COMMENT | Manchester, the next Windy City?

Schemes such as Trinity Islands, Deansgate Square and Meadow Islands will give Manchester a new skyline, characteristic of other global cities, writes Joseph Berthoud of Hilson Moran. But could this cause complications like London has experienced in its square mile, where skyscrapers are blamed for creating strong winds which have toppled pedestrians?

There are two main ways to assess the wind impact of a skyscraper. One is by placing a scale-model in a wind tunnel, such as those used by Formula One teams. The other (also used in F1) is computational fluid dynamics, which uses a virtual model to simulate the wind. Both methods have similar costs and times to test a single design. However, when multiple iterations are required CFD starts to show real benefits.

During design development, the choice of method can depend on project approach. A wind tunnel best suits a design workshop, where members of the design team attend on the day of testing to discuss strategy. However, a project can experience frequent changes in the following weeks. Subsequently reorganising this workshop each time can be costly and cause delays. CFD best suits an iterative approach, in which a new design can be retested quickly and cheaply as the project evolves.

When testing the final design, each method produces extra data that the other might miss. Wind tunnels produce extra data about turbulent gusts that CFD can only provide at great expense. CFD will tell you how much cumulative turbulence there is, but not whether it is made of one big gust, or many little ones. CFD results show wind conditions from 100,000 points across the area around a development. Wind tunnels will typically use sensors in only 100 locations and could miss some important features. Either way, neither is a one stop shop.

Wind gusts can be deflected by a building causing a ‘downwash’ effect at street level

Why are these crucial tools not always implemented? Cost could be a factor; but for a major tower, an extra analysis at the end of the design is marginal compared to the total costs of the building.

Bad press associated with wind issues around buildings often sees the problem as just one of comfort, which of course at times it can be. However, high winds can be a significant safety problem and cause injury to pedestrians as well as creating an unwelcoming environment for workers, staff or consumers.

Solutions are best identified and integrated earlier in the design process and doing so can be more cost effective. If necessary, there are ways to mitigate the impacts of wind near the end of the design process. Trees, particularly evergreen species, have been shown to significantly decrease wind effects. The use of porous structures such as fences, trellises, banners, or even public art can also be effective.

The largest clients and developers who are experienced in building these structures understand the need for wind analysis as early in the design process as possible. For Manchester, the building of these new towers is of huge importance. They create jobs, commercial and residential space and add to the city’s attractive and dynamic nature – benefits which could be undermined by a poor wind environment. Designing and testing from the start to work with the wind supports the benefits these new towers will bring.

Regardless of the method chosen, it is important to use an alternative method to test the final design, giving you a complete picture, and allowing you to pick up on any missed implications. Early intervention will always be the favoured approach over retrofitting.

• Joseph Berthoud is principal sustainability consultant at Hilson Moran