Flash floods and urban flooding: European scale
Vulnerabilities - flash floods due daily rainfall extremes
Flash floods caused on average 50 casualties per year in Europe, 70% of the total number of deaths due to floods (8). The fatalities mainly occur in countries surrounding the Mediterranean Sea, where large population density exists at the coastal areas due to the important urbanization processes in this area during the last decades (9). Particularly in Spain, almost 90% of the victims due to floods are caused by flash floods (10).
Changes in river flow extremes at a +2°C global warming are currently of central interest as this is the global target defined by policymakers to lower international greenhouse gases emissions. The impacts of a +2°C global warming on extreme floods (due to daily rainfall extremes) have been assessed for Europe for 1 in 10 and 1 in 100 year events. This was done for a combination of different models (global and regional climate models, hydrological models); the changes in floods were compared with the reference period 1971 - 2000 (2).
For floods (due to daily rainfall extremes) the results indicate a clear North to South gradient in future changes in flood magnitude:
- South the 60°N line: a strong increase in flood magnitudes, due to changes in extreme rainfall, except for some regions in Bulgaria, Czech Republic, Poland, the western Balkans, the Baltic countries, and southern Spain where no significant changes can be detected. Floods are even increasing in areas such as southern Mediterranean where the average discharge is projected to decrease (3).
- Above the 60°N line: a relatively strong decrease in flood magnitude in parts of Finland, NW Russia and North of Sweden with the exception of southern Sweden and some coastal areas in Norway where increases in floods are projected. Projections of decreasing flood magnitudes are mainly due to the decreases in snowpack in areas where most of the floods are caused by spring snowmelt in combination with rainfall. Increases in flood magnitude in Scandinavia are mainly seen in coastal areas where the rain-fed floods will increase (2).
Intense rainstorms are moving more slowly, causing more floods
Intense rainstorms can flood cities and produce deadly flash floods. In previous decades, these extreme precipitation events were quite common in especially southern Europe. In the summer of 2021, we’ve seen such an event over parts of Germany, Belgium and the Netherlands, causing more than 200 casualties and billions of Euros in damage. With global warming, these events are expected to occur more frequent throughout Europe for two reasons (23).
The first reason is an obvious one. Warmer air can hold more moisture, about 7% per degree of warming. That’s why these intense rainstorms are typically a summer phenomenon. The second reason may come as a surprise. Rainstorms are expected to move more slowly with global warming, exposing a certain area to the extreme precipitation of one event for a longer period. This slowing-down is linked to the difference in temperature between the poles and the tropics. This difference determines the strength of winds high in the atmosphere. A larger difference leads to stronger winds. But the poles are warming much faster than the tropics, and this difference is getting smaller. With slower winds, rainstorms move more slowly from one place to another (23).
Not all intense rainstorms are moving slowly. Only about 3 to 4% are slow-moving events. Changes in their frequency of occurrence are extremely important, however. These are the rainstorms that are most devastating because they can accumulate extremely large volumes of water in a small area, causing flooding (23).
By the end of the century, intense rainstorms may become 7 times more frequent in Europe, according to a recent study. The part of these storms that are moving slowly may become 14 times more frequent. This projection is based on a high-end scenario (RCP 8.5) that most scientist consider unrealistically high. The results do issue a warning, though: A future reduction in the speed of rainstorms could enhance the risk of flash flooding to a much higher level than one would expect from just the higher moist content of warmer air. The higher frequency of (slowly moving) intense rainstorms will affect all of Europe. Just like now, southern Europe will still experience the highest frequency of cases in the future (23).
Vulnerabilities - Urban drainage
An urban drainage system may damp or amplify changes in precipitation, depending on the system characteristics. The combined impact of climate change and increased urbanisation in some parts of the North Sea region could result in as much as a four-fold increase in sewer overflow volumes (6). For Roskilde (Denmark), for instance, a 40% increase in design rainfall intensities was found to increase the current level of damage costs related to sewer flooding by a factor of 10 (4). The actual change in cost will depend on catchment and sewer system characteristics. The impacts of climate change on sewer flood and overflow frequencies and volumes show wide variation. Studies indicate a range from a four-fold increase to as low as a 5 % increase, depending on the system characteristics (5).
Floods and overflows occur when runoff or sewer flow thresholds are exceeded. Given that the response of the sewer system to rainfall may be highly non-linear, the changes in the sewer response may be much stronger than the changes in rainfall. And the impact ranges can even by wider when studying the impacts of sewer overflows on receiving rivers. Sewer overflow mainly occurs in summer and as models project the likelihood of lower river flow in summer in northwestern Europe, dilution effects in the receiving water might be less, thus increasing impacts on river water quality and aquatic life (6). Changes other than those related to climate may also occur in urban areas, such as increased urbanization and more pavement surfaces, and affect or strengthen urban drainage impacts (7).
At many places climate change both increases droughts and heavy rainfall. Too little and too much water are part of the same problem. They may be part of the same solution too: by redesigning the storm water infrastructure of cities.
When it rains hard, the sudden volume of water can overwhelm urban drainage systems and lead to flash flooding. Slowing down this discharge and collecting part of it in reservoirs can overcome this. In fact, by storing it we can safe the water for the future, and we can turn troublesome storm water into a resource. Vegetation planted in and around the reservoir purifies the water because the pollutants become trapped in the soil or the roots, and contaminants are taken up by tissues or even broken down into less harmful substances. The cleaner water can then be further purified and allowed to percolate into underground aquifers beneath the city, where it can serve as a drinking-water source during droughts (1).
The necessary adjustments can be fit in the city’s infrastructure quite easily. By digging a trench, for instance, alongside the pavement filled with gravel and soil, and planted with trees. The trench acts as an intermediary step between the storm drain and the sewer, a sponge, and slows and purifies the runoff to protect the city. There’s an extra advantage to more trees in the cities: their shade relieves the heat during summers (1).
In addition to these adjustments of the urban drainage system, several other measures can be taken that strengthen these positive effects of reducing urban flooding and increasing water storage in aquifers: pervious paving increase infiltration, and green roofs capture storm water, allowing much of it to evaporate before directing the filtered excess into the city’s drainage system (1).
Green versus engineering: (bio)infiltration
Urban green infrastructure may be effective as an integral component of storm water adaptation measures to mitigate climate change-induced flooding (12). The relative efficacy and costs of green infrastructure and conventional engineering adaptation approaches has been quantified for two communities in the Midwest USA (11). Grey infrastructure approaches include upsizing existing storm water pipes and containing excess flood volume in underground storage chambers. Local-scale green infrastructure practices focus on increasing the proportion of watershed runoff draining to bioinfiltration areas. Utilizing over-curb surface storage in areas where structures would not be impacted was also considered as a non-structural adaptation strategy (11).
Model results show that adaptation strategies incorporating storm water (bio)infiltration may reduce overall adaptation costs while providing a more comprehensive suite of benefits than grey infrastructure alone. For a 10-year design storm, substantial reductions in flood volume were predicted by applying bioinfiltration areas to only 10% of a ‘traditional, built-out urban area’. The flood storage volume of existing green infrastructure was much larger, however, than the flood volume that could be handled by introducing bioinfiltration. This led to the conclusion that for the study site preserving the hydrologic connectivity of green infrastructure was a more robust adaptation approach than engineered infiltration approaches (11).
Vulnerability of urban areas to storm water flooding varies according to regional climate patterns and site-specific factors such as topography, drainage system configuration and the capacity of soils and other natural or engineered storage elements to store excess runoff (13). Therefore, the efficacy of green infrastructure versus conventional engineering adaptation approaches will also vary from one urban area to another. Climate-proofing urban storm water networks calls for tailor-made solutions that fit the specific context of a certain urban area.
Green versus engineering: green roofs
Urbanization modifies the hydrologic cycle. Rainfall rapidly runs off the impervious surfaces of buildings and roads, and concentrates in in the drainage network. When it rains hard, and long, peak flows in the drainage network may be exceeded and surface flooding results. In most parts of the urbanized world, stormwater runoff increases for two reasons. First, due to the ongoing urbanization and, hence, change of pervious in impervious surfaces. Second, because global warming increases the intensity of extreme rainfall events. A logic response may be to increase the discharge capacity of drainage networks. This is very expensive, however, and municipalities are trying to avoid this. They are seeking for other options to cope with more extreme rainfall events. One of these options is implementing green roofs.
Whether green roofs are a good alternative to adapt to more extreme rainfall events depends on their performance in stormwater runoff mitigation. This performance has been evaluated for a municipality in Milan, in northern Italy, a city with a combined system, designed to collect both sewage and urban stormwater during rainfall events. The current stormwater infrastructure of this municipality is undersized or non-existent, a situation common in many municipalities globally. For this performance evaluation, the impact of both spatially homogeneous and heterogeneous installations of green roofs was studied. In the latter case, green roofs were concentrated where the drainage network is more prone to high degrees of filling. Calculations were made for different combinations of rainfall intensity and duration (14).
The results show that implementing green roofs can be a valuable strategy to reduce stormwater runoff to the urban drainage networks, both in terms of flow peak and volume. The larger the roof area that is covered with green roofs, the larger the mitigating impact. Also, the impact is larger when green roofs are concentrated near parts of the drainage network that are more prone to high degree of filling. Thus, spatially heterogeneous installations of green roofs may be a more tailor-made solution than homogeneous installations. There is a downside, however. The strategy seems to be most effective for frequent storms of smaller magnitude. For infrequent storms of larger magnitude, green roofs alone are not able to significantly reduce stormwater discharge. For these extreme events, stormwater discharge capacity of urban areas is determined by the characteristics of the sewer infrastructure. It will be necessary to include structural measures such as sewer relining, enlargement of the diameters, or simplifying network layout to cope with these extreme situations as well (14).
The benefits and price tag of greening European cities
Green roofs can be an important measure in the context of the European green deal. The benefits of green roofs include the reduction of storm water runoff by retaining precipitation (15), less energy demand for the cooling of buildings (16), and lower air temperatures in the city on a hot day (as a result of transpiration by the plants on the green roofs) (17). In addition, green roofs enhance sequestration of carbon dioxide and pollutants from the atmosphere (18), reduce noise in buildings (20), increase habitat for especially birds and pollinators (19), and improve well-being for the local population (21).
The opportunities and limitations of green roofs as a tool for sustainable urban development in Europe have been assessed in a model study. The study focused on the period 2070–2100, compared with the reference period 1990–2013, for a moderate (RCP 4.5) and high-end (RCP 8.5) scenario of climate change. In this study, roofs were turned into green roofs at a scale that agreed with 35% of the total European urban impervious surfaces (22).
According to the results, green roofs cool surfaces by between 2.5°C and 6°C, and air temperature in the city by about half of this surface temperature reduction. Cooling energy saving of buildings could be about 92 TWh/year. The combined effect of carbon dioxide sequestration by biomass growing on green roofs, and energy savings can be up to 55.8 Mtons per year. Transpiration of rainwater from these green roofs could potentially reduce urban runoff by about 17.5%, and thus help to avoid urban flooding during rainstorms (22).
There is a price tag attached to this, however. The benefits that could be monetized by the authors cover less than half of the estimated costs of greening. There are additional benefits that might be worth such extra cost, according to the authors. These benefits are related to biodiversity, water quality, health, wellbeing and other aspects (22).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.
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- Roudier et al. (2016)
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- Susca et al. (2011); La Roche and Berardi (2014), both in: Quaranta et al. (2021)
- Issa et al. (2015), in: Quaranta et al. (2021)
- Whittinghill et al. (2014); Kuronuma et al. (2018), both in: Quaranta et al. (2021)
- Colla et al. (2009); Fernández Cañero and González Redondo (2010), both in: Quaranta et al. (2021)
- Van Renthergem and Botteldooren (2009), in: Quaranta et al. (2021)
- Blackhurst et al. (2010); Orisini et al. (2014); Baldock et al. (2019), all in: Quaranta et al. (2021)
- Quaranta et al. (2021)
- Kahraman et al. (2021)