Early Warning System for Tree-Fall Hazards on Railways: An Example of a System Developed for the Czech Railway Infrastructure Administrator

An early warning system Stromynazeleznici.cz (trees on railway tracks) has been developed to assist the national rail infrastructure administrator (Správa železnic, SZ) in managing the hazard of tree falls. A forecast of the tree-fall hazard on a 3-hour basis for the following three days is provided. The model incorporates data from weather forecasts (Aladin model) and a tree-fall susceptibility layer which delimits the locations where falling trees are capable of crossing railway tracks.

The tree-fall susceptibility layer is prepared from the raster of a normalized digital surface model. One-meter cells contain information about the absolute height of the surface above the relief model. All non-vegetated areas (all types of buildings, tall objects, bridges, masts, etc.) and areas with low vegetation that do not pose a hazard are filtered out. Impact zone buffers are defined for the remaining vegetation areas according to the actual height of the vegetation. The final output is a proportion of the length of railway lines per unit section which are threatened by falling vegetation.

Stromynazeleznici.cz contains tree fall evidence for recording, presenting, and exporting incidents. The forecast is based on a regression model programmed in R (server solution Project R). A multivariate logistic regression was chosen as the most suitable approach to construct the model according to cross-validation results and practical requirements. The following characteristics were selected as explanatory variables in the logistic regression: maximum daily wind gust, soil saturation index, snow index, the occurrence of thunderstorms, the season, the range of altitudes in the vicinity of the rail track, the median height of trees along the railway tracks, and the length of the rail track section with trees along the rail track.

The hazard level of tree falls is calculated for the "hectolines" (i.e., 100-meter segments) of the railway track. These are then aggregated into three levels of administrative units defined by SZ. The hazard level is calculated for three-hour intervals, covering a 45-hour forecast period – resulting in 15 time slots for each hectoline (the rail network in Czechia consists of 94,759 hectolines). The forecast is updated four times a day as new meteorological data become available.

The data is stored in a database and presented in the form of graphs, tables, and an interactive map. Hazard information can be found on the map: the tree-fall hazard level is represented by a five-level colour scale for individual administrative units. When zooming in, the risk is shown in relation to the hectolines. A timeline is located at the bottom of the screen, allowing users to switch between different time slots or aggregated time windows. Clicking on an administrative unit or hectoline will display the forecast and details for the selected element. The map also offers additional thematic layers — fallen trees, a layer showing vegetation susceptibility to falling onto the railway track, a tree health layer (derived from the Sentinel-2 data), and a forest tree species layer.