River floods Austria
Austria: Vulnerability - Trends in the past
A general increasing trend in flood magnitude in Austria has been found especially in catchments smaller than 500 km2 (37). Others concluded, however, that so far no trend can be observed of an increased frequency and severity of extreme climate events in the context of a long-term climatic change (27,31). The severe floods in 2002 and 2005, and the extreme hot and dry summer 2003, however, highlight the potential impact of weather-related disasters in Austria (24). Flood losses already account for two-thirds of all economic losses in the European Alps due to natural disasters in the period from 1980 to 2005 (35). Alpine countries suffered from economic losses of € 57 billion caused by natural hazards—only in the period from 1982 to 2005 (35).
Austria: Vulnerabilities - The 2005 flood
During the second half of August 2005 severe flooding occurred in the northern central Alps, especially in Tirol, Vorarlberg, Salzburg, Steiermark, Kärnten and Niederösterreich (26). Following heavy precipitation on August 21 major Swiss cities like Interlaken, Luzern and the capital Bern were flooded. On the following day a number of rivers in southern Bavaria including Lech, Iller, Isar, Inn and also Danube flooded vast areas. Amongst other cities Garmisch-Partenkirchen, Kempten and Regensburg were affected by major inundations. In some regions the river gauges even exceeded the level of the century flooding of 1999 (21).
The flood that occurred in the second half of August 2005 at the northern Alps resulted from extreme rainfall on saturated soils (21,26). The extreme rainfall itself was caused by a rather rare meteorological situation implying the advection of warm and humid air masses from the Mediterranean that circled eastward around the Alps and cooled down on the northern side which led to condensation and continuous rainfall. Furthermore, due to forced lifting at the northern slopes of the Alps precipitation locally reached outstanding intensities (21). Precipitation intensity strongly varied locally: in Vorarlberg, 50-230 mm rain fell in 24 hours (the average volume being 120-220 mm during July-August) (26).
Locally, infrastructure (telecommunication a.o.) was destroyed, and freshwater supply and waste water treatment plants were damaged. Total damage for Austria was estimated to be € 555 million. 5 people drowned (26). Total monetary losses were € 2.6 billion in Austria, Switzerland, and Germany (34).
In Vorarlberg almost all rivers flooded at water levels that exceeded 100 years return periods. Slightly lower flood levels, however, had also been reached in 1999 and 2002. That same year total damage in Vorarlberg was estimated to be € 178,2 million. In Tirol flood return periods were estimated to be 100-1000 years. Bridges, roads and buildings were destroyed. In 2006 total damage in Tirol was estimated to be € 264 million. The tourist city of Lech was one of the places that suffered most damage; the city centre flooded and 40 buildings were severely damaged when the river Lech reached its highest discharge ever recorded (26).
The river discharges by far exceeded the boundaries of flood protection. Designed protection level of the civil engineering works was a 100 years return period. Return periods of the peak discharges were estimated ranging from several years, to over 100 years, and even up to 5000 years (26).
Given long records of station observations within the affected region neither a long term positive trend of monthly mean August precipitation (which can be used as an indicator for water saturation of soils) nor positive trends in the very extreme daily precipitation sums can be detected. Therefore, this individual event cannot be regarded as a consequence of long-term global climate change (21).
Austria: Vulnerabilities - Projections for the future
In Austria, due to the complexity and the topographically induced variability of the Alpine climate, regional climate models are especially uncertain regarding the prediction of future changes in frequency and magnitude of floods (40). A comprehensive study of climate change impacts on flood frequency shows no clear climate signal for Austria (41). In a comprehensive study of climate change adaptation in Austrian water policy (41), flood peaks with a return period of 100 years were assumed to change by -4 % (decrease) to +10 % (increase) for the period 2021-2050 compared with the period 1976-2007).
Regional climate change scenarios indicate an increase of precipitation intensity and most models also show an increase in extreme precipitation especially in winter in the alpine region (22). Several studies focus on the so-called Vb cyclone. This weather pattern caused heavy damage in Austria in 1999, 2002 and 2005. Scenarios indicate a decrease of the probability for this weather pattern in the future (23), but no information about a potential increase in intensity can be gained from the GCM (24).
Case study river Lech
River runoff was simulated for the Lech basin (1000 km2), located in the Northern Limestone Alps for present (1971–2000) and future (2071–2100) climate conditions (two GCM’s, statistical downscaling, A1B and A2 scenarios). Future projections show a reduction of the intensity of mean annual floods by about 15%. For summer floods, this decrease is more pronounced by about 25%. In contrast, for the intensity of winter floods an increase by 50% is projected. For spring and autumn no clear tendencies were obtained (32).
The approach projects a slight decrease of annual maximum flood events up to a return period of around 10 years. For the most extreme events, instead, no clear signals were obtained as these projections are highly uncertain. The timing of the annual maximum floods significantly extents from around 7 months under present conditions to 12 months in the future scenarios (32). In addition to the consequences of climate change, the expansion of settlement and industrial areas into the floodplains will probably increase flood risk along the river Lech in the next decades (33).
The spatial development of the assets at risk in the Alpine Lech Valley, particularly of residential areas, due to land use changes has been studied over a historic period (since 1971) and up to possible shifts in the future (until 2030). Historical, present, and future land use data were combined with three inundation scenarios: return period of 30, 100 and 200 years. ‘Worst case’ inundation scenarios were assumed by disregarding technical protection in the inundation scenarios and deriving the total loss to assess and compare the upper limit of potential damage over time. The analysis revealed land use changes in the alpine study area like urbanization and the decline of agriculturally used grassland areas. Although the major agglomeration of residential areas inside the flood plains took place before 1971, a steady growth of values at risk can still be observed until now. Even for the future, the trend is ongoing, but depends very much on the assumed land use scenario and the underlying land use policy. The annual growth rate of the damage potential of residential areas amounted to 1.1 % between 1971 and 2006; this growth rate is projected to continue by 1.0% between 2006 and 2030 (‘constant values,’ i.e., asset values at constant prices of reference year 2006) (36).
Europe: casualties 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 (30).
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
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 (30).
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 (28).
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 (29).
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 (29).
A more integrated approach to the management of floods should address all phases of the flood risk cycle, including activities and measures on prevention, protection, preparedness, emergency response and recovery after the flood event. Integrated flood risk management calls for the co-operation of all public authorities and other parties concerned. Promoting public participation and awareness-raising are key prerequisites for the successful implementation (25).
Flood events in the past few years have shown that even well-built flood control systems may fail if the discharge exceeds design values. This residual risk must be reflected in the planning process and has to be properly communicated to the public that is potentially affected (25).
Within the next ten years a special effort should be made to identify inundation areas and to speed up hazard zone mapping (25).
Recently, the Lech River has been widened and renaturated, which may have reduced the flood peak. For the upcoming years, flood control strategies are limited to the improvement of existing dams or construction of pebble and boulder traps close to the built-up areas as the Lech River was declared as ‘Natura 2000’ protection area in 2000 (36). For the river Lech adaptation by non-structural measures such as stricter land-use regulations or enhancement of private precaution may be capable of reducing flood risk by around 30 % until 2030 (38,39).
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.
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Austria.
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