Site Analysis

An assessment of wind speeds and reliability may also influence decisions about micro power generation.

In assessing a site for wind, consider the speed (average and peak) and direction of wind affecting the site at different times of year. As well as the prevailing wind direction, consider which direction the strongest, wettest, coldest, most humid and most annoying winds come from, whether any winds come straight from land or sea, and any local effects.

Assess the shelter provided by existing buildings and planting – will these remain over the life of the building? Can the effect of the wind be moderated by the shape and orientation of the building, future planting and specifically designed windbreak fencing.

Climatic conditions may also determine how the design deals with wind – for example, in hotter humid climates wind can be used to provide a beneficial passive cooling effect.

Wind speed

The wind speed affecting a specific site will be influenced by:

• general wind speeds in the region
• how exposed the site is
• the terrain – for example, wind gathers speed as it passes over or between hills, or over open expanses of water or flat land
• ground roughness – wind slows down as it passes over rougher terrain, and accelerates over open and flat expanses of water or land
• effect of the sea or other water expanse – for example the cooler sea (relative to the adjacent land) generates onshore winds in the afternoon
• neighbouring buildings – wind speeds are lower when a site is surrounded by tall buildings, and higher on parts of a building that is taller than others around it; wind can also gather speed as it is funneled around buildings or through gaps between buildings
• neighbouring vegetation – wind is slowed when the site is surrounded by trees
• building height – parts of buildings which are taller than the surrounding buildings or shelter will be exposed to higher wind forces.

Climate change may result in more extreme weather events and designing for higher wind speeds should be considered as part of the design brief.

Under NZS 3604: Timber-framed buildings, buildings are classed as:

• below 32 m/s – low wind zone
• 32-37 m/s – medium wind zone
• 37-44 m/s high wind zone
• 44-50 m/s – very high wind zone
• over 50 m/s – requires specific design.

The standard divides New Zealand into two ‘wind regions’, and some areas are classed as ‘lee zones’ (i.e. zones where landforms create localised (down-wind) wind acceleration and therefore higher wind speeds). The design wind speed a specific site will be subject to depends on these two factors, as well as ground roughness, site exposure and location within the landscape (topography). A map of the wind regions and lee zones can be found in NZS 3604.

The design wind speed for the given site determines the amount of bracing required under NZS 3604 to resist the lateral wind loads on the building. Bracing must be calculated for both wind and earthquake loading – design the bracing for whichever of the two loads is greater.

Wind zones are, of necessity, based on a number of simplifying assumptions, such as the assumed height of a future building, wind direction, and very general topographic and sheltering information. Sometimes, the wind zones may under or over estimate the site wind speed. In these cases,  it may pay to have an engineer do an assessment based on NZS 4203 General structural design and design loadings for buildings. This may be a good option especially if he or she is already carrying out other work on the project.

Finding information

A site visit may give clues to wind speed and predominant or strongest wind direction. It can also be worth asking around the neighbourhood.

Indicators of higher wind speeds are:

• a general lack of developed planting
• stunted tree and shrub growth
• wind break fences on adjacent properties
• wind shaping of existing planting.

In addition:

• The local council should be able to tell you the wind zone of any property within its boundaries. Some local authorities have this information online. A land information memorandum will often contain information about the property’s wind zone.
• The National Institute of Water and Atmospheric Research (NIWA) provides climate station data for its 30 climate stations throughout New Zealand, including wind speeds, number of gale days per month, wind roses and more. The NIWA website also has a free climate overview, and NIWA can provide site-specific information on enquiry or using its Climate Explorer web tool. Climate information is also available from the MetService website.

For your clients

To help your clients understand the impact of climate on design, refer them to www.smarterhomes.org.nz/siting-landscaping/climate.