Hill shape has a huge effect on wind speed

The same factors that make New Zealand an ideal location for generating wind power - the hilly geography and its location in the path of the “roaring forties” - also mean that wind loadings are an important consideration when it comes to our building codes.

Hill shape has a significant effect on wind speed, and can increase its force up to three-fold compared to flat terrain nearby. This makes the hill shape multiplier very important when taking wind actions into consideration for constructing buildings and structures on hilly sites.

WSP Opus Research is part of the New Zealand Wind Engineering Research Consortium, which is investigating how to reduce the vulnerability of built infrastructure to wind damage by improving design wind speed analysis procedures. The other members of the consortium are NIWA, University of Auckland and GNS Science. The consortium has received funding from the Natural Hazards Research Platform to undertake this research.

In 2011 the consortium investigated the wind speed hill shape multipliers measured at a line of nine anemometer masts in Belmont Regional Park near Wellington. Wind speeds were also predicted with computational fluid dynamic computer modellng and wind tunnel testing in order to compare the results to the existing loading standard.

WSP Opus Researchperformed the wind tunnel testing for this project.  While the results from all three research components were generally in agreement, they differed from the wind speed hill shape multipliers calculated through the existing loadings standard. Differences in the way that the organisations interpreted the standards also created marked differences in the multipliers they calculated.

These comparisons will be used to provide a database of information to improve analysis procedures used by structural engineers to calculate design speeds for hilly sites.