Will more frequent river floods cause widespread malfunction of European road networks?
More frequent river flooding is a climate hazard in Europe, threatening its road transport infrastructure.
This study investigates whether the potential level of disruptive impact on the road networks of European countries could reach a socio-economic tipping point – with an abrupt and large loss of road-network function. To answer this question, the researchers investigate the robustness of road networks under unfavourable flood combinations in the European Member States.
In July 2021 Germany, Belgium and the Netherlands experienced large-scale flooding due to extreme rainfall events and overflowing rivers. These types of flooding events are now between 1.2 and 9 times more likely than in the cooler pre-industrial climate. Many roads and nearly all bridges in Germany’s Ahr Valley were severely damaged in the July 2021 floods, hampering reconstruction and crisis response, as well as effecting economic recovery of the area.
This research applies a form of statistical analysis to large-scale flooding events in Europe, to see if it can inform climate adaptation policy. The approach identified locations and regions prone to ‘abrupt and disproportionately large loss of network functionality, due to unfavourable flood combinations’, whilst noting whether nation-scale tipping points were realistic. Thus, the study advances knowledge on the emerging climate-driven flood threat in Europe and highlights a new form of analysis to identify road-network regions most prone to this threat for targeted action.
The researchers assessed the robustness of the national road networks of 30 European countries and the probability for socio-economic tipping points. They modelled ten-thousands of flood combinations, and their impact on road-network performance. They used a uniform 100-year flood map, from which they sampled combinations of small flood events, across all countries. For a given location, such an event has a 1 in 100 chance (1% probability) of being equalled or exceeded in any given year. But the researchers note that the chance that one of these events will happen somewhere in a Member State in any given year is much larger. They add that some of these events will do hardly any damage, while others may be very disruptive.
To identify policy-relevant tipping points the researchers used a stepwise approach:
- Steps 1-3 focused on whether river floods would cause tipping points in European road networks. They applied three road-performance metrics: isolated trips, preferred routes disrupted, and travel time increasing over remaining trips. Step 3 identified a tipping point using the following practical perspectives:
1. A large river flood event at multiple locations, concurrently disrupting routes between economic centres beyond thresholds for the three road-performance metrics.
2. A high likelihood of a small-scale flood event that has a more extreme impact, compared to other events of the same size.
- Step 4 identified the tipping points from many combinations of concurrent floods.
- Step 5 introduced impact assessment, comparing susceptibility for different European countries, and effect on national road network performance.
- Step 6 identified those countries more vulnerable to flooding and informed national road operators of vulnerable critical network links.
The results show that the road networks of Greece, Albania, Croatia, Norway, Serbia, Latvia and Austria are relatively vulnerable, whereas Belgium, Estonia, Lithuania and Portugal are comparatively robust. The researchers suggested several factors contribute to a countries’ vulnerability: large flood exposure, an unequal ratio between country length and width, 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 due to river flooding, but regional-scale tipping points can happen. The researchers suggested that flood-proofing the weak spots could benefit national road operators.
The study’s relatively simple modelling approach is useful for large-scale flood risk studies, to note the impact of road and other network disruptions. On a smaller scale, the inventory of disruptive outlier events (small-scale floods which have greater functional impact than expected), can be used by national road operators in European countries, to note individual road segments that are most vulnerable to flooding and make targeted infrastructure adaptations to ensure flood resilience1. Furthermore, the researchers suggest that investment banks and policymakers can use the study’s findings on road-network robustness to inform climate adaptation policies.
- Further preventive measures that policymakers can take to avoid flood disruption to roads:
- Interventions to improve road functionality (e.g. elevate the road embankment, strengthen bridges, use more flood-proof pavements, take measures to prevent erosion, etc).
- Interventions at road network level (e.g. add new roads and bridges to the network at the most critical locations, so that there is an alternative if a vulnerable road is hit).
- Interventions at the system level (e.g. increase stocks of factories and avoid just-in-time deliveries, so that production can continue even if the road is temporarily unavailable).
The role of emergency management is also very important say the researchers. For example, when a heavy precipitation event is expected, it is very important to issue a warning that people should stay at home. Traffic chaos will happen well before the network capacity collapses due to inundation, which might severely hinder/hamper all emergency response activities. The study shows the vulnerability of road networks to river floods. Comparison with the degree of vulnerability of other transport or communication networks is not included in the study. Furthermore, there is no consensus in academic literature to conclude whether road networks are more vulnerable than other transport or communication networks.