A risk-analysis procedure for urban trees subjected to wind- and rainstorms

Urban trees can play a crucial role in developing sustainable, safe and resilient cities, but at the same time they can pose risks. With an engineering approach, the study deals with a procedure able to quantify the risk that a tree subjected to the action of extreme wind events, without (windstorm) or with (rainstorm) ongoing precipitation, can cause by falling on potential targets. Such a Quantitative Risk Analysis (QRA) is based on the combination of four components (hazard, exposure, vulnerability and damage) that can be modeled more or less accurately depending on the objectives of the analysis. The innovative aspect is the characterization of the analysis, by means of two factors: the “Scale”, to describe the spatial width over which the QRA is performed, and the “Degree of Knowledge” (DoK), to state the accuracy used to estimate each QRA component. Firstly, QRA components and principles behind such a distinction in DoKs are explained. Then, the proposed procedure is tested by means of a practical example, where a sensitivity analysis considering different DoKs for hazard is carried out. Despite further studies are required to improve the procedure reliability, the results are encouraging. Even considering low DoKs, the procedure is able to quantify the risk that can be used in comparative terms, especially useful when quantifying the efficacy of risk mitigating actions.