3.2.2 Loss of lateral connectivity

3.2.2 Loss of lateral connectivity (flood protection and drainage on floodplains)


Wetlands and floodplains play a particularly important role in the ecological integrity of aquatic ecosystems. By providing habitats for life stages of aquatic organisms, they are significant in ensuring or achieving the good ecological status of adjacent water bodies. Wetlands and floodplains also play a significant role in flood retention (EEA, 2018).

Studies have shown however that 70-90 % of European floodplains have been environmentally degraded as a result of structural flood protection, river straightening, disconnection of floodplain wetlands, agricultural land use and urbanisation over the past two centuries. The largest pressures on floodplains are linked to hydromorphological pressures, land use and pollution (EEA, 2019).

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Flood protection structures play a key role in this context. Flood events are one of the most common and most dangerous natural hazards affecting European society with almost 3 700 flood events having occurred in Europe between 1980 and 2015 (EEA, 2016). Since decades, European countries have taken flood protection measures that mostly involve conventional engineering flood protection structures to mitigate the catastrophic consequences of floods. At the same time, flood protection structures and measures (such as levees, retention basins, channel straightening, removal of vegetation and sediment) are among the main causes for hydromorphological alteration and ecological impairment of rivers, in particular by disconnecting river channels from the floodplains and modifying riparian zones. 

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Further pressure on the river-floodplain system is exerted by activities that drain excess water from the soil to increase areas suitable for crop production. Land areas may also be drained to serve for forestry or coastal and urban development. Drainage for agriculture has led to major losses of wetlands throughout Europe and is related to several hydromorphological pressures such as channelization of rivers and channel deepening (Vartia et al., 2018). In Europe, 35 % of wetland loss between 2000 and 2006 was due to conversion to agriculture (EEA, 2012); only in south-western Sweden, almost 70 % of wetlands have been lost due to drainage over the last 50 years (Franzén et al. 2016). In many European countries mainly in northern and central Europe, more than 40 % and up to 100 % of farmland is being drained (based on data from ICID, undated).[1] 

[1] http://www.icid.org/imp_data.pdf

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In the second RBMPs under the WFD, almost 15 000 surface water bodies (about 10 % of total) are affected by physical alterations of their channel, bed, or riparian area due to flood protection and/or agriculture in 21 of the WFD countries. In addition, flood protection and/or drainage for agriculture are the reasons for designating almost 7 500 water bodies as heavily modified in 26 European countries.

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Both flood protection infrastructures and drainage affect floodplains and the connectivity of rivers and streams to floodplains, as they cause changes to the land area surrounding water bodies. This can have major implications for the integrity of both riparian and aquatic ecosystems (Amoros and Roux, 1988; Junk et al., 1989, Junk and Wantzen, 2004). In a natural system, lateral connectivity between rivers and their floodplains allows the exchange of water, sediment, biota as well as nutrients. The loss of lateral connectivity leads to the loss of key habitats and as a result to the decline of species and biodiversity both on the floodplain itself and in the aquatic environment. Further, physical processes are disturbed related to the natural water retention capacity of floodplains as well as sediment dynamics.

Artificial bank protections that serve flood protection (embankments, levees or dikes) affect the morphology and dynamics of the river channel by restricting the channel width and the sediment supply from the river banks. Bank reinforcement and levee construction can also lead to bed incision because of the resulting high flow velocities; in its turn, bed incision reduces the connectivity between the river and its floodplain (lateral connectivity). The reduction of this lateral connectivity damages the functioning of the riparian zone and reduces productivity, nutrient exchange and dispersal of biota more widely across the floodplain.[2]

As far as land drainage is concerned, natural channels have been straightened and deepened for surface drainage ditches with significant effects on channel morphology, instream habitats for aquatic organisms, floodplain and riparian connectivity, sediment dynamics, and nutrient cycling (Blann et al, 2009).[3] Further, the regular maintenance of drainage ditches and rivers (via dredging and weed cutting) leads to physical disturbances and morphological changes in water bodies (Vartia et al., 2018).

[2] REFORM wiki “Embankments, levees or dikes“, available online at http://wiki.reformrivers.eu/index.php/Embankments,_levees_or_dikes#Useful_references

[3] https://www.researchgate.net/publication/241682569_Effects_of_Agricultural_Drainage_on_Aquatic_Ecosystems_A_Review

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Figure 8             Embankments for flood protection (left) and agricultural drainage (right)

 Photos not included  

Notes: Insert notes here

Sources: Left photo on embankment (Rinaldi et al. 2016), right photo on drainage (Swedish Board for Agriculture & Swedish Agency for Marine and Water Management, 2015)

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Measures and management challenges  

The restoration of bank structures, the reconnection of floodplains or backwaters (such as oxbows and side channels) and the restoration of wetlands are key measures applied in river basin management planning to restore lateral connectivity between rivers, their riparian area and the wider floodplain (EEA, 2018). For example, in the international Rhine basin, about 125 km² of floodplains were reactivated by 2012 with a target of more than 150 km² by 2020. In addition, measures were taken to increase the structural diversity of approximately 100 km of river banks by 2012 with a target of 800 km of banks by 2020 (ICPR, 2015). With increasing awareness on the importance of floodplains, the numbers of examples of restoration measures or works aiming to improve river-floodplain systems' functioning are rising (EEA, 2019).

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The improvement of lateral connectivity between rivers and their floodplains is a key element for the achievement of the environmental objectives of the WFD. Especially multi-benefit measures which support the achievement of environmental requirements of various environmental policy instruments beyond the WFD, such as the Floods Directive, Birds and Habitats Directives and the Nitrates Directive are particularly relevant to the restoration of disconnected wetlands and floodplains. For example, buffer strips can be beneficial for reducing pollution (included in the Nitrates Directive and the good agricultural and environmental conditions of CAP cross-compliance), for improving riparian habitats, reducing hydromorphological pressures as well as increasing water retention and mitigating the impacts of floods.

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River restoration measures aiming to give more room to rivers are also important for floodplain restoration as well as for the prevention of flood disasters. A targeted ‘Room for the River’ Programme was established in the Netherlands, consisting of over 30 projects that were completed at the end of 2018. The key of the Room for the River approach is to restore the river’s natural floodplain in places where it is least harmful to protect those areas that need to be defended from floods.[4]

It is difficult to predict how exactly pressures on European floodplains and lateral connectivity of rivers may develop in the future. Climate change though, in particular in northern Europe, is bound to lead to increased precipitation and flood events. In its turn, this may require further mitigation measures linked to flood defences as well as increased drainage leading to increased pressures on floodplains and lateral connectivity (EEA, forthcoming).[5] At the same time, European targets of the new Biodiversity Strategy need to be met, whereby the restoration of floodplains and wetlands is mentioned as a means for restoring at least 25 000 km of free-flowing rivers by 2030 (EC, 2020).

Despite the obvious importance of floodplain restoration, it has not been systematically included in river basin or flood risk management plans yet. For developing more strategic approaches to floodplain restoration in the future, it will be important to develop a more coherent knowledge base on floodplains and a more targeted approach towards financing this type of restoration (EEA, 2019).

[4] https://www.dutchwatersector.com/news/room-for-the-river-programme

[5] EEA, forthcoming, Water and Agriculture report.


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