The Danish Road Directorate is responsible for 5% of the public road network in Denmark. This corresponds to about 3,800 kilometres of the overall 70,000-kilometre road network in Denmark. However, more than half of all Danish transport is via these national roads. A central task for the Danish Road Directorate is to make these roads safe and accessible for road users. Due to the extreme precipitation events that Denmark has experienced in recent years, and which will only be more frequent and more powerful in the future, the Danish Road Directorate has directed particular focus on preventing road flooding. The Directorate is working on the basis of the UN's A1B scenario, which predicts an annual precipitation increase of 11% in 2050 and of as much as 22% in 2100, relative to the meteorological baseline period 1961-1990.
The Danish Road Directorate already has a good idea of where problems typically occur in the road network during extreme rainfall events. However, the Directorate needs more knowledge about which stretches of road will be particularly vulnerable in the near and in the far future. Therefore, the Directorate has had an analysis method developed which can accurately identify which parts of the road network will be at the highest risk of flooding. This mapping has been included as a part of the Directorate's climate change adaptation strategy. The mapping will be used to prepare a decision basis consisting of a model which can identify the risk of flooding within a radius of 2 kilometres from the roads. The Directorate uses the concept of 'blue spots' to denote vulnerable places in the road network. In other sectors, the blue-sport concept is often used to denote hollows and depressions. Normal blue-spot mapping can be compared to pouring water onto an impermeable plate. However, such mapping only indicates where in the landscape hollows will allow water to pool up. "Our model takes this further. We define a blue spot as a stretch of national road where the probability of flooding is high and where the impact of flooding is significant," explained Christian Axelsen MSc, who works in research and development at the Danish Road Directorate. "Probability is determined by looking at factors such as the design capacity of the drainage system, precipitation statistics and local soil conditions. Impacts are determined by socio-economic factors such as the number of road users influenced by a flooding event. In order to be identified as a blue spot in our model, we have to have a stretch of road that is sufficiently prone to flooding and where flooding will have considerable impacts," said Christian Axelsen.
Illustration of the maximum extent of an identified blue spot.
The model is placed in a GIS environment in which several different data types combine to identify blue spots. This is a three-tiered model, which, step-by-step, ultimately identifies blue spots in the Danish national road network. In brief, the model first screens an area for hollows and depressions. Then, it analyses those most at risk. At the third tier, the catchment area and existing drainage capacity is included in the analysis. The second part of the equation incorporates the likely impacts of flooding. If a stretch of road is flooded where the annual average daily traffic (AADT) is low, then this stretch of road will not meet the criteria for identification as a blue spot. The threshold value for when flooding is considered to cause considerable impacts has been set to 10,000 road users per day. In order to calculate the impacts for future scenarios, the model is also fed with traffic projections and population growth data. "By using a tiered analysis, sub results from each tier of the analysis will mean that by far the majority of potential blue spots are eliminated for further analysis, because many of them will turn out not to meet the Directorate's criteria for blue spots," explained Christian Axelsen.
Impact matrix used to grade identified blue spots. This matrix helps identify which blue spots are to be given priority focus and resources. In the Directorate's Tier-1 screening, the model indicated around 500,000 hollows in the terrain along state-owned roads. At Tier 2, the number was reduced to around 5,000 real-risk flood locations. At Tier 3, the number had been reduced even further after additional analyses and inspection. By using this model, 11 locations have now been identified which meet the Directorate's criteria for identification as blue spots. The survey has also showed that by 2050 there will be an additional 20 blue spots, and by 2100 yet another seven to eight. The blue spots are located throughout most of the Danish national road network, however the majority are in the Greater Copenhagen area.
The solutions required to adapt the identified blue spots to the future climate conditions depend on the specific characteristics of the individual location and on what will provide the best adaptation for the money spent. "We go out and inspect the identified blue spots sites, and, on this basis, we come up with a recommendation which takes account of our service targets and what will provide the most economical solution at the individual location. We try to avoid any excavation work if there are other solutions which both the Directorate and the road users can accept. For example, the problem may be managed by installing a fixed pump, increasing the response capacity with mobile pumps or by cleaning drainage and sewers more often," said Christian Axelsen. The Directorate's model has already attracted interest from abroad. Thus, the model has been used in Ireland, Sweden and the Netherlands, and the model has also attracted interest from the US. Christian Axelsen believes that Danish municipalities may find inspiration in the model as well, even if it cannot be transferred directly to municipal conditions. "Municipal roads will often turn out to have significantly different characteristics and to possess a far greater complexity in an urban setting. Of course, we encourage municipalities to find inspiration in the approach used in our model. And they are welcome to draw on our knowledge about the catchment area around national roads. Managing water requires cross-disciplinary efforts, so we may also benefit from the experience of municipalities," he said.