Burst banks: floods

Professor Jähnig, intense and prolonged rainfall is likely to become more frequent due to climate change. Are we going to have to get used to flooding?

Unfortunately, yes, at least if we continue to do things as we have in the past. Floods are actually natural events in intact riverine landscapes, which over thousands of years have created unique biodiversity and resilient ecosystems. They are even a prerequisite for vital ecosystem functions – such as groundwater recharge. 

By straightening rivers and building dikes too close together, humans have created rivers in which floodwaters swell faster and higher. Rather than allowing the water to expand in the floodplain and partially seep away, large volumes of water flow faster towards the sea. While this certainly had advantages, for example for navigation and agriculture, it also poses considerable risks – as we can see today. 

Conventional technical flood control does not offer absolute protection. It interferes heavily with the structure of rivers, it is expensive to maintain and difficult to adapt to the increasing number of flood events. As a society, we therefore need to take a different approach to our rivers and floodplains. 

Dykes and artificial reservoirs should increasingly be replaced or at least complemented by nature-based solutions. These new approaches are usually multifunctional, i.e. they serve several regulatory, social or economic objectives simultaneously. In the case of floodplains, for example, they benefit climate adaptation, recreation, nature conservation and biomass production. It is important to find a good mix and to redesign the landscape, including our forests and cities, to better absorb rainfall and retain water for the hot season when water is scarce.

After the severe floods of 2013, the German Federal Government and the German Federal States adopted a National Flood Protection Programme. Dr Pusch, how do you rate what has been achieved?

The aim of the programme is to reduce river flood levels by 10 or even 50 centimetres over long stretches. This would certainly be a major success for flood protection, but it is uncertain whether this capping of flood waves will be sufficient in the face of climate change. It is disappointing that two-thirds of this flood retention is to be achieved through new polders, and only one-third through near-natural flood protection measures such as dyke relocation. Ten years later, in 2023, of the 168 spatially significant partial and individual measures planned, 66 were still in the design phase, 46 in the preliminary planning phase, 18 in the approval or contract award phase and only 26 in the construction phase. 

There are countries that are much better positioned in terms of flood protection, such as the Netherlands. For almost 30 years, the Netherlands has been moving back large sections of dykes and creating floodplains as part of the “Room for the River” programme. In Germany, on the other hand, the active floodplain area increased by only 0.1 per cent per year between 2009 and 2020. It must be said that most of the money spent on flood protection in Germany is still being invested in raising and reinforcing dykes. 

From a scientific point of view, however, it would be advisable to limit the use of dykes specifically to the protection of settlements, and not agricultural land, as this increases the risk of flooding for settlements and important infrastructure. Potential compensation payments to farmers for giving more space to rivers are far outweighed by the avoided flood damage to settlements. Revitalised floodplains will once again provide a wide range of ecosystem services such as nutrient retention, carbon sequestration and, last but not least, habitat for a high diversity of species.

On the subject of biodiversity: Dr Wolter, being a fish ecologist, do you think that more intact floodplains automatically result in more species and more stable populations?

Natural or restored floodplains are undoubtedly some of the most species-rich habitats in Germany. Oxbow lakes and shallow floodplains, for example, create zones with less turbulence, where fish larvae can develop undisturbed and poor swimmers can thrive. 

The Red List of endangered freshwater fish and lampreys in Germany, which we updated together with partners in 2023, also shows how important these habitats are. More than half of our native species are now considered endangered or already extinct. This means that the threat to native freshwater fish and lampreys has worsened significantly over the past fourteen years.

Even the trout (Salmo trutta) has been reclassified as an endangered species. It too could benefit from natural flood protection – provided it starts in the upper reaches. The trout – and many other species of flora and fauna – would also gain habitat if these stretches were more richly structured, with many more obstacles such as dead wood or large stones, providing habitat diversity through deep pools, wide and narrow stretches or small riffles. 

Not only have the main waterways been straightened, but also many of the upper reaches of rivers in order to drain off rainfall as quickly as possible. In terms of flooding, this means that the straighter these small tributaries are, the faster a flood wave will reach the downstream areas – giving local residents little chance to react.

Professor Tetzlaff, water balance and land use are inextricably linked. Besides restoring floodplains, are there other ways to increase the water retention of our landscapes?

Drainage of farmland and wetlands, removal of floodplains, deforestation, extensive monocultures and sealing have indeed reduced the ability of our soils to absorb water and reduced the retention capacity of the landscape. The result is faster run-off, especially during flood events.

This can be counteracted by various land-use strategies that improve the water-retention capacity of the soil and slow down run-off: for example, afforestation with mixed forests. This is particularly effective in mountainous areas where flooding occurs in the higher river reaches. A 70 percent forest cover in a catchment area retains twice as much water as a 10 percent forest cover. Trees also increase evaporation rates. However, both of these factors also lead to less runoff during dry summer months, which can affect other ecosystem services. Moving away from extensive monocultures in favour of diversified agriculture and forestry (agroforestry, mixed forests with differently aged trees) and intercropping is another possibility to increase soil infiltration rates.

All these measures can, depending on the region, effectively mitigate small and medium floods and reduce damage. In the case of larger floods, which are expected to become more frequent as a result of climate change, land use in the catchment area tends to play a minor role. Nevertheless, increasing forest cover and restoring natural water retention capacity are positive steps towards improving the resilience of our landscapes to floods – but also to droughts.