Ms. Helmreich, when it comes to climate change, the concept of the “sponge city” is often brought up. You prefer to speak of a water-conscious city. What’s the difference?
Brigitte Helmreich: The comparison with a sponge is flawed, because it implies storing water in the city for the long term and then “squeezing it out” when water shortages occur. Instead, our research on water-conscious cities focuses on using diverse, holistically planned measures to achieve the closest possible balance to the natural water balance.
What does the natural water balance look like?
In undeveloped areas such as meadows or forests, depending on the location, more than 80 percent of rainwater is often stored in the soil and plants and then evaporates. The remainder seeps into the ground or runs off. This means that evaporation usually accounts for the largest share. Furthermore, groundwater can recharge when rainwater seeps into the ground.
What is the situation in urban areas?
When such areas are populated, with roads and houses, a large part of the surface becomes sealed. This means that the water can no longer be stored in the soil or plants and thus cannot evaporate. Instead, the rainwater runs off. This disrupts the water balance. In addition, asphalt surfaces heat up on hot days and do not cool down at night. When we often have long dry periods due to climate change, there is a lack of evaporation by plants and therefore also a lack of cooling. The city heats up more and more. In addition, the sealed surfaces mean that the sewer system becomes overloaded within a short period of time during heavy rainfall, leading to flooding. Cities are increasingly struggling with these problems.
How does the concept of a water-conscious city counteract this?
Green facades and green roofs ensure that less water runs off and more evaporates. Infiltration channels along streets or on buildings can seep, store, and evaporate rainwater. Furthermore, surfaces that don’t necessarily need to be sealed, such as parking lots, can be made more permeable. This way, we can promote the natural water balance.
Another major topic is multifunctional areas that can be used as temporary storage for rainwater, but which also serve a secondary function. In Rotterdam, for example, basketball courts are deliberately flooded during heavy rains to relieve pressure on the sewer system. Such solutions are becoming increasingly important.
What approaches do you pursue in your research?
One of my research projects focuses on pollutants that are released by rainwater from non-metallic roofs—i.e., tile, concrete, or wood shingles—and can then seep into the groundwater as they seep into the ground. We use infiltration basins or technical treatment systems to reduce the input.
In another project, we are investigating how urban trees can adapt to climate change through optimized location. We are testing how much substrate a tree needs to grow in the limited space available in the city and how this can be mixed with water-storing materials to provide the tree with water during a dry period. Another topic is cisterns, small water reservoirs that supply plants even during prolonged droughts. However, we are not only researching such technical solutions; we are also examining the city from a holistic perspective, together with other disciplines.
What constitutes a holistic view of the city?
To stay with the example: By planting more and better-watered urban trees, we not only strengthen the local water balance. We also increase biodiversity. And we contribute to a better quality of life because the city is less hot and urban greenery makes communal spaces more attractive. Greening roofs and facades also goes well with energy-efficient buildings. This makes the city more climate-resilient overall. To consider these aspects, we cooperate with experts from ecosystem research, architecture, landscape planning, and the social sciences. And we work together with local authorities and citizens. In urban planning, we must keep the bigger picture in mind from the very beginning.
Copenhagen and Freiburg, for example, are already well on their way to becoming climate- and water-conscious cities. What can other communities learn from this?
Many municipalities have recognized that changes are necessary. However, these changes won’t happen on their own; they require legal regulations, for example, for green roofs on public buildings. Furthermore, construction is often carried out according to the logic of “as cheaply and as quickly as possible.” This ignores the long-term benefits of green roofing and sustainable water management measures. A green roof not only has environmental benefits but also contributes to the building’s insulation, for example. We finally need to do some honest math here and include operating and follow-up costs.