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.