Water For City

Professor Helmreich, the concept of the "sponge city" often comes up when discussing climate change. You prefer to use the phrase "water-sensitive city". Where does the difference lie?

Brigitte Helmreich: The sponge analogy is inadequate, since it implies that water is stored on a long-term basis in the city and then "squeezed out" of the city again when water becomes scarce. By contrast, our research on the water-sensitive city is about using a variety of different and integrally planned measures to come as close as possible to the balance of the natural water supply.

What does a natural water supply look like?

Depending on the location, soil and plants in undeveloped areas such as meadows and forests can often store more than 80 percent of stormwater and then release it through evaporation. The remainder of the water drains or flows away. This means that evaporation accounts for the largest share, while groundwater is replenished by stormwater drainage.

And what's the situation like in urban areas?

When these areas are settled and developed with streets and buildings, a majority of the natural ground surface is sealed. This means water can no longer be stored in soil and plants, and thus can no longer evaporate. Instead, water from precipitation simply flows away. This interrupts the water supply cycle. On top of that, the asphalt surfaces heat up on hot days and don't cool down at night. When climate change brings us frequent long dry periods, then there's less evaporation from plants and thus less cooling. As a result, cities are heating up more and more. And in case of heavy rain, runoff from the sealed ground surfaces overloads the sewers within a very short period of time, resulting in flooding. These problems are becoming more and more frequent in today's cities.

How does the water-conscious city concept help here?

Planted facades and green roofs reduce the amount of stormwater which flows away without evaporating. Infiltration swales along streets and buildings can collect, store and evaporate precipitation runoff. And surfaces which don't necessarily have to be sealed, for example, parking lots, can be designed to be more permeable. This is one way we can support natural water supplies.

Another important issue is multifunctional areas, which can be used to buffer stormwater runoff in case of heavy rains but also serve a different function. For example, in Rotterdam, outdoor basketball courts are intentionally flooded during heavy rains to relieve the burden on the storm drainage system. Solutions like these are gaining in importance.

What approach does your research take?

One of my research projects looks at pollutants which are dissolved by stormwater runoff from non-metallic roofs, i.e., from ceramic or concrete tile roofs or wood shingle roofs. These pollutants can then reach the groundwater. Here we use infiltration swales and technical treatment plants to reduce the amount of pollutants reaching the groundwater.

Another one of our research projects is investigating how urban trees can be adapted ideally to climate change by optimizing their location. We're testing how much substrate a tree needs to grow in the limited amount of space available in a city. We're also looking at how to equip the space with water-retaining materials that will supply water to the tree during dry periods. Another topic has to do with cisterns. These small water storage containers can also supply plants with water during longer dry spells. However, our research looks at more than technical solutions like these; we also consider the city together with other specialized disciplines in order to gain holistic perspectives.

What does a holistic perspective on the city look like?

To stick with our example: We're not only reinforcing the local water cycle with more and better irrigated urban trees. We're also contributing to biodiversity and raising the quality of life, since cities won't be as hot anymore and shared urban areas make cities more attractive. Planting roofs and facades also fits well with energy-efficient buildings. This makes the city more climate-resilient. To take these aspects into account, we're collaborating with experts from ecosystem research, architecture, landscape planning and the social sciences. We're also working together with public authorities and residents. When it comes to urban planning, we have to keep our eyes on the overall picture from the very beginning.

Copenhagen and Freiburg are examples of cities that are well on the way to becoming climate-conscious and water-sensitive cities. What can other communities learn from them?

Many municipalities have realized that changes are necessary. But these changes don't take place by themselves, they require legal regulations, for example, regarding roof planting for public buildings. Construction is often guided by the principle of "as fast and inexpensive as possible", turning a blind eye to the long-term benefits of planting and sustainable water management measures. A planted roof not only benefits the environment, but it also helps insulate the building, for example. Here, we finally have to be honest and include operating expenses and follow-up costs in our calculations.