UNSEALING PLACES: HOW FORMER TRAFFIC AREAS BECOME FUTURE-ORIENTED GREEN OASES

WHY UNSEAL SPACES?

Unsealing improves water management by allowing infiltration and long-term storage of rainwater, promotes biodiversity and improves soil health. Heat islands are cooled off and growing trees store CO2. The surrounding environment also benefits from unsealed spaces: Green spaces invite people to linger, improve the quality of life in the entire neighbourhood and strengthen the urban fabric.

NEW FREEDOM FOR THE GROUND

Unsealing means making the ground permeable to water. In the process, sealed surfaces that are impermeable to water and air are broken and replaced with materials that allow infiltration. Green spaces, gravel, water-resistant coverings or paved tiles with green spaces in between – these are permeable and promote sustainable rainwater management. There are a variety of materials to choose from, and the possibility of vehicle traffic is not ruled out. In the planning process, a distinction is made between fully permeable and semi-permeable surfaces.

Fully permeable surfaces are green areas on natural soil or gravel areas. Of course, water can seep through these surfaces. The water is absorbed by the soil and can evaporate or flow through the soil layers into the groundwater.

Surfaces that are open to infiltration are often found on heavily utilised areas. Here, pavements are laid for pedestrian and vehicle access that can still absorb rainwater thanks to their joint width and store it in underlying layers. If a sponge city is installed under these surfaces, the rainwater can seep into these storage bodies and remains available there for trees.

Unsealing, benefits the soil, water management and climate – and improves local quality of life. With combined initiatives, car parks can be transformed into biodiverse parks, asphalt deserts into places to spend time.

THE EXAMPLE OF TULLN: TRANSFORMING A CAR PARK INTO A GREEN PARK

This was achieved on the Nibelungenplatz square in Tulln, Austria. This urban community with roughly 17,000 inhabitants is not far from the capital city of Vienna, and is well-known as a garden town due to the abundance of garden facilities. Here, a car park of roughly 8,000m2 in front of the Tulln Town Hall was redesigned into a green park and multi-purpose square. In this way, the town has reinforced ist image as a garden town while also reacting to the needs of the public. Residents were involved in the design of Nibelungenplatz from the very start. In 2021, a public participation process was launched including public voting. This was followed by the competition phase in 2022 and design based on the results of the participation process. Finally, construction was carried out from May 2023 to June 2024.

Nibelungenplatz is situated between the Aupromenade along the Danube River and the local Town Hill which is housed in a former Minorite monastery. Part of the square was maintained for vehicle traffic and continues to act as a car park, but the space was greened and secured with infiltrating paved tiles. Towards the Danube, an open park landscape now unites the former monastery building with the riverbank. Thirty-eight new trees and large-scale unsealing generated a lot of media attention. Newspapers reported about the “Miracle of Tulln”, of sprouting greenery in a former car park. These reports and the positive response among the population demonstrate the need for more projects like this.

ST. PÖLTEN: MAKING THE RING GREEN

In St. Pölten, the capital of Lower Austria with roughly 50,000 inhabitants, another pioneering project is currently underway that is inspiring imitators both in Austria and internationally. The promenade ring that surrounds the old town of St. Pöltner was previously reserved exclusively for vehicle traffic. Now along its 2.3 kilometres, a multi-functional footpath and bike path network is under construction with shaded playgrounds and rest areas, gastronomy zones and open recreational areas. This strengthens the urban fabric and gives the city a new, modern identity.

The new trees are also planted in extensive sponge city areas that allow plenty of room for roots and collect rainwater. Along the promenade ring as well, the key design criteria come from a participation process: For this redesign project, the city collected ideas and concerns from the population during two years. Implementation is now being carried out step-by-step to allow the local residents to get used to the new segments. The first construction section in the southern area of the city was completed in summer 2024 and has already received lots of positive feedback. In the coming years, work will begin on the western and eastern sections of the ring.

SPONGE CITIES IN PRACTICE

Thanks to its special, extensive substrate structure, the sponge city gives more space for roots underground. A surface that is open to infiltration but can be walked and driven on is laid on top, allowing water to enter the sponge city and remain available there for the trees. In Tulln, eleven of the 38 newly planted trees are in similar spots with increased pressure of use, i.e. car parks or multi-purpose areas, where there is frequent vehicle traffic. These trees were planted in underground sponge city areas. Trees in green areas or large beds do not require this support.

As part of integrated rainwater management, the underground water storage systems in Tulln and St. Pölten proved extremely effective in fall 2024, when days of rainfall in the region provoked historic flooding in many places, while both the new Nibelungenplatz and the completed section of the promenade ring withstood the heavy rains without a problem. In St. Pölten, according to the Austrian national weather and climate service, the Central Institute for Meteorology and Geodynamics (ZAMG), 447 mm of precipitation fell in just a few days. This is roughly half the annual level of precipitation. On the promenade ring, this enormous quantity of water was able to fully seep away, considerably alleviating the situation for the town’s sewer system. With a view to future-proof urban planning and climate adaptation in communities, this project has been rightly regarded as pioneering. It is a shining example of how climate-conscious urban planning even on a small scale can have large effects, and delivers practical findings for cities and communities worldwide that are facing similar challenges.