BD-R-2: Restoration of natural flood plains | Umweltbundesamt

BD-R-2: Restoration of natural flood plains

The realignment, renaturation or slotting of dykes carried out since 1983 has produced an increase in natural flood plains. By their connection to water courses or water bodies and by restoring the dynamics of natural flooding, new and – in terms of nature conservation – valuable habitats have been created for numerous rare and endangered animal and plant species. Likewise, alluvial forests have been created which are just as important for nature conservation.

Increase in natural flood plains benefits biological diversity

The maintenance of wetland or water-rich habitats and their connectivity in the landscape is of preeminent importance for the conservation of climate-sensitive species. Near-natural alluvial meadows play a key role in this context. Alluvial meadows are lowlands which are clearly characterised by the dynamics of a river and its water regime, and in which phases of high water levels alternate with low water levels. These dynamics induce an abundant mosaic of various habitats which explains the high diversity of animal and plant species in these zones. Typical forms of vegetation in alluvial meadows are alluvial forests characterised by tree species such as willows, oaks or elms which cope well with flooding, even if it lasts for some time.

The restoration of natural flood plains involves substantial synergies between species conservation and biotope conservation as well as floodwater protection. Apart from heightening dykes, the installation of holding ponds or other technically-based measures, the durable restoration of natural flood plains is considered an effective component of comprehensive floodwater risk management (cf. Indicators WW-R-2 and RO-R-3): Where rivers can spread into such flood plains in case of floodwater events (cf. Indicator WW-I-4), run-off is slowed down and the floodwater wave attenuated. In the circumstances of climate change and associated changes in the run-off dynamics of river catchment areas, the restoration of natural retention areas plays an increasingly important role, also as a measure for adaptation to climate change, in order to prevent floodwater damage, for instance in settlements, transport infrastructures or agricultural areas

Such newly gained flood plains had previously been in many cases under intensive agricultural use. Where areas are re-exposed to natural floodwater dynamics, they can be re-colonised by many species of plants and animals typical of alluvial meadow habitats. This includes numerous rare and endangered species which are adapted to the special conditions of frequently alternating water levels, such as beaver (Castor fiber), otter (Lutra lutra), kingfisher (Alcedo atthis), sand-martin (Riparia riparia), marsh harrier (Circus aeruginosus), various species of duck – especially those sensitive to disturbance – as well as numerous species of dragonfly and amphibians. Besides, habitats that can flood naturally such as alluvial meadows, provide important links in biotope networks and within the system of protected areas designated under Natura 2000.

By means of renaturation, realignment or slotting of dykes in respect of 79 rivers nationwide, an area of 7,100 ha of former alluvial meadows was, in the period from 1983 to 2020, reconnected with the natural flooding dynamics of water courses. As a result they are, during floodwater events, flooded without any human intervention or control. Any installations of controlled floodwater polders or other controlled flooding mechanisms used in alluvial meadows were not taken into consideration. The annual net gain depends on the size of completed projects in the year concerned which makes this figure variable. The distinct annual increases over the past three years should be viewed as positive.

In 2021, the nationwide inventory established in a report on the condition of alluvial meadows concluded that, of formerly approx. 1.6 million ha of alluvial meadowland on riversides, only approx. 511,900 ha today remain available as retention areas for floodwater.¹²⁹ Compared to those figures, the restoration of naturally flooded alluvial meadowland in the period from 1983 to 2020 covers a relatively small terrain. Consequently, the requirement for action remains great.

Discussions concerning the importance of restoring natural flood plains and the renaturation of alluvial meadows for adaptation to climate change, contained in the action field entitled ‘Biological Diversity’ demonstrate how close the connections are between climate protection, adaptation to climate change and the development of biodiversity. In this context, specialists maintain that the indirect impacts of climate change in terms of the implementation of climate protection and adaptation measures are greater than the direct impacts caused by global warming and changes in the precipitation regime. Measures such as the installation of renewable energy plant, the production of cultivation biomass, the implementation of agro-forestry measures, the restoration of natural flood plains for floodwater protection and the renaturation of moorlands for strengthening their CO2 sink function – which at least partly benefit both climate protection and adaptation to climate change – all these factors bring about both positive and negative consequences for biodiversity.

^{129 - BMU – Bundesministerium für Umwelt, Naturschutz und nukleare Sicherheit & BfN (Hg.) 2021: Auenzustandsbericht 2021 – Flussauen in Deutschland. Berlin, Bonn, 71 pp.}