River floods Portugal
A trend of decreasing flood magnitudes has been found for Spain and Portugal for the period 1956–1995 (26). No clear trend was found in the occurrences of flash, river and urban floods in Northern Portugal over more than 150 years, according an analysis of a DISASTER databasecovering floods and landslides over the period 1865-2016 that caused deaths, injuries, missing, displaced or homeless people in mainland Portugal during the period of 1865-2010 (32). According to this database, floods in mainland Portugal caused 1012 deaths, 478 injured, 13,372 displaced and 40,283 homeless people in this period (33). Approximately 22% of the recorded floods resulted in human fatalities (34).
Floods have been the most deadly natural disasters in Portugal during the last century, followed by earthquakes. Large river floods are caused by heavy rains associated with a westerly zonal circulation that may persist for weeks. The system of dams within the basin reduces the frequency of flooding, but cannot `tame' the river. The dam system has even contributed to an increase in the peak flow, as in the 1979 flood. Nevertheless, these floods are not a danger for the human population (21).
In contrast, flash floods are more dangerous and deadlier than these large river floods, as demonstrated in 1967 and 1997. They affect the small drainage basins and are caused by heavy and concentrated rainfall, created by convective depressions (active cold pools or depressions caused by the interaction between polar and tropical air masses), active in the south of the country, in the Lisbon region, Alentejo and the Algarve. Deforestation, soil impermeability, chaotic urbanization, building on floodplains, the blockage of small creeks or their canalisation, and the building of walls and transverse embankments along the small creeks all contribute to the aggravation of this kind of flood (21).
The Tagus River
Climate change will affect high and low flows of large rivers around the globe. To what extent extreme flows will change differs from one region to another. These changes have been assessed for five large rivers from three continents (27). Two in Europe: the Rhine River (at the Lobith station in the Netherlands) and the Tagus in Portugal (at Almourol), one in Africa (Niger at Lokoja), and two in Asia (Ganges at Farakka and Lena at Stolb). The assessment was made for the near future (period 2006-2035), mid-century (2036-2065), and end-century (2070-2099), compared with the reference period 1981-2010. A total of 20 climate projections was made: five climate models (GCMs) linked to five hydrological models times four different scenarios of climate change ranging from a low- to a high-end scenario of global warming (the so-called RCPs 2.6, 4.5, 6.0 and 8.5).
In this study high and extremely high flow volumes were defined as the 10 % and 1 % highest discharges of all discharges in a certain period (the so-called Q10 and Q01). Likewise, low flow volumes were defined as the 10 % lowest discharges (Q90). Model projections indicate that both high and low flow volumes will decrease for the Tagus (up to −50 % in 2100). This projected reduction follows the general reduction in precipitation over the region. Similar changes to low flows in the Tagus River were shown in other studies (28,30,31). With respect to high flows, however, opposite results (future increases in high flows) have been reported for the Tagus in a previous study (29).
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 (25).
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 (25).
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 (22).
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 (23).
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 (23).
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 (24).
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 Portugal.
- Hoyois and Guha-Sapir (2003), In: Anderson (ed.) (2007)
- Anderson (ed.) (2007)
- Mitchell (2003)
- Barredo (2009)
- Höppe and Pielke Jr. (2006); Schiermeier (2006), both in: Barredo (2009)
- Höppe and Pielke Jr. (2006), in: Barredo (2009)
- 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)
- EEA, JRC and WHO (2008)
- Wang et al.(2005), in: EEA, JRC and WHO (2008)
- Milly et al. (2005), in: EEA, JRC and WHO (2008)
- Hisdal et al. (2007), in: EEA, JRC and WHO (2008)
- Ramos and Reis (2002), in: EEA, JRC and WHO (2008)
- Barnolas and Llasat (2007), in: EEA, JRC and WHO (2008)
- Lehner et al.(2006); Dankers and Feyen (2008b), both in: EEA, JRC and WHO (2008)
- Christensen and Christensen (2003); Kundzewicz et al.(2006), both in: EEA, JRC and WHO (2008)
- Palmer and Räisänen (2002), in: EEA, JRC and WHO (2008)
- Kay et al. (2006); Dankers and Feyen (2008), in: EEA, JRC and WHO (2008)
- Andréasson, et al. (2004); Jasper et al.(2004); Barnett et al.(2005), all in: EEA (2009)
- Arnell (2004); Milly et al. (2005); Alcamo et al. (2007); Environment Agency (2008a), all in: EEA (2009)
- Dankers and Feyen (2008), in: EEA (2009)
- Comisión de Coordinación de Políticas de Cambio Climático (2007)
- Ciscar et al. (2009), in: Behrens et al. (2010)
- Kundzewicz (2006)
- Kundzewicz (2002)
- IPCC (2012)
- Mediero et al. (2015)
- Pechlivanidis et al. (2017)
- Prudhomme et al. (2014); Roudier et al. (2015), both in: Pechlivanidis et al. (2017)
- Roudier et al. (2015), in: Pechlivanidis et al. (2017)
- Eisner et al. (2017)
- Krysanova et al. (2017)
- Santos et al. (2018)
- Zêzere et al. (2014), in: Santos et al. (2018)
- Pereira et al. (2016), in: Santos et al. (2018)