River floods Italy
Vulnerabilities - Italy
From 1991 to 2001 about 12.000 landslides and more than 1000 floods have occurred in Italy. The major flooding events in 2003 affected more than 300,000 people and caused an economic damage of more than 2 million Euros. Besides, there are many smaller flooding events which damage agricultural areas and urban areas, causing significant damage but no human victims (21).
The total costs of reducing the risks of floods and landslides in Italy is estimated at 42 billion Euros. But this estimate does not take into account the higher risks from climate change scenarios, for which no assessment currently exists. Past experience shows that the 28 large floods that hit Italy between 1939 and 2004 caused 694 victims, made 1.5 million people homeless, affected 2.85 million people and caused 32.7 million US$ worth of damages (22).
A technical report by the Ministry of the Environment and Land Protection (3) quantifies the areas with high risk of flooding: they cover an area of 7774 km2, corresponding to 2.6% of the national territory. The most dramatic floods in Italy occurred in the Po (1951, 1994 and 2000) and Arno river basins (1966).
Alterations of the Alpine hydrological system due to changes in precipitation, snow-cover patterns and glacier storage that, further modifying run‑off regimes, will lead to more droughts in summer, floods and landslides in winter and higher inter-annual variability. In the Italian central Alps, rivers could experience an increased winter run-off by 90% and a decreased summer run-off by 45% (35).
Vulnerabilities - Italy - Po River
Current flood risk
In Italy, the most vulnerable area is the River Po basin in northern Italy (22). The central-eastern Po Plain in northern Italy is a rapidly subsiding sedimentary basin that hosts about 30% of the Italian population and 40% of Italy's total productive activities.
Subsidence rates range from 0 to 7 cm/year, the maximum occurring in synclinal areas at the Po Delta and near Bologna. In the eastern Po plain the recent effects of human activities on subsidence have been judged to be as great as those resulting from long-term natural processes. It is suggested that the main factor controlling modern subsidence is water withdrawal, which was particularly intense during the second half of the 20th century, coinciding with accelerating economic growth. There is a clear-cut correlation between flood frequency and rapid subsidence. In contrast, few floods occurred in low subsidence areas. The anthropic-caused increase in subsidence has now greatly increased the potential for additional flooding (24).
The present-day network of the Po River is commonly divided into four main stretches: the Upper Po (with a drainage area of approximately 37,000 km2), the Middle Po (68,000 km2), the Lower Po (70,000 km2) and the Po Delta (25).
The river morphology has evolved from a geometry with irregular meandering channels controlled by discontinuous levee alignments and riverbank protection (early nineteenth century) to a geometry with artificial meandering or straight channels that are controlled by flood corridors and continuous riverbank protection (26). The present levee system, which was completed during the 1960s by the “Magistrato per il Po”, is a flood canal along the final 420 km of the Po River watercourse (including also the final stretches of its tributaries), with a remarkable channel storage capacity in the Middle Po stretch. The major outcomes of the progressive expansion of existing levee systems are a decreasing flood proneness of the Upper Po sub-basins (e.g., Sesia, Tanaro and Ticino rivers) and an increasing vulnerability of Po Plain areas (Lower Po, Po Delta) to hydrological hazards, which is seen as an increasing value of the flood peak with given probability (27).
The annual regime of the Po River, which is characterized by two low-water periods (winter and summer) and two flood periods (late fall and spring), is strongly influenced by the seasonal pattern of precipitation. The first flood period reflects the intensification of rainstorms in late fall, while the second flood period reflects the contribution of snowmelt processes in the most elevated portions of the catchment (28).
Concerning the seasonal to interannual response to climatic forcing, a robust dependence of wintertime precipitation and discharges on the state of the NAO is assessed on a centennial time scale, this dependence resulting in stronger (weaker) precipitation and higher (lower) discharges during negative (positive) anomalies of the NAO index (25).
Agriculture is the main land use over the Po Plain, which explains the huge total length of the network of artificial canalisations over the basin (about 16.750 km), and in particular the impressive 85 km-long Cavour irrigation canal, which was opened in 1866 and diverts up to 110 m3/s from the Upper Po to the Ticino (25).
Average Po River discharge is about 1,500 m3/s. After 1916, flooding episodes with daily peak discharge above 8,000 m3/s occurred, specifically in 1917, 1926, 1928, 1951, 1976, 1994, 2000 and 2008, with the absolute maximum discharge observed on 20 May 1926 (9,780 m3/s). The minimum daily discharge observed during the 1807–1916 period was recorded on 12 May 1817 (277 m3/s). After 1917, at least five drought events culminated in a discharge minimum below 300 m3/s: in 1938, 1949, 2003, 2005 and 2006, the latter event coinciding with the minimum daily discharge ever observed (168 m3/s, on 21 July 2006) (25).
The development of infrastructures along the fluvial network to protect urbanized and farming areas in alluvial plains has increased average river discharge and flood peaks (26). In particular, until the late 1950s, the defence system was made of discontinuous levee stretches (mostly erected since the late nineteenth century) that allowed a natural damping of the peak-flow discharge of the Upper Po tributaries. River discharge increase is also due to the change of the damping effect of many great sub-alpine lakes on high discharges because of lake management, specifically because of the regulation of the water surface level by sluice-gates.
In the period 1831–2003, precipitation has significantly increased in winter whilst changes in the rest of the year are apparently negligible; evapotranspiration has increased in summer; discharge values have increased in winter and strongly decreased in summer (25).
The Po River flood of 2000
The flood of the Po River on 13-16th October 2000 was the most intense event of the last 200 years in the Piedmont area. Values for precipitation larger than 600 mm in 96 hours have been reported. With this event the Northwest of Italy experienced one of the largest floods on record. Some rivers’ flood values have reached the 200 years return period curve. Part of the city of Turin was damaged. Only a few people died (29).
According to the media Po River floods in the past have been largely due to ‘human greed, incompetence and complacency’ and should not be called natural disasters (30). The death toll of the 2000 flood was 28, 21 of them drowned in Italy.
According to the article in the Guardian:
For six days rainstorms ravaged Italy's northeast and the Swiss Alps, turning streams into rivers, rivers into torrents and streets into canals.
In the past, swollen rivers drained harmlessly into fields that lined their banks. That was why they were called flood plains. But farmers flattened, cultivated and smoothed the fields so the water surged into towns instead. A 1994 law forbade any new buildings within 150 metres of rivers. But developers edged right up to the banks, illegal but easy to arrange with the connivance of corrupt zoning officials. Unbelievably, some buildings were actually sited on river beds. Others were plonked on landfills near rivers, narrowing their flow.
The beds of tributaries of the Po river were cemented over, cramping the flow and making it faster. The World Wildlife Fund and other environmental groups repeatedly warned that hydraulic projects in the Dora Baltea river in the northeast were dangerous. But warnings were ignored. A state of emergency remained in force in the northeast, where road, rail and telephone links were cut. Factories, schools and shops were closed in many towns and cities.
"Out of respect for the victims of this and past tragedies, authorities should spare us their attempts to justify decades of inappropriate measures and inaction with references to the cruelty of the weather, to the country's complex fluvial system and to the fables of natural disaster," said the Corriere della Sera newspaper. An independent panel of experts who studied the Po river said the calamity was "strongly dependent upon the actions of man". … Deforestation, unregulated construction and inadequate drainage maintenance were also blamed for the death of more than 140 people in a mudslide that buried the southern town of Sarno in May 1998. What emergency crews are really clearing up is the rubble of the state.
According to the media the 2008 Po River flood was also due to “too much building, concreting-over and canalization”.
Future flood risk
A very strong increase (locally more than +40%) in the 100-year flood level of the Po River is projected by the end of the century (2071-2100) under the SRES A2 emissions scenario. For the Po River, the projected future return period of a current 100-year flood is projected to be less than 20 years. Less strong increases in extreme flood levels were also found in several other rivers on the Italian peninsula (37).
Projections of future flood risk in Italy are not univocal, however. Others researchers project a mean decrease of annual average flood risk by 2030 of approximately 18%, based on several models and two emission scenarios (SRES A1B and A1B-2016-5-L), with projections ranging from a decrease of 40% to an increase up to 20%. By 2100 the models become more evenly divided between increases (up to 100%) and decreases (up to 75%), although a majority still projects a decline; the mean of all projections for 2100 is a decrease of average annual flood risk of 10% (38).
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 (39).
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 (39).
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 (32).
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 (33).
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 (33).
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 (34).
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 - Italy
Adaptation measures in Italy include (36):
- A law on the implementation of hydro-geological protection. It requires the authorities responsible for hydrological basins management to detect risk areas, set prevention plans and establish regulations to avoid additional risk due to anthropogenic factors. It is also the legal basis for the identification and funding of urgent preventive measures.
- A government directive on the establishment of an integrated warning system at the national and regional level that includes (1) the monitoring of hydro-pluviometric data and water availability, (2) a group of national experts in seasonal weather forecasting and climatology with the aim to update the scenarios for the next three-month period, (3) the implementation of a network of centres for data processing, supporting decision-making for civil protection and warning for hydro-geologic and hydrologic risk, (4) the promotion, financing and coordination of technical and scientific initiatives aimed at widening the knowledge base on extreme weather events and at applying it to the development of early-warning, evaluation and real-time monitoring tools, and (5) the implementation of a national Radar Plan for nowcasting.
In Italy, in the wake of the floods that plagued the northern part of the country in the fifties (Polesine, Po valley) and in the sixties (Florence, Arno river catchment), a process was set in motion aimed at developing a new integrated approach to water management – at the catchment level - suitable to serve as a framework designed to prevent, mitigate, prepare for, respond to and recover from the effects of floods and other water – related disasters. This framework, known as “River Authority”, was designed to cope with water management and flood hazard mitigation issues within each of the main Italian catchments (31).
With regard to the plans, it is important to remind that the “River Authorities”, for the most important catchments, define the “A”, “B” and “C” bands which are drawn around the river to border the different hazard levels:
- the “A” band is positioned to the limits of the flooding due to a discharge equal to the 80% of the one of 200 years return period;
- the “B” band is the same, but with a discharge 100% of 200 years return period;
- the “C” band is positioned to the limits of the flooding due to a discharge with return period equal to 500 years, or, if available, an historical value which gave a disaster in the area.
While within the limits of the bands “A” and “B” the urbanisation is ruled by the “River Authority” (and strictly prohibited or limited to necessary maintenance of existing building), the activities within the “C” band have to be ruled by the Municipalities. The “C” band is divided in different regions with a different degree of hazard (and so with different rules related to the possibility of urbanisation), because they are usually very large and the impediment of using that large areas would bring an economic loss (31).
In Milan the flood prone area of the Lambro river has been divided into areas with 4 different risk levels, ranging from 1 (less dangerous) to 4 (extremely dangerous) (31):
- in the areas with hazard in class 1, no particular reasons against further urbanisations have been determined;
- in the area with hazard 2 (medium risk), new urbanisation is still possible, but the Municipality may require specific studies about hydro-geological features, and it may also be required to build defensive structures;
- in the area with hazard 3 (high risk) applications for new buildings must be equipped with documents concerning the hydro-geological conditions; together with plans for hydraulic and structural safety;
- in the area with hazard 4 (very high risk) no new urbanisations are allowed, and works are permitted only in order to reduce the vulnerability of existing buildings. Strictly forbidden are all the chemical and petrol-chemical activities along with garbage dumps.
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Italy.
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