Science - May 27, 2010

Not much choice on a bare town square

Marion de Boo

You are either buffeted by winds or you melt in the sun on the streets and squares of modern towns, says landscape architect Sanda Lenzholzer in her PhD thesis. She argues that designers should take more notice of a town's microclimate than they have been doing. That will also help reduce delinquent behaviour

Fountains have a psychological effect
Her contemporary redesign for Alkmaar's Canada Square looked very pleasant. Both clients and visitors were full of praise for the beautiful open stage for the new theatre. But the mood soon changed to one of complaining about the chilly wind. ‘A problem no one had thought about - neither the client nor myself', explains university lecturer in landscape architect Sanda Lenzholzer ten years later. ‘In the end they enclosed that beautiful, chic podium with windshield on all sides. It was like sitting in a cage!', she explains in fluent Dutch with a hint of a German accent. ‘What did I learn from it? That if you don't test a design idea like that against aspects of the town's climate, it won't last long.' This experience got the designer thinking. On 18 June Sanda Lenzholzer is due to get her PhD for her research on designing for a more comfortable microclimate in cities, making use of trees and green outside walls and roofs.
Another example: Adriaan Geuze's design for the Schouwburgplein in Rotterdam was lauded by architecture critics the world over. But the general public avoid the cold, windy square with its slippery, raised floor.
Lenzholzer: ‘Bare, chilly squares were the fashion for a long time. I hope that people are changing their minds about that now. People like to leave squares totally empty so that markets and fairs can be held on them. But there are markets on a nice, atmospheric square like the Lange Voorhout too. And the Oranje fair is held there every Spring. The canopy formed by the trees adds to the atmosphere on the Lange Voorhout square. The variation of sun and shade gives the public a choice on whether to keep cool or warm up - nice for the elderly in particular. On a bare, empty square there's no choice', says Lenzholzer. ‘As a designer I always wonder how people experience the public spaces I have designed. There are clearly cross-connections between atmosphere, climate and the experience of space. And microclimate certainly contributes to the way you experience a space.'
Lenzholzer went in search of design guidelines for improving microclimates. Urban designers ought to pay more attention to the climate in the city and to keeping the temperature comfortable. Their designs should keep open spaces cool in the summer and sheltered in the summer.
‘Microclimates in cities are coming in for attention internationally, but in the Netherlands I am among the pioneers. For my doctoral researche I spent many days on Dutch city squares asking questions to thousands of people. Ordinary citizens know a tremendous amount about the microclimate in their own cities and everyone thinks it's important. That is nice, and encouraging too. This subject is definitely relevant to society. But the alarming thing is that urban designers and managers are hardly aware of the problem at all, which means that valuable knowledge among ordinary people goes largely unused.'
Only in the last couple of years have the gaps in our knowledge about city climates been acknowledged. We are catching up with the help of big national research projects such as Climate for Space, and Knowledge for Climate. Along with the water problems in the Netherlands, themes such as heat in the cities and thermal comfort are also being addressed now for the first time. However, many of the measurements being taken are purely physical ones. Less attention is paid to issues such as atmosphere and sensory perception.
In the course of her doctoral research Sanda Lenzholzer sought to identify which aspects of atmosphere and microclimate a designer can influence. She found that the proportions of a square, the choice of materials on and around the square, and the degree of openness all contribute to the atmosphere. People have a preference for warm colours, while cold colours make them feel uncomfortable. Her conclusions are of interest both for the redesign of existing squares and for new neighbourhoods.
Urban shelter belt provides refuge
Sanda Lenzholzer designed a model for an urban shelter belt, a serious of linked rows of 25 metre-high trees with transparent screens between them at certain calculated distances. An urban shelter belt of this kind would make a square significantly more comfortable, Lenzholzer argues. Over the course of the day and through the different seasons, visitors to the square could always pick the most comfortable spots. She would like to experiment with this.
Plenty of trees and glass windshields can improved the thermal comfort on a square. Sanda Lenzholzer thinks it would be interesting to realize an urban shelter belt and take measurements and gauge public opinion on it. She is not afraid of graffiti and vandalism. ‘As a designer, you are always going to face a dilemma. If a nice, brand-new design attracts a lot of attention, vandals and gangs of teenagers will move on of their own accord. But if you make everything completely lout-proof, it won't be attractive enough to local people, so they won't come and you'll actually end up attracting more undesirable elements.'
Lenzholzer took measurements over the seasons of the microclimate at various spots on the squares and at various times of day. This resulted in climate maps. They showed for example that wind speeds were higher in the middle of the square than below a low building. There was also more wind along through roads. Gusts of wind make a big impression on visitors. Anyone who has ever been blown off their bicycle will remember it and avoid the place where it happened. This 'negativity bias', as psychologists call it, is a very human trait that increases our survival chances.
As well as being too windy, cities can be too hot. Lenzholzer: ‘What surprised me was that people didn't complain about the heat in public spaces, not even those surveyed during the heat wave of 2006. My guess is that heat waves are still too abstract for people in the Netherlands, because they are rare.'
Fountains have a psychological effect above all. They create a feeling of coolness and relaxation, and a pleasant atmosphere. The measurements suggest, however, that they do not keep things cool as effectively as trees. Trees with a screen-like crown, such as plane trees, offer the best shade as well as working like umbrellas in the rain.
Via new projects such as Knowledge for Climate, and Space for Climate, Sanda Lenzholzer translates her findings into practical terms. She is also working on the European Interreg project Future Cities, in which European cities collaborate on climate readiness, with an eye on heat stress and thermal comfort. ‘For Arnhem and Nijmegen city councils, city climate is a new and inspiring theme.' The first step is to analyse the city climate together with MSc students of Landscape Architecture. ‘On the basis of geographic data on the availability of water and greenery, we can already arrive at design recommendations', says Lenzholzer. ‘You don't have to measure everything precisely.'
Research is being done on how you can retain rain water and make use of greenery to prevent overheating and improve air quality. Green roofs are mainly helpful for water retention in the city, but they do not seem to have much impact on the microclimate at street level. ‘A model has been made together with the students: urban airconditioning, in which water features help to provide cooling for the city, depending on the wind direction.
Lenzholzer: ‘That research provided design criteria for improving the microclimate in the urban area. It produced some eye openers for the council. Our research has brought about a change of mindset: City climate is more important than people realize.'
Canyons in the city
Sanda Lenzholzer's Wageningen colleague Vincent Kuypers of Alterra is also doing a lot of research in the Arnhem-Nijmegen conurbation. His research is done in the framework of the European Interreg project Future Cities. In Arnhem, an existing industrial estate is being cleverly restructured and extended in line with new design principles. In Nijmegen, smaller changes are being made, such as the introduction of green roofs.
‘Air pollution in the city is related not only to traffic density, but also to the nature of the compact city itself', says Kuypers. The concept ‘compact city' is clearly in need of revision.' Cities turn into islands of heat because the city warms up tremendously during the day and the heat cannot escape at night. Dark and asphalted surfaces contribute the most to this. The night temperatures in cities can be ten degrees higher than those in the surrounding areas. ‘Stagnant air gets polluted faster and that causes problems, especially in hot summers. For decades this has been seen as an ecological side effect of the city. It is only recently that more attention has been paid to the health side of it.'
Kuypers: ‘We prefer to do solution-oriented research. For example, we know how to create more shade on the streets.' On a narrow street, a double row of trees reaching the gables can create the urban canyon effect: the lack of ventilation means the air doesn't circulate. Kuypers: ‘This problem was recognized long ago, but I have yet to meet the city councillor who announces that they are going to cut down trees to improve the air quality. It is high time we dealt with the vegetation in the city differently.'
Residents on busy, dusty streets have to clean their windows all the time. Green vegetation to a height of about six metres is not only cooling but also traps some of the dust. In Tokyo and Madrid, this is already being done on a large scale to improve the street climate. ‘If you are going to convince the town councils, you do have to be able to explain the usefulness of this kind of vegetation properly', says Kuypers. ‘And as well as vegetation, water also helps cool down a city. Climate belts and climate boundaries are shifting north. Eventually we will have a climate here like that of the city of Bordeaux , where it is well over 30 degrees in the summer. That is really not pleasant.'