Eisenreich (2005)
29. DETEC (2008)
30. Ciscar et al. (2009), in: Behrens et al. (2010)
31. Kundzewicz (2006)
32. Beniston et al. (2011)
33. FOEN (2007), in: Beniston et al. (2011)
34. IPCC (2012)
35. Castellarin and Pistocchi (2012), in: Mediero et al. (2014)
36. Merz et al. (2007), in: Kreibich et al. (2015)
37. Fleischhauer (2005); Zimmerman et al. (2005), both in: Kreibich et al. (2015)