Drag coefficient in urban trees

Key message The drag force is frequently responsible for tree fall; however, its calculation depends on a rarely known parameter, namely, the drag coefficient. This paper reviews this topic. The pressure exerted by the wind on trees can generate large drag forces, which have been considered the most frequent causes for mechanical failure in trees. To calculate these forces, one of the main required parameters is the drag coefficient. However, the mechanisms in plants for reacting to wind pressures differ from those that are considered in analyses of civil structures, since plants have complex architectures and higher flexibility in the composition of elements (crown, stem and roots). The static or dynamic analysis is another factor that deepens the complexity of the discussion of the drag coefficient since the wind is dynamic; the response of the tree to this load is also dynamic. In addition, the existing information is scattered and varies according to the form of analysis and the simplifications that are adopted by the authors, thereby rendering complex the decisions regarding which methodological path to take to calculate the drag coefficient and how to measure the influence of the values accurately. The main objective of this work was to present and discuss factors that affect the calculation of the drag coefficient in trees using results from the literature. The results were analyzed and discussed using comparisons among studies and through the authors' interpretations. It is hoped that the results will provide information that enables researchers to identify ways of simplifying methodologies for determining the drag coefficient, without reducing the complexity of the topic, and, based on this knowledge, to infer the errors of the embedded results.