Extreme wind speeds over a Sitka spruce plantation in Scotland

Abstract

Hourly-mean wind speed, gust factors and turbulence intensity at the top of a Sitka spruce plantation (of maximum canopy height 16 m) were recorded continuously over a 15 month period. The maximum hourly-mean wind speed recorded was 13.2 m s −1, the turbulence intensity was in the range 0.33–0.54 depending on mean wind speed and the gust factor was 2.1 on average. The cumulative distribution of wind speeds was found to follow the Weibull distribution with parameters k = 1.55 and c = 3.46 m s −1. From this distribution the parameters of the Fisher-Tippett function describing the cumulative distribution of the annual extreme hourly wind speeds were estimated. Hence the most likely extreme in a 50 year period was calculated to be 16.8 m s −1. The characteristics of the variation in wind speed were found to be in acceptable agreement with those from theoretical and empirical descriptions of the effect of surface roughness on wind speed developed for the UK building industry. Turbulence intensity was found to be lower than predicted by theory at high mean wind speed. Closer agreement with the experimental results was achieved when the surface roughness in the theory was allowed to decrease with increasing mean wind speed, as is likely to happen when trees bend. It is concluded that the methods developed by the building industry for predicting wind climate are an adequate basis for calculating the risk of wind damage in coniferous plantations. This conclusion is important because there are few long-term measurements in forested areas.