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3.3.4 Restoration to cope with urban flood risks

Flood events in European cities: Causes and challenges

Flooding in inland urban areas has two main causes: failure of the urban drainage network to remove rainwater fast enough, causing accumulation; and flooding by an adjacent river as a result of rainfall in the catchment upstream. The former may be associated with insufficient system capacity by design and/or operation, as well as to poor physical condition of the infrastructure. The latter can be exacerbated by urban growth and land use change outside the main urban area (i.e. upstream peri-urban and adjacent rural areas) as precipitation that was previously absorbed or slowed down by vegetated land can then run off on the sealed surface of suburban infrastructure and through bare winter fields (Petts et al. 2002). In those cases where the two processes converge severe flooding may ensue in the highly populated urban centres.

Inland flood events can be classified and described based on the source of the run-off water (e.g. rainfall, snowmelt or a combination of both) as well as the intensity and duration of the associated rainfall. Cities located close to mountainous areas can also be affected by several other types of flooding such as flash floods which occur as a result of the rapid accumulation of run-off waters from the higher upstream areas (caused by extreme rainfall, cloud bursts, landslides, the sudden break-up of a dike or failure of flood control works). Around the world, the majority of cities are located towards the lower end of river catchments and in coastal areas, often making them vulnerable to all these types of flooding, sometimes in combination (Huntley et al. 2001).

In the urban context, sealed surfaces short-circuit the natural water cycle and increase the rate at which run-off water reaches the drainage network, saturating the system and intensifying floods (Anthonj et al., 2014). These more intense floods have become more frequent in recent decades, and they have strong implications on the quality of life in urban areas as they shut down basic infrastructure and interrupt economic activity, and in more extreme cases may destroy homes, businesses and public infrastructure (EEA, 2016a).

Many European cities have to deal with flood risk management issues on a regular basis and most of them have a history of catastrophic flood events. For example, in 1910 Paris was hit by a flood catastrophe, whereby the River Seine rose 8 meters above its ordinary level. Thousands of Parisians evacuated their homes as water infiltrated buildings and streets throughout the city shutting down much of Paris's basic infrastructure (1910 Great Flood of Paris, n.d.). The recent flood episode of June 2016 shook the city once more, with the river level rising to 6.1 meters above ordinary level causing the shutdown of transport and electricity systems.

In early 1995 large areas of cities located along the Meuse River in the Netherlands found themselves under water. Heavy rain events combined with snow melt from the mountains raised the river’s water level, flooding areas around the city of Maastricht and south of the city of Nijmegen. Around 75,000 people living along the Meuse and Waal rivers had to be evacuated (ESA, n.d.)