Dynamics and aerodynamics of deciduous urban trees
Strong winds cause a great deal of damage to trees, and these falling trees inevitably damage property, communication lines and may even cause fatalities. This thesis presents an investigation into tree stability. The following genera were found to be most commonly occurring in extensive surveys in the south of England following the strong winds of October 1987: Acer (Maple), Aesculus (Horse chestnut), Fagus (Beech), Quercus (Oak) and Tilia (Lime). The Fagus and Tilia were found to be more prone to uprooting whilst the Aesculus and Quercus were more susceptible to crown breakage. Trees of all sizes were observed to suffer crown damage and/ or uprooting. However, with the exception of trees above 30 m in height, trees of every height were also found undamaged by the high winds. Smaller trees (5 m) were rarely recorded in the storm damage surveys, but this was thought to be due to the smaller amount of damage caused by them rather than being representative of the actual number damaged. The literature review revealed little information on broadleaf trees but suggested that uprooting was a dynamic process. Observations of a Platanus (Plane) in high winds revealed natural frequencies of 0.25 Hz whilst the tree was in leaf and 0.8 Hz when it was without leaves. Damping coefficients of 0.25 and 0.08 respectively, were also measured. High wind speeds were recorded whilst the tree was without leaf, but greater tree displacement was recorded at the lower wind speeds when the tree was in full leaf. Forced oscillation experiments produced similar values of natural frequency and damping. Additionally, with static loading experiments to determine load/ deflection curves, values of drag coefficients were determined. These were calculated to be 0.8 whilst the tree was in leaf and 0.2 without leaves. The value for the tree in leaf is similar to those determined for conifers by Mayhead (1973a). The uprooting of stumps showed self-seeded trees to be more stable than those planted in soil pits which had developed pit-bound roots. Uprooting by winching at heights of 0.5 m-1m, enabled these comparative conclusions to be drawn, but was thought not to realistically simulate the uprooting process of the wind acting on the trees' crown.