Storm-Induced Tree Failures and the Effect on Emergency Service Accessibility in Brunswick, Germany

Severe storm events pose a growing threat to urban infrastructures. In particular, falling trees can block streets, which significantly disrupts the functionality of the road network and limits the operability of emergency services. Here we present framework to assess this impact of severe storm scenarios on the road network of the city of Brunswick, Germany.

For our assessment, we apply fragility curves for trees to estimate the probability of failure under a certain storm scenario. We use fragility curves that are parametrized on tree height classes. To translate tree failures into road blockage, we define a closure logic which determines if a fallen tree results in a blocked road segment by combining information on tree height and road width. For this, we use tree height data from the city tree cadaster and estimate road width by measuring the width of representative, single lanes for several road class types in OpenStreetMap.

We construct a probabilistic map of road closures across the urban network for the selected storm scenario using a reliability-based approach by combining the individual failure probabilities of all trees located along a road segment. This probabilistic closure map is converted to a deterministic disruption map by drawing a random sample of open or closed streets based on their calculated probability of failure, resulting in one realization of a disrupted road network for the city. The cascading impact on emergency service response is illustrated by calculating isochrones for ambulance dispatch locations, such as fire stations. Finally, we compare the isochrones of the disrupted network scenario with the normal scenario, which allows us to quantify changes in accessibility of emergency services.

The presented framework provides a quantitative measure of the robustness of the city road network for tree-induced road closures under extreme storm events. It supports the identification of critical roads and thus enables the systematic assessment of cascading effects on emergency service accessibility. Overall, the framework is helpful for risk-informed urban planning, emergency preparedness and adaptation strategies.