Modified wind speed due to topographic effects

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King, A.B.; Revell, M.; Carpenter, P.; Turner, R.; Cenek, P.; Flay, R. 2012 Modified wind speed due to topographic effects. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/07 29 p.

Abstract: The aim of the research described in this report is to reduce the vulnerability of New Zealand’s built infrastructure to wind damage through provision of improved design wind speed procedures. Wind flow in New Zealand is strongly influenced by the hilly terrain over which it passes, with both valleys and hill crests experiencing stronger, and in some instances much stronger, wind speeds than over flat terrain. Increased wind speeds are a potential hazard for towers and pylons used to support both infrastructure and communications equipment which are often located near or on hilltops. In New Zealand, at locations far from any wind measurements, design winds are frequently estimated for such proposed structures by applying the AS/NZS 1170.2 loadings standard. A relevant question is – “How accurate is it?” In an attempt to answer this question this report presents results from a specific experiment comparing measured wind speedups over the rugged Belmont hills in the Wellington area of New Zealand with wind speedups estimated from the AS/NZS 1170.2 loadings code from two organisations, through computer (Gerris and WASP) modelling, and through wind-tunnel modelling. It was found that computer modelling with CFD code Gerris and wind tunnel calculations agree remarkably well with observations and differentiate considerably better between areas exposed or sheltered by the local terrain compared to applying the CFD code WASP or applying the AS/NZS 1170.2 loadings code. Furthermore it was found that the AS/NZS1170.2 predictions differed significantly between the organisations. However, it should be noted that for a single location this increased accuracy does come at considerable computational cost. (auth)