Risks and probabilities mapped out
The study of blue spots was a response to parliamentary questions about the vulnerability of Dutch infrastructure given the increase in extreme weather events. Deltares looked at the risks, probabilities and impact of possible problems with excess water. The study looked at the probabilities and consequences of three categories of extreme weather:
- major flooding resulting from dike failure;
- local flooding caused by intense rain or long periods of rain;
- problems with water during very heavy showers on the road.
It was found that there is only a minor risk of local flooding as a result of long periods of rain. On the other hand, very heavy showers on the road can result in more problems because this is a more likely event. As a result, road users can get into difficulties with, for example, aquaplaning, and it will be more dangerous to drive. Major floods can have major consequences. Unusable roads can lead to economic failure costs. Even so, the risk profile is comparable to that for aquaplaning because the risk of major floods is very small.
Changes to road network for evacuation, and to the design shower
In addition, the study looked at whether the road network can be used for evacuation purposes during floods. It emerged that the Dutch road network was not designed to cope with floods and that the roads will not generally be usable after a dike breach. If that happens, the roads will either be flooded or too damaged to use. On the basis of the findings, Rijkswaterstaat will, in the future, design roads to cope with more extreme rainfall in order to minimise problems with water. The ‘design shower’ has been altered: from approximately 11 millimetres to 14 millimetres of rain in 5 minutes.
The study of blue spots was used in the European ROADAPT project to establish a European directive for road management agencies, setting out how they should manage the impact of climate change.