River restoration and flood protection More nature, less concrete

How can cities better prepare for extreme weather while enhancing the quality of life for their residents? Hydraulic engineering expert Prof. Boris Lehmann discusses innovative river restoration projects in Europe and the vision of a more sustainable urban future.

Professor Lehmann, what is it about rivers that fascinates you?

Rivers, along with their floodplains, epitomise nature’s dynamic and ever-changing evolution. They have surrounded and nurtured the very roots of our civilisation! The world’s four earliest civilisations all flourished along rivers and their fertile alluvial plains: Mesopotamia along the Euphrates and Tigris in present-day Iraq, the Harappan culture along the Indus in present-day Pakistan, China along the Yangtze and Yellow River and Egypt along the Nile.

How did the use of rivers change when humans became sedentary?

With the advent of sedentary lifestyles, man’s reliance on rivers expanded significantly. Rivers became vital sources of food (drinking water and fish), transport routes (rafting and shipping), providers of materials and energy (hydropower), disposal systems (sewage and waste) and spaces for recreation. As land cultivation increased, our perception of rivers changed: once seen as wild and dangerous, they were “tamed” or straightened. However, time has shown that interventions like this often failed to deliver the desired stability. Instead of creating controllable systems, these measures resulted in monotonous, artificial watercourses that lost their natural dynamics. As a consequence, flood waves were no longer able to spread out into their original floodplains, accelerating and intensifying in force as they travelled downstream.
 
The consequences are becoming increasingly dramatic. “Once-in-a-century” floods are now occurring with alarming frequency.

The rapid pace of climate change is also increasing rainfall intensity, which is compounding the situation further. As an expert, I view the “space for rivers” approach not just as a beautiful vision but as an existential necessity. It is critical for protecting our cultural landscapes and ensuring their resilience to climate change.

River restoration sounds very promising: bringing nature back into the heart of cities. But what does this really mean, and how can it be achieved in densely built urban areas like Paris or Frankfurt?

In Paris, for example, the banks of the Seine have been reclaimed and transformed into pedestrian-friendly spaces. This not only improves water quality, it also creates inviting areas where people can gather and enjoy their surroundings. River restoration involves implementing measures that restore bodies of water – such as streams and rivers – to a more natural state, by making their structure and flow more diverse and dynamic.

Why should we restore rivers to their natural state?

Rivers have always played a key role in shaping landscapes, and to do this they need space. A living river is not just a habitat for all kinds of plants and animal species, it also serves as a crucial source of water during dry periods, and acts as an effective natural drainage system during floods. The Netherlands is an impressive example of how this approach can be implemented in densely populated areas. The Room for the River programme expanded natural floodplains along the Rhine and Meuse by strategically relocating dykes. These areas now act as natural flood bypasses, while also creating new spaces for recreation and wildlife.

How can restored rivers help protect cities from flooding?

Restored rivers serve as effective natural flood protection systems. By reconnecting oxbow lakes and floodplains, they provide space for floodwaters to spread out and slow down. A notable example is Vienna’s Danube Island, which was created not only as a recreational area but also as a critical flood-relief measure for the city.

How can cities better prepare for extreme weather events such as heavy precipitation?

Heavy rain is an enormous challenge for many urban areas. Rapid runoff can cause flash floods in a very short space of time. These drainage corridors follow topographical “valley paths” and are characterised by strong currents. Rapid surface water runoff causes flooding, which can occur far away from watercourses and have different characteristics than river-bound flooding. One proven way of tackling this problem is the concept of the “sponge city”. Numerous small-scale measures are implemented to retain as much rainwater as possible, allowing it to sink into the ground. Copenhagen has introduced this approach in several parts of the city. Features such as green roofs, infiltration systems and greened-up spaces help slow down rainwater runoff, holding back as much water as possible and allowing it to soak into the ground. This not only diminishes the risk of flooding but also enhances resilience to drought.

Cities are often densely developed. How do they manage this challenge?

Lyon, for example, has successfully resisted dense urban development by restoring the banks of the Rhône. By relocating car parks and roads, the city created space for recreational areas and near-natural waterways. Large-scale inner-city river restoration projects always require space – often necessitating the costly reorganisation of existing infrastructure and buildings. Cost-intensive projects like this typically require dedicated funding. In Germany, state or federal garden shows are used for this purpose.

How would a city in 2040 look if river restoration were a key focus of urban planning?

We would see a corridor winding its way through the city – much like the green and blue corridors already realised in Copenhagen and Munich. Munich’s Isar River is a prime example of how natural river landscapes can be integrated into densely built urban areas. In 2040, such corridors in many other cities would include easily accessible bodies of water surrounded by rich vegetation and with high water quality. Less visible, but equally important, would be the many individual, decentralised measures of the sponge city concept, which help to store rainwater and mitigate flash flooding during heavy rainfall. From an ecological perspective, the structurally diverse watercourse would provide numerous animals with opportunities to migrate through the city or establish habitats there. River restoration – whether on the Seine, Danube or Rhône – greatly enhances the quality of life for both humans and wildlife.