DCSIMG

Leeds wind tunnels: One off or warning?

Fire crews survey the damage after a lorry was blown over at the foot of Bridgewater Tower. PIC: Bruce Rollinson

Fire crews survey the damage after a lorry was blown over at the foot of Bridgewater Tower. PIC: Bruce Rollinson

Fears of freak winds around Leeds’s tallest building arose last week when a pedestrian was killed by a lorry blown over. Rod McPhee looks at potential causes and lessons

Back in 2005, as pre-recession Leeds limbered up to become Britain’s newest skyscraper city, one expert warned of a danger which didn’t exactly enter the public consciousness.

In an interview with the Yorkshire Evening Post, Tony Rees, head of architecture at Leeds Metropolitan University raised the spectre of how our weather could be in conflict with an ascending skyline.

“Collections of high buildings are well known to create their own mini-environments,” he said. “seriously affecting wind patterns around them. They can either create terrible or wonderful parts of a city.”

Almost six years after making the comments, the potential for devastation in the situation he described were seen in Leeds.

Last Thursday a passer-by was crushed by a toppling lorry and the accident appears to have been caused by a strong wind sweeping across the junction of Water Lane and Neville Street in Leeds city centre.

“The phenomenon of mini-environments is pretty well established among the architectural community,” Mr Rees said this week.

“What looks like happened here is a combination of high winds, the geography of the area, the layout of the streets and the design of the building, all coming together in this unusual incident. These incidences are very difficult to predict but at the same time they’re not entirely unpredictable.”

Thursday’s tragedy raises the question of whether it happened because of the mini-environment created in an area that contains the city’s tallest building, Bridgewater Place.

In the six years since it opened, it has gained a reputation for creating a freakishly windy space at the base of the 367 ft tower.

In windy weather, pedestrians often struggle to stay on their feet and, as last week’s unusually high winds hit the city, it appears they lifted the lorry involved in last week’s accident off the ground too.

Mr Rees said: “All kinds of factors can cause winds to blow things over, or even create a force of suction depending on how they are channelled.

“How they are channelled depends on what they come into contact with, what tunnelling effect occurs. Either way they can be extremely powerful forces.”

The shocking death was certainly enough to prompt Leeds City Council – who gave planning permission to the project – to issue a strong statement.

The authority’s chief executive Tom Riordan said: “As part of the original planning application, a wind assessment was carried out on behalf of the developer and this indicated that the impact the building would have on wind speed would be minimal.

“However, since the building was completed, there have been unforeseen wind effect issues around it. The developers and architects have been working with us to resolve this since the extent of the problems became known to us.

“The council has acted to make the area safer for pedestrians during high winds, installing extensive lengths of railings along the footpaths in the area.

“The developers and architects are undertaking work with an international wind engineering consultancy who have carried out wind tunnel testing, computer simulations and technical analysis which is enabling us to identify the most effective solution available to resolve the ongoing issues. This highly complex work is still in progress.”

“Following the tragedy on Thursday afternoon, however, we are looking urgently at other ways of making the area safer.”

There are other local examples of mini-environments being created by buildings.

When the Merrion Centre first opened in 1964, shoppers complained of the wind tunnel effect of wind gusting in and out of the open-topped complex. It became so extreme that the owners were forced to install a roof to limit the effects.

To this day the city centre still has pockets where extreme winds make life uncomfortable for pedestrians.

On Bond Street and Commercial Street, high-sided buildings frequently create a wind tunnel effect which blows across Park Row down onto Infirmary Street.

And on Clay Pit Lane, in the area at the base of the SkyPlaza tower, there are times when these turbulent winds are obvious.

The interaction of stormy weather and buildings has created destruction before. In 1965 high winds destroyed three giant cooling towers at the nearby Ferrybridge Power station. It was said to have been caused by a vortex which bounced between the towers.

But should we dismiss the incidents as rare freaks of nature?

Professor Stephen Garrity, from the University of Leeds’ School of Civil Engineering, is unsure.

“I don’t know if these are freak winds,” he said. “They actually happen fairly consistently, though not every day.

“But it’s like any event in nature: We talk about a one in 10,000 year flood – but that could happen tomorrow.

“So we may have had a one-in-100-years wind gust there, and it just happened early on in the life of the building. It might never happen again, it might happen again three or four times, it’s very difficult to predict.”

But should those who constructed the building have had some sense of the potential dangers?

“Even using computer modelling and wind tunnel modelling, you may only be able to identify wind tunnel areas once the structure is built.” he said. “Models can’t always pick up on localised effects.”

One of the most site-specific issues is that of the solitary location of Bridgewater Place. It creates such an extreme effect partly because there are few skyscrapers surrounding it to take some of the force of any wind blasts.

Which is why cities like New York have relatively few problems despite having so many tall buildings.

Prof Garritty said: “In somewhere like Manhattan there’s a huge density of high-rise buildings so there’s very little space for the wind to permeate and flow through. Because of the density it flows over the mass of buildings.

“Whereas in Leeds there are more open spaces, and when you get wind coming off the corners of buildings it gets buffeted around, you get what’s called ‘vortex shedding’ – you could call it rough wind.

“That wind can get blown down, then blown horizontally and that’s what appears to have happened in this instance.”

So should we panic about the threat of a recurring vortex?

“I don’t think so,” said Prof Garrity “The idea of a vortex sounds very dramatic but it’s not going to sweep across Leeds destroying the city.

“I think the lorry just happened in the wrong place at the wrong time and the problems could potentially be solved just with some minor landscaping or minor alterations to Bridgewater Place.

“Something as simple as, say, building a wall by the pavement could stop this downward wind hitting the ground and moving horizontally onto the road.

“Although this is something we’re new to in Leeds, partly because tall buildings are a new phenomenon for us, they aren’t exclusive to Leeds. They can happen anywhere and probably do happen everywhere.

“It’s very easy to look at things like this with the benefit of hindsight but it’s very difficult and complicated to predict these things.

“The fact is that if humans put an obstruction in the path of wind, then wind will find some way of getting around that. If it gets trapped in some way then the consequences can be disastrous.

“There is a lot of complicated science and mathematics involved in predicting how nature reacts to the man-made world but the fact is that nature is also random in many ways – and man just has to react to that.”

 
 
 

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