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3.3.4 Restoration to cope with urban flood risks

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.)

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Dresden is another city that has repeatedly suffered from the impacts of flooding, with the latest major events taking place in 2002 (a centennial flood) and 2013.      

Recent flood events   in the city of Dresden

In August 2002, triggered by a long period of intensive   rainfall in central Europe, the Elbe River and one of its tributaries, the   Weißeritz, flooded parts of Dresden as well as other downstream villages.   Reaching 9.40m, the water level was the highest in record since 1275 and   exceeded the former maximum of 8.77m which was recorded in 1845 (Grollmann   and Simon, 2002 as cited in Ulbrich et al., 2003). Damages were estimated at   80 million Euro for community services, 300 million Euro for flood protection   infrastructure, about 45.6 million Euro for agriculture and forestry, all   these on top of the damages to the central railway station as well as other public   and private buildings (EEA, 2016a). This was an event that affected several   countries in central Europe, and the total economic loss across the region   which was associated with the floods amounted to over 14.5 billion Euro   (Ulbrich et al., 2003). A renewed threat surged in late May 2013, when   prolonged and intensive rain visited the city once more. The water reached   similar levels to the ones registered in the 2002 events, putting to the test   the flood prevention and flood risk management measures that had been   implemented after the centennial flood. Overall, severe consequences for the   city were avoided thanks to better, quicker and more effective official   communication; effective use of mobile and stationary protection walls; and   increased retention capacity and appropriate run-off pathways (UNU, 2013).

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Floods are particular amongst other forms of disasters since the vast amounts of water that they leave behind can aggravate the immediate damages caused and extend the time of recovery. Apart from the immediate damages to infrastructure and property, crop destruction and disease are also common impacts of flood events (ESA, n.d.). Within cities, extreme events of flooding can have a number of impacts including material, economic and health impacts. Therefore, there is overall consensus that urban areas need to be made more resilient to flooding especially in the face of climate change.

At present, European cities also face specific climate change challenges which are and will continue to sharpen flood risk in urban centres. In a warmer climate, projections show a further increase in the risk of river floods in many western and central eastern European areas and in urban drainage flooding in particular western and northern Europe (EEA, 2016a; 2012b). In this context, appropriate run-off pathways should be ensured when elaborating new development plans and when defining urban drainage systems’ capacity. This should help to enhance the resilience of urban centres against future flood events and reduce their social and economic impacts.

In most urban centres where buildings encroach to the edge of the rivers, flood risk has often been managed by encasing rivers in concrete with many culverts. These constraints result in river maintenance difficulties and reduce the ability of channels to cope with increasingly heavy summer rainfall. Many concrete-lined channels were designed to accommodate major flooding (i.e. every 20 – 30 years). This may no longer be adequate due to predicted climate change impacts. For this reason, flood risk managers are now increasingly committed to creating space for floodwater where possible through river restoration activities (London River Action Plan 2009).

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Responses to urban flood risks: Links to river restoration

Increased urbanisation brings higher concentration of people, economic activities and assets, subsequently resulting in higher disaster risks (UNISDR, 2012). In terms of flood risk and resilience to flooding the importance of strategic planning and design becomes evident when reflecting upon the main causes for floods in inland urban areas described above. Design, operation and maintenance of the urban drainage network, as well as the mode and direction of the city’s expansion are all variables that can be controlled or at least influenced by strategic planning. In this context, strategies and measures to protect and restore natural areas, especially in those parts of the city at higher risk of flooding, can become key to reduce disaster risk and ultimately the impacts resulting from catastrophic flood events.

Cities can carry out a number of actions to enhance their protection against flood damage. These can include 'grey' flood protection and appropriate urban designs but also green solutions, such as providing more space for the city rivers. In urban areas where appropriate spatial planning and disaster risk reduction efforts are undertaken (for instance, sensible building codes are developed and respected, human settlements are established away from floodplains or steep slopes and infrastructure and services are suitable), the consequences of flood events and other disasters are kept to a minimum (UNISDR, 2012). Sustainable urban drainage, stormwater management and green roofs are also long-term approaches to managing surface and groundwater by reducing the rate and volume of run-off.

As part of the United Nation’s “Making Cities Resilient Campaign” the City of Venice elaborated a plan that encompassed structural and bio-geomorphological elements for flood protection. The plan includes a wide set of measures ranging from greener solutions like the reconstruction of wetlands and reclamation of polluted sites, through softer solutions like flood monitoring, early warning and public awareness raising, to more traditional interventions like city pavement elevation and urban maintenance (UNISDR, 2012). In several other European cities, flood protection measures are combined with the restoration of river banks and the redevelopment of waterfront areas to support recreation and green urban planning.

The need to address the risk of flooding has actually been one of the main triggers for restoration activities in case studies reviewed for this report. For example, persistent flooding events (often resulting in substantial financial damages) have triggered restoration activities in the urban areas of the River Waal in Nijmegen, the River Quaggy in London, the River Isar in München, the River Dyle in Leuven and the River Glinščica connected to the Podutik reservoir in Ljubljana.

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Room for the River in the city of   Nijmegen

In the   east part of the Netherlands, a sharp bend within the course of the river   Waal coincides with the location of the city of Nijmegen. Being enclosed by   an urban area, the limited space for the river to discharge all the incoming   water, especially during extreme events, results in a dangerous bottleneck   (FDC, 2015). This has already caused major flooding events in the past (1993   and 1995) and continues to pose a threat (Gemeente Nijmegen & i-Lent,   2015).

Under the   Room for the River programme driven by the Dutch government, an integrated   initiative is underway that will reduce flood risk in the city of Nijmegen by   moving an existing dike 350m inland, digging an ancillary channel to give the   river more room, and building bridges across the new channel. This measure   will be coupled with the creation of a river park and the redevelopment of   waterfront areas, boosting the further development of the city and providing   new opportunities for leisure and recreation (NWRM, n.d.). The long-term   involvement of a range of stakeholders, the commitment of the local   government and extensive planning and assessment efforts have gained the   project the necessary public support to start the implementations (STOWA,   n.d.).

Nijmegen   has linked the river project to plans to expand the city on the northern   banks of the River Waal. The new city bridge over the river and the Room for   the River project will change the lie of the land. In future, instead of   turning its back on the river, Nijmegen will embrace it. In 2011, the plan   received the Waterfront Award in New York. In addition to increasing high   water level protection, Room for the River has been a catalyst for urban   planning (https://www.unesco-ihe.org/sites/default/files/13270-rvdr-brochure-governance-engels_def-pdf-a.pdf).

The new   river park created on the River Waal as a result of the “room for the river”   interventions in Nijmegen serves as a new public space (more than 80 hectares)   in the heart of the city, where people of Nijmegen and Lent can enjoy the   presence of water on a daily basis, do sports and experience the floodplains.                                                                        

Photo: @Municipality   of Nijmegen

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Restoration of river banks and opening of the   city of Munich towards its river

Hydraulic   regulation measures introduced in the 19th century resulted in a   gradual degradation of the ecology, flow conditions and water quality of the   river Isar flowing through Munich. These modifications also increased the   risk of flooding and damage to properties located at lower altitudes, as well   as limited public access to the river (Arzet and Joven, n.d.; RESTORE,   2013).  

In   response to these issues, the Isar Plan was launched in 1995 as an initiative   that integrated the goals of flood-protection, ecological restoration,   landscape design and recreational use (Arzet and Joven, n.d.; Reiss-Schmidt,   2014). An 8 km stretch of the river that cuts across the city of Munich has   been renaturated (Wulf and Schaufuß, 2013; Arzet and Joven, n.d.), including   renaturation of the river banks, the enhancement of access routes and setting   the flood defences back from the river bank. The benefits of the project   included improved discharge of floodwater/lower risk of flooding, an almost   natural river flow, enhanced aesthetics, better access to visitors, improved   water quality and a restored habitat that supports local fauna and flora   (RESTORE, 2013; City of Munich DUPBR, 2005).

Water   quality in the River Isar has improved up to bathing water standards thanks   to wastewater treatment plants. In addition, flood protection measures have   been integrated with an attractive landscape design. As a result, a high   number of people visit the River Isar, especially during the summer, as Munich   is now a city with an 8 km bathing site.   

River Isar   after restoration. Photo: @ Wasserwirtschaftsamt München

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Very often a city’s current layout with its existing infrastructure and settlements may hinder the implementation of measures like floodplain widening and de-culverting of urban streams. The complexity and financial burden commonly related to major maintenance and upgrading works on the urban drainage network can also constrain a city’s scope of action. However, the increased frequency and intensity of flood events will continue to call for disaster risk reduction measures in many European cities. In situations where an increase in flood protection is necessary, but the room for action within the city’s boundaries is limited, combining inner-city measures with planning at the larger scale to include nature-based solutions in upstream areas can be an alternative. The measures taken to increase resilience in the city of Leuven, historically challenged by flooding, gives a good example of this.

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Increasing   resilience to floods in the city of Leuven

The flooding of the Dyle River in the city of Leuven has   always been an issue as the naturally-occurring steep slopes and historical   deforestation upstream of Leuven easily lead to rapid increases in flow rate   and water level during high precipitation. These floods occurred after heavy   rainfall or after sudden thaws following cold winters, resulting in the   rivers overflowing. The most extreme flooding event in its recorded history   took place in 1891, which led to a third of the city being flooded, and it   remains a reference point for the river’s destructive potential. Since then,   flooding events (e.g. in March 1947 leading to extensive flooding in the   upstream municipality) have remained a regular occurrence and prompted a   number of interventions.

In the 1970s, the idea to   protect the city of Leuven began with the design of traditional hydraulic   solutions, in particular a large flood reservoir in Neerijse valley upstream   of Leuven. The poor water quality and the agricultural land use (still very   important in that period) made it a requirement that the flood reservoir   would not take up more space than was absolutely necessary. During the design   period of this traditional hydraulic solution, growing environmental   awareness led Leuven to explore more nature-based solutions that take the   ecological health and landscape value of the river valley into consideration   (La Rivière 2014).

Using new modelling   software and taking into account the ecological requirements, the protection   measures were tested, yielding successful results. The tested measures   included a controlled flood reservoir in Egenhoven and natural, uncontrolled   flood zones in the Neerijse valley (La Rivière 2014.).

In this new approach, instead implementing a hard   engineered solution with artificial flood reservoirs, the natural processes   has been restored resulting in “wet” valley floors along the River Dyle   upstream of Leuven. Infrastructure works are kept to a minimum and are   intended to 'guide' the river rather than contain it (La Rivière 2014).

In addition to the provision of areas to store water   upstream of Leuven, it may be possible to slowly enhance the capacity of the   channel network within the city over the years, providing that there is   sufficient support within Leuven. Although the capacity enhancements will be   relatively small, when these are coupled with flood resistant and resilient   constructions, they will help to reduce the amount of storage required upstream.   In the EU project Flood Resilient City (FRC), the Flanders Environment Agency   implemented a number of measures to enhance the capacity of the channel   network within the city. The existing quay-walls were improved to maintain   the discharge capacity in the city centre. This was done in close contact   with the riparian owners. In that way they became more aware of the river   flowing next to or close to their houses. At one location it was possible to   build a terrace alongside the river and a small park for people to enjoy.   During high water the steps can flood. This increases the capacity for the   river in the city centre. Additionally the terrace helps to make more people   in Leuven aware that the Dijle is a living river, and that there is an   ongoing threat of flooding (Source: http://www.floodresiliencity.eu).         

Terraces alongside the River Dyle in the city of Leuven. Photo:   @Flemish Environment Agency

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Resilient cities should be well integrated into their surrounding hinterlands through green and blue infrastructure (i.e. forests and other natural areas, rivers, lakes, parks, green roofs, etc.), providing people with the opportunity to reconnect with nature despite ongoing urban growth. This type of multi-functional approach linking flood risk reduction objectives with the protection of a city’s heritage and increased quality of life through the integration of the wider landscape is well exemplified by the restoration of the River Guadiana in the city of Mérida.

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Restoration of the   River Guadiana in Mèrida

In Mérida, capital of the region of Extremadura in Spain,   the overall area adjacent to the Guadiana River was suffering a progressive   environmental degradation caused by invasion of the river margins by the   adjacent landowners, uncontrolled excavations for the extraction of gravel   and sand, dumping of debris and rubbish and degradation of the natural   vegetation. While the urban section of the river suffered from these problems   to some extent, further issues were also prevalent. One of them was the   presence of urban infrastructure very close to the river, which called for   their protection against floods. Furthermore, the Montijo dam (a dam   constructed for irrigation purposes and located downstream of the city) caused   frequent oscillations of the river’s water level, subsequently resulting in   dramatic visual impacts on the urban landscape. This became an issue as well   since Mérida is a historic city with an important archaeological and   monumental heritage (declared a UNESCO World Heritage site in 1993). Some   elements of this heritage are closely related to the river, as the monumental   Roman bridge, the longest (ca. 800 m) Roman bridge that still stands today.

In the late 1980s the “Confederación Hidrográfica del   Guadiana” (Spanish Water Authority in the Guadiana River basin area, a part   of the Ministry of the Environment) decided to act in the area to solve, or   at least reduce, the series of problems faced by the city and its river. In   the urban areas the project allowed to properly integrate the riverbanks with   the city, paying special attention to the aesthetic and archaeological   aspects and taking advantage of them to provide the citizens with new green   zones (parks) which in turn are compatible with occasional flooding and   prevent the improper use of the areas adjacent to the river. The restoration project on the River Guadiana was   linked to the Urban Plan of Mérida.

Urban integration has been   one of the most important targets of this restoration project and achieved to   a high extent. Currently, most of the urban river banks have become parks or   riverside promenades, allowing an adequate transition between the city and   the river and embellishing the environment surrounding the existing   monuments. The restoration measures   have been very well accepted by citizens, who widely use both the urban and   suburban restored areas for sports and recreation (e.g., walking, trekking,   cycling, fishing and kayaking).

 

                           
   

                            

   		    

 

   
   

 

   		    

 

   

River   Guadiana at Mérida after the restoration measures. Photo: @ Confederación Hidrográfica del Guadiana

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