An Assessment of the Static and Dynamic Factors Involved in Windthrow

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Abstract
Static parameters of the windthrow process were measured by winching over ten Sitka spruce trees growing on a windthrow-susceptible site in the Scottish Borders. The critical wind speeds to cause uprooting were calculated using measured wind profiles and assuming static loading of the crown. Values obtained greatly exceeded the wind speeds recorded during a gale which caused damage. The ranking of stabilities of the ten samples was: Suppressed trees >gt; sturdy dominants >gt; slender dominants. Recalculation using measured damping ratios, and assuming that the gust frequency coincided with the tree frequency to cause resonance, reduced the values to within the range of the maximum recorded gust. Displacements of three sample trees, estimated from accelerometer data, were compared with simultaneous wind speed recordings. Perfect resonance was not observed but large oscillations built up over a period of 2–3 cycles. Estimates of the dynamic load factors varied from 0.5–5. The larger gusts tended to be associated with the smaller factors. Increasing the dynamic load factor from 1 (equivalent to a static load) to 2 reduced the critical wind speed by approximately 40 per cent. The influence of sway direction and canopy contact on the damping ratio of the three accelerometer trees was investigated. The results suggest that canopy contact comprises a greater portion of the damping ratio of small trees than that of dominants.

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