Green city: a few bushes are not enough to refresh

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The MFO Park in Zürich Oerlikon: a large park landscape that goes up in the
The MFO Park in Zürich Oerlikon: a large park landscape that goes up in the air. (Image: Zürich Tourism)

Extreme heat makes life barely bearable, especially in cities. Greenery is supposed to help regulate the temperature. Experts from the Lucerne University of Applied Sciences and Arts explain how we can go green in terms of urban climate.

Planting an avenue here, greening a roof there - Peter Schwehr, Head of the Competence Center for Typology & Planning in Architecture at the Lucerne University of Applied Sciences and Arts, thinks little of it. The expert for urban and neighborhood development is a great advocate of green cities, as trees not only provide shade but also contribute to cooling by evaporating water. Schwehr stresses, however, that "we have to keep an eye on the big picture." His goal is the sponge city: a city that is designed in such a way that, in the event of heavy rainfall, the water does not simply disappear into the sewage system, but is able to soak it up and retain it - just like a sponge. "When the sewers are full, the water floods the city. Once that water is drained away, it’s no longer available when it’s needed." One solution may be parks that are built so they can turn into a pond during heavy rains without causing any damage.

Like many experts at the Lucerne University of Applied Sciences and Arts, Schwehr is concerned about climate change. Meteorologists agree that we are facing more hot days and more severe weather events. And our cities are not prepared for them. Overheating on the one hand and flooding on the other are the result. "What we need against this is not more walls, but more greenery and new concepts for our buildings and communities, according to the motto ", says Schwehr. This is not necessarily implemented today, but at least hardly doubted anymore.

Green facades

Buildings with green facades are an eye-catcher. It is also undisputed that vertical greenery protects interiors from summer heat. Urs-Peter Menti, a professor at the Institute for Building Technology and Energy, points to Singapore, where green facades are widely used. "Up to eight degrees less are measured there on the surface of buildings with green facades - this saves energy for cooling rooms." But he also emphasizes, "It’s not about the individual building. It is important to think in terms of areas and quarters and to ask about the building in its context." Building technology expert Urs-Peter Menti takes a pragmatic view: "We at the Institute for Building Technology and Energy want to know where the greatest effect can be achieved with the least expenditure of money or gray energy: What exactly is the benefit if the surface of the parking lot is white instead of black? And also: What effect does a green facade have?"

Where do the trees belong?

Even if the architectural remedies against heat islands are known in principle - plants, moving water, shading and a positioning of buildings that makes air exchange possible - more precise information is needed for the planning of a development: Where do the plants belong? What are the consequences of a development facing east or west in a particular location? What is the effect of an avenue of trees or a green south-facing façade? These questions must already be answered in the planning process - subsequent adjustments can normally hardly be financed.

It would therefore be desirable for architects, landscape architects and planners to have a model available as early as the project design phase that can calculate in a matter of seconds the effects of a change in the building’s position, a greened facade or an avenue of trees. This is exactly what Markus Koschenz from the Institute for Building Technology and Energy and his team have developed. To verify the model, the researchers collected data using drone measurements in the Suurstoffi area. "The measurements have confirmed: The model reproduces reality well," Koschenz is pleased to report. The introduction of a first version is therefore imminent.

The two projects "Neighborhood Climate Modeling" and "Neighborhood Climate: Socio-economic Implications of Heat Islands" are supported by the Interdisciplinary Thematic Cluster (ITC) " Space and Society " of the Lucerne University of Applied Sciences and Arts, in which also other projects in the environment of the focus topic "Housing & Sustainability" are funded, the modeling project also by the State Secretariat for Education, Research and Innovation SBFI.

Plants in the wind

On the Horw campus of the Department of Engineering & Architecture, one has encountered all kinds of climbing plants since 2018. Civil engineer Kilian Arnold is responsible for this. He likes to garden and has also learned a lot about plants at the department. But the research questions posed by Arnold and his team at the Building Envelope and Civil Engineering Competence Center have little to do with plant care. Since his diploma thesis, he has been concerned with the forces at work when climbing plants are strung up on ropes and thus grow skyward along the facade. In various research projects, he looks at different aspects of the problem, because one thing he has learned is that when too many factors come together in a project, it becomes difficult to determine cause and effect precisely.

At the facade test stand on the Horw campus, there is a test stand that is around twelve meters high and covered in greenery up to the top. It actually originates from a project that has already been completed. But one effect is still clearly visible today: Some plants wrap themselves around the cables in such a way that they are bent and thus shortened - in some places so much that the wire has simply been torn out of its attachment. On one side, each of the wire ropes is attached to a concrete block. Originally, they were all at the same height. In the meantime, differences of up to 10 centimeters are noticeable because the wisteria, for example, has a firm grip on the wire rope.

The hall behind the façade test stand is roaring, because Kilian Arnold’s current project is underway here. He wants to check what a wind speed of 45 meters per second does to wire rope and plants. After all, these systems have to cope with dynamic wind forces, which present the climbing system with static challenges. In order to design such systems to withstand the strongest expected wind forces, so-called drag coefficients of the climbing plants must be known. This knowledge is a prerequisite for a numerical calculation model.

At 45 m/sec, the wind sweeps across the Swiss midlands only about every 50 years. But when it does, rope and plants should not be an additional source of danger. For his diploma thesis, Kilian Arnold was able to measure wind speeds of up to 30 m/sec and thus check his calculation model. Now he has a stronger wind tunnel at his disposal for his project "Wind forces in overgrown rope facades". Arnold and his team are investigating 14 different climbing plants for this purpose. To start with, the researchers are working with plastic plants. He won’t bring the real ones into the wind tunnel until he knows everything is going as planned. "I can only use the living plants once, after that they’ve lost all their leaves," he explains. Which is just as well, because the plants sacrifice leaves at high wind speeds to ensure their survival. The result of the calculations and checks should be available by 2025 at the latest: In the form of a guideline that can be used to plan safe facades greened with climbing plants.

Plants or photovoltaics?

Behind the hall with the wind tunnel, Gianni Settembrini from the Institute of Building Technology and Energy has set up a test stand. Many of Settembrini’s projects revolve around the question of how we can better equip our building stock for the increasingly warmer summers in Switzerland. The "GreenPV" project is looking at whether it makes more sense to use facades for greenery or for photovoltaic panels - in other words, should a building use greenery or electricity to cool down on hot days. "In the roof area, the benefits of both greening and photovoltaic systems are already being increasingly utilized. The building façade, on the other hand, has great potential that has been largely untapped," Settembrini says, explaining his project, which is supported by the Swiss Federal Office of Energy (SFOE). By the end of 2023, with the help of literature research, simulations and measurements, recommendations with practical solutions for the optimal use of facades are to be formulated, which not only take into account energy aspects, but also include financial, design and social effects.

Urban greening also includes numerous other aspects: For example, the Stadtklang project, which was completed in 2018, also looked at how planting in courtyards, for example, affects the acoustic conditions in a development. And the project "Seeking Birdscapes", which deals with the perception of bird calls, is currently completed. Plants also provide a habitat for numerous animal species and ensure that birdsong can still be heard in the city.

Author: Senta van de Weetering
Images: Sponge City: Competence Center for Typology & Planning in Architecture (CCTP), drone flight: Seya Eggler, green facades: hslu