Making central Wellington less windy

This Issue This is a part of the Urban design feature

By - , Build 99

Proposed changes to the Wellington District Plan will make it easier for developers and designers to assess the effect of their projects on wind at street level.

For the last 10 years the Wellington District Plan has required wind tunnel testing of all new central business district buildings taller than 18.6 m.

However, this height rule – intended to create certainty for developers – has also created some unexpected anomalies. It means that even projects that will have no effect on wind at street level, such as adding an extra storey to an already very tall building, still require a wind tunnel test.

A recently-proposed change to the District Plan is intended to address this. It will make wind tunnel testing more discretionary in some cases. The Wellington City Council is also using this revision of the District Plan to revise its wind performance criteria. The aim is to make the criteria more accessible to designers and planners so that they can consider the effects of wind along with other concerns such as historic values and urban design issues. A decision on both changes will be made later this year.

Wind in urban areas

Although tall buildings do not necessarily cause wind problems, poor aerodynamic design can significantly affect conditions at street level. This is not a new problem. The 12-storey Hope Gibbons building in central Wellington caused wind problems at ground level in the 1920s. Traffic wardens were regularly stationed on the windy corners to help pedestrians across the road.

Even in the 1960s ropes were still being placed at windy corners in central Wellington to stop people from inadvertently stepping in front of passing traffic as they struggled to maintain their balance.

By the 1980s, however, Wellington – along with Auckland – had introduced wind performance requirements to help reduce the effect of buildings on wind at street level in the CBD. Unlike Wellington, Auckland does not have a height requirement; rather the decision as to whether a wind tunnel test or an expert opinion is needed is made on a case-by-case basis.

The District Plan

Wellington City Council’s ‘Wind Design Guide’, published as an appendix to the District Plan, is still one of the best available internationally. However, like most aerodynamics guides it is more helpful in describing what not to do than providing advice on how to reduce the effects of wind. At Victoria University’s School of Architecture work is now underway on a document looking at ways of achieving the latter. The aim is to evaluate the likely benefits of features such as canopies or breezeways which vent the wind through the building at an upper level.

Wellington’s current District Plan has been written to achieve two main goals. The first is to limit maximum wind speed to an acceptable safety level. This has been set at a gust of 18 metres per second (18 m/s or 64.8 km/hr) for a period of no more than one hour a year. Wind gusts stronger than this are considered unsafe.

The second is to prevent ‘wind creep’. This is the process by which each new building in a street makes the wind problem slightly worse, though not unsafe, until eventually the safety level is breached. At present Wellington has two creep criteria. The first is for areas where maximum gust levels are currently under 10 m/s, and the second for areas where maximum gust levels are under 15 m/s. In each case, new buildings must be designed to make sure that the existing levels of either 10 m/s or 15 m/s are not breached.

Magnifying effects of small changes

Even relatively small changes to maximum wind speeds can have a significant impact on the general windiness of a site. For example, a park where the maximum gust level is 14 m/s is likely to have another 4,000 hours a year where the wind is strong enough to make it uncomfortable to sit there. Increasing the maximum gust level to 16 m/s will increase the number of uncomfortable days to 5,500 or more. An extra 1,500 hours or 60 days per year when the park might be uncomfortable is not a trivial change. It does not require a ‘wind expert’ to determine this is not a ‘de minimus’ effect under the RMA.

For non-experts the effect of these apparently insignificant differences in maximum gust speed can be hard to understand, especially when they are being weighed up against other issues such as heritage or urban design values. The proposed changes to Wellington’s District Plan are intended to make the issues clearer. In the future, comfort and creep criteria will be defined using an hours-per-year format. The District Plan change also allows the introduction of specific comfort criteria for parks in this same hours-per-year format. This is similar to the process required by the Auckland District Plan to establish ‘Wind environment categories’. In both cases, wind tunnel tests must measure the total number of hours per year that wind from all directions exceeds certain specified speeds. This approach makes it possible to assess things such as whether a reduction in northerlies might offset an increase in southerlies.

To find out what levels of wind people find comfortable, Opus Central Labs and Victoria University carried out surveys of behaviour and comfort responses in Midland Park and other outdoor spaces in Wellington. These suggest that an acceptable comfort limit would be a wind speed of 2.5 m/s (10 km/hr) exceeded no more than 20% of the time.

New ways of measuring wind

As part of the District Plan change, Wellington City also considered changing the testing requirements to allow Computational Fluid Dynamics (CFD) computer modelling as well as traditional wind tunnel tests. However, while it seems inevitable that in time CFD tests will become an acceptable alternative, this matter is still under consideration.

Among the issues being weighed up is the fact the CFD modelling requires a lot of computing power. In addition, wind tunnels have been rigorously tested and proven to be reliable predictors of wind gusts. These generic studies have been backed up by measurements made in Wellington streets. The literature so far lacks similar studies into urban CFD in general, and for Wellington (and Auckland) in particular.

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