Tall buildings can affect the direction and strength of wind at ground level.

Wind flows faster higher above the ground, and tall buildings can deflect these fast flows towards the ground. These impacts can affect how comfortable people are in the street and public spaces, and in some situations down drafts can be dangerous.

The wind environment is complex and is affected by the surrounding buildings as well as the wind conditions in the city in general. New buildings may also affect the wind flow onto and around established buildings and may, for instance, result in increased (or decreased) downdrafts from tall buildings some distance away.

For these reasons, wind assessments will be required as part of resource consent applications for tall buildings in some parts of the central city to make sure the streets and public spaces remain safe and comfortable places.

Types of wind assessment

A wind assessment provides the evidence needed for the Council to be certain that new buildings will not cause adverse impacts on ground-level public space.

It is a written report from a wind expert demonstrating the impact on ground conditions around the site. In most cases, wind modelling is required as part of the assessment.

Wind modelling is a form of wind assessment where the effects of Christchurch winds are simulated on a model of the building and its surroundings. It is carried out in a wind tunnel or by computational fluid dynamics (CFD), a form of computer modelling.

In some low-risk circumstances, the Council will accept desktop wind assessments prepared by a suitably qualified wind expert instead of modelling. However, if a desktop assessment shows that comfort or safety standards may be breached, wind modelling will be required.

When you'll need a wind assessment

In the City Centre Zone, wind modelling is required for buildings above 28m. The matters of discretion in rule 15.13.2.6 a. x. G. require proposals to demonstrate:

The mitigation of the adverse impacts of wind caused by tall buildings on the safety and comfort of people, whether stationary or moving, at street level and in other public open spaces including Cathedral Square, Victoria Square, the Otākaro Avon River Corridor, the Margaret Mahy Family Playground, any public open space zoned Open Space Community Park Zone, Central City Heritage Triangles and other parks, and any mitigation measures proposed, demonstrated through the use of wind modelling and analysis.

A wind assessment may also be required for buildings above 25m in the Central City Mixed Use and Central City Mixed Use (South Frame) zones to show that there is no adverse effect on the public spaces in and around the zone, such as the South Frame and Ōtākaro Avon River Corridor. Desktop assessments will usually be accepted for buildings between 25m and 30m, with modelling required for buildings above 30m.

Buildings certified by an urban designer

In some circumstances, buildings may be certified for compliance with rule 15.13.3.1 by an urban designer from Council’s approved list.

The certifier must provide a complying wind assessment report to Council as part of the certification process.  If the building does not comply with the wind standards, it is not able to be certified.

Who prepares a wind assessment

Wind assessments need to be prepared by a suitably qualified and experienced wind expert.  This can be:

  • A mechanical engineer with at least 5 years experience in modelling wind in the built environment.
  • A qualified meteorologist with at least 5 years of experience in computational fluid dynamics.

The expert is to state their experience when providing a wind assessment. The expert may also be requested to demonstrate expertise, or example, by supplying a portfolio of completed assessments.   

The Council may request a peer review of the assessment at its discretion.

When to carry out wind modelling

Wind modelling should be completed early in the design process, especially for tall buildings. 

Changes in the form of buildings can often moderate its impact. Slender towers with podiums are an effective way to allow wind to flow through at higher levels, whilst deflecting downdrafts away from the street.  Verandas may help improve conditions at the base of the building.

Tall and wide buildings without podiums or verandas are unlikely to meet the standards.

Required standards for wind modelling

Internationally recognised standards known as the Lawson LDDC wind standards are to be met. These provide an expected maximum wind speed that should be experienced most of the time. For instance, on footpaths, the wind speed should be no more than 6m/s for 95% of the year.

Wind assessments should be run using an accurate model of the city that includes buildings within a radius of 200m of the proposed site and must demonstrate that the standards are met within a radius of 100m, or that conditions are no worse.

For 95% of the year, the following average speeds should not be exceeded at ground level, using the Gust Equivalent Mean measure:

  • 4 m/s at the boundary of the site street frontage for the width of the footpath, or within public open spaces such as parks or public squares.
  • 6 m/s within any carriageway adjacent to the site.

Additionally, new buildings must not result in dangerous wind speeds, which are those exceeding 15m/s more than 0.3% annually at ground level.