The problem with the skyscraper wind effect
The City of London is promising that high-rise buildings will be monitored to ensure they don't make conditions unbearably windy in surrounding streets. But why do skyscrapers have this effect and what can be done to alleviate it?
Anyone who has ever walked near a very tall building in the middle of a city on a windy day will have noticed a strange effect.
The wind is often much more intense around the base of the tower.
And the growth in high-rise structures is generating more concerns. The City of London Corporation has promised a more "rigorous" assessment of developers' predictions of ground winds, following complaints about strong gusts outside the 20 Fenchurch Street Building, better known as the Walkie Talkie.
"I almost got blown over the other day walking up past the building," a sales assistant working nearby said earlier this year. "When I got around the corner it was fine. I was scared to go back."
Toronto in Canada has suggested bringing in by-laws to ensure planning for skyscrapers takes into account the risk of street winds.
In Leeds, 35-year-old Edward Slaney was crushed after strong winds toppled a lorry near the 32-storey Bridgewater Place, the city's tallest building, in 2011. This was one of several incidents, some resulting in injuries, reported to the council.
Accelerated winds near skyscrapers are caused by the "downdraught effect", says Nada Piradeepan, an expert on wind properties at engineering consultancy firm Wintech. This happens where the air hits a building and, with nowhere else to go, is pushed up, down and around the sides. The air forced downwards increases wind speed at street level.
There is also an acceleration of wind around the side of the buildings if it has completely square corners.
And, if several towers stand near each other, there is an effect known as "channelling", a wind acceleration created by air having to be squeezed through a narrow space. This is a form of the Venturi effect, named after the 18th-19th Century Italian scientist Giovanni Battista Venturi.
"These different effects can combine to create faster-moving wind. It's complex," says Piradeepan. "The downdraught effect is most strong where buildings stand face-on to the prevailing wind, which in London is from the south west." More rounded buildings, such as London's Gherkin, don't have quite the same downdraught effect and don't encourage an increase in wind speed around them, as the air doesn't accelerate around corners, he adds.
The City of London has fewer skyscrapers than New York but much of its layout is based on medieval street patterns. Its narrower roads mean it concentrates the wind through channelling more than happens in New York's generally wider streets and avenues, says architect Steve Johnson.
Architects test skyscraper designs in wind tunnels to ensure there would be no damage to structures. But the potential effect on people living and working down below is becoming more of a focus for study, says Johnson.
Dubai's Burj Khalifa, the world's tallest building at 828m (2,716.5ft), underwent "micro-climate analysis of the effects at terraces and around the tower base" before opening in 2010.
In Toronto, the broadcaster Global News measured gusts of between 30kmph (18.6mph) and 45kmph (28mph) at one corner of the 55-storey Four Seasons Hotel. It detected wind speeds of just 5kmph (3.1mph) slightly north of the building.
As the air at higher altitudes is colder, it can create chillier micro-climates when downdraught from skyscrapers reaches street level. This can be welcome during hot spells, but less so in winter. And, as buildings go higher, the speed of air hitting them rises, increasing ground winds below.
Skyscraper-affected airflow is a relatively new phenomenon in cities like London and Leeds, which were mainly low-rise until recently.
This is not so in New York, where, more than a century ago, residents were complaining of the winds caused by the face of the Flatiron building, then considered tall at 93m (305ft). It was said to lift women's skirts above their ankles, attracting young men not used to such public exposure. In 1905, a salacious (for the time) film of this phenomenon was made.
As long ago as 1983 in New York, engineering consultant Lev Zetlin called for laws to counteract the effects of buildings on street wind.
The City of London Corporation is not going this far, but it is changing the way it works with developers. The level of wind predicted by developers and that which actually occurs can differ "somewhat", says the corporation's head of design, Gwyn Richards. So there's going to be independent verification of studies carried out by developers to ensure they're as "rigorous and resilient" as possible, he adds.
The problem is that, where buildings causing downdraught problems have already been built at great expense, they can't simply be demolished.
Among the solutions on offer are screens to shield people from the wind at street level or even the use of more trees and hedges to break up air flow.
In Leeds, the city council last year granted permission for angled shelters near the base of Bridgewater Place, known as "baffles". But Lindsay Smales, senior lecturer in building, planning and geography at Leeds Beckett University, has said he doubts much can be done "once you've built a tall building like that to mitigate the problems of micro climate and the effect of the wind".
As downdraught happens most where buildings are square-on to wind, would changing their angles be a good idea?
Johnson is inspired by the example of a far more low-rise place, the seaside resort of Whitstable in Kent, famed for its oyster trade and now home to offshore wind farms. Some of its street layout was designed to be at 45 degrees to the prevailing wind so that there's not such a wide section facing it, he says.
"None of these problems are new," Johnson says. "The ancient Greeks and Romans knew something about the effects of wind on buildings. It's just that, unlike today, they didn't try to build enormous skyscrapers."
Giovanni Battista Venturi (1746-1822)
Italian scientist who was a professor at the University of Modena in Italy
Researched sound and colour, but is most famous for his work on hydraulics
He first noted the effects of constricted channels on fluid movement
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