Buffer areas are created for water masses, natural areas and recreational areas: An analysis of case studies in Germany and the USA shows that the restoration of floodplains can ensure efficient flood protection with multiple benefits. The researchers therefore now recommend removing hurdles so that areas can be made available to promote ecological flood protection.
When humans did not intervene to regulate, many rivers regularly flooded large areas – these floodplains are called floodplains. The frequent flooding there is not a problem for nature – on the contrary: these landscapes are the habitat of many animal and plant species. But then humans also began to spread in these regions – and finally put nature in its place: where rivers regularly burst their banks, dykes and other artificial structures were often built to control the water masses. However, as is becoming increasingly clear, these concepts are less and less able to protect the cultural landscape from the increasingly extreme flood events associated with climate change.
The further expansion of dams and canals does not seem to be a good solution: “Conventional technical flood protection has a major impact on the water body structure, is expensive, usually rigid and cannot be easily adapted to the increasing flooding caused by climate change,” says Co-author Sonja Jähnig from the Leibniz Institute of Freshwater Ecology and Inland Fisheries. In the meantime, the concept of “flood protection through flooding” is now being used: rivers are to be given more flood plains in certain areas in order to achieve a buffer function in the event of flooding. Among other things, this can prevent the levels downstream in urban areas from exceeding critical levels.
Successful projects analyzed
In the meantime, there are already a number of projects worldwide that are intended to give rivers more space again. Jähnig and her international colleagues are now showing that such measures can, under certain circumstances, achieve several improvements at the same time: the increasing risk of flooding as a result of climate change is buffered and the environment and the population can benefit significantly. The researchers present successful “multi-benefit projects” in Germany and California, about which they have collected, evaluated and analyzed extensive information.
As an example for Germany, the team examined a project in which a dike relocation on the Elbe near Lenzen ensured the restoration of flooding potential. Among other things, large alluvial forest areas were created again. As the researchers report, it has now been shown that the project has reduced the flood peak by almost 50 centimetres, resulting in far-reaching protective effects. “This has not been measured so clearly to date and has refuted the position that relocating dykes does nothing for flood protection. Since then, similar projects have been implemented in other rivers in Germany,” says co-author Christian Damm from the Karlsruhe Institute of Technology. The project also had a significant ecological success: different habitat types were restored in the area and numerous animal species returned.
Potential also in densely populated areas
The second project examined in Germany showed that river and floodplain renaturation is also possible in urban areas. It was an eight-kilometer-long river renaturation of the Isar, which stretches from the southern city limits of Munich to the city center. “The Isarplan illustrates the multiple use approach and stands out due to a very cooperative planning process in which the population was also actively involved,” reports co-author Jürgen Geist from the Technical University of Munich. Accordingly, the goal of reducing the risk of flooding, gaining habitats and providing recreational benefits has been achieved. The researchers explain that the risk of flooding fell mainly because the river was given more buffering capacity with a width of at least 90 meters instead of the previous 50 meters. The measures were also able to form gravel banks, which benefit endangered fish species, among other things.
In the USA, the researchers analyzed the flood bypasses in the Sacramento river basin. In this case, too, clear multi-benefit effects emerged: “The Yolo bypass is a model for a well-managed socio-ecological system. Public-private partnership works well there. Species protection, flood protection and agriculture can be reconciled – and all of this in the immediate vicinity of a large city,” says Jähnig. The dyke removal and floodplain renaturation on the Bear and Feather Rivers were also successfully implemented in order to increase local flood protection. In both American cases, reducing the risk of flooding was the main driver for the project – and restoring natural ecosystems followed partly as a welcome side benefit, the researchers report.
More funding needed
As part of their study, the researchers also show the factors that can promote or inhibit multiple use projects. Because the presented examples of success were the result of an interaction of several factors and required committed perseverance in order to be able to be implemented, the researchers summarize. “This also has to do with the fact that there is still comparatively little practical practical knowledge from such multi-use projects, but one is confronted with relatively large administrative and legal obstacles. It is therefore important to analyze successful examples and to prepare the success and risk factors for other actors who also want to implement such projects,” says Geist.
According to the researchers, it is becoming increasingly clear that ecological flood protection is more cost-effective than previous approaches and has great synergy potential. That is why politics and administration should now promote these projects more. In the case of Germany, Jähnig says: “It would be desirable if the responsible political and administrative levels at federal, state and local level developed efficient approaches to provide the necessary areas,” says the researcher.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries, specialist article: Front. environment science doi: 10.3389/fenvs.2021.778568