Will river floods ‘tip’ European road networks? A robustness assessment

River flooding is a major climate hazard for Europe and a threat to its road transport infrastructure. Its impact on road network interruptions is mostly unexplored, while some have suggested that national road networks may experience tipping points. This study assesses the robustness of road networks in European countries and their potential for a tipping point: an abrupt and disproportionally large loss of network functionality, due to unfavourable combinations of floods. Ten-thousands of flood combinations are sampled and their impacts on road network performance are assessed, following a methodological approach inspired by percolation analysis.

The results show that Albania, Croatia, Serbia and Austria are relatively vulnerable, whereas Belgium, Estonia, Lithuania and Portugal are relatively robust.  Several factors contribute to a country’s vulnerability: large flood exposure, an unequal ratio between country length and width, the presence of mountains, and main road corridors located in the floodplains of rivers. In small mountainous countries, such as Slovenia, Macedonia and Albania, the modelling showed that some small-scale floods may disrupt 32 to 41% of the preferred routes between economic centres, causing severe traffic disruption. More broadly, the modelling in this study showed that tipping points in European countries – in the sense of nationwide network fragmentation – seem unlikely to be due to river flooding, but regional-scale tipping points can happen. Flood-proofing the identified weak spots could result in quick wins for national road operators.