Danish Road Directorate - Blue Spot Analysis

The Danish Road Directorate (DRD) developed a Blue Spot Analysis to identify roadways vulnerable to flooding under present conditions and future climate projections and to inform its climate change adaptation response. A “Blue Spot” is a section of road where there is both a high risk of flooding and significant consequences from flooding. Road flooding is the primary climate adaptation challenge for Denmark and is the focus of the DRD’s climate adaptation planning.  The DRD’s strategy to adapt to road flooding is to manage flooding that occurs, improve roads when financially feasible, and to prevent climate-related impacts whenever possible.  The DRD uses Blue Spot Analysis to identify critical projects and assess the cost effectiveness of road improvements.

The DRD conducted the Blue Spot Analysis on the Danish national road network for current climatic conditions and projections for 2050 and 2100.  The methodology to identify Blue Spots contains four levels of analysis, with each level adding data layers to the analysis that help most accurately identify present and future floodprone areas and the consequences of flooding:

  • Level 1: Mapping of terrain depressions identifies low-lying areas in proximity to roadways where rainfall will collect assuming no infiltration into the soil. This is the first step to identify locations that are potential flooding threats to roads. This step utilizes a nationwide digital terrain model assembled by LiDAR data and with a resolution of 1.6 meters; this stage of analysis assumes no precipitation is absorbed by the terrain.
  • Level 2: Vulnerability calculates rain sensitivity of areas identified in Level 1, based on impermeable surface area. It produces a risk map that identifies the amount of rainfall required to fill particular depressions assuming a certain percentage of impermeability (ranging from 20 to 100%). At this stage, a national soil map is used, which helps distinguish between differing levels of natural infiltration. The analysis also factors in demographic data that helps indicate urbanization and acts as a proxy for predicting imperviousness of surfaces. Climate change factors and precipitation statistics are also taken into account; for example, perviousness of soil is expected to be lower in 2050 and more so in 2100 due to more frequent heavy precipitation events that cause soil already to be saturated.
  • Level 3: Hydrological Analysis models surface reservoirs and depressions to identify pathways, catchments, and ponds in risk areas. In doing so, it assesses water flow on the surface and in drainage systems to more accurately calculate flood risk. By adding in calculations that account for hydrological connections, this step of analysis can help predict how a given rainfall event will affect flood depths and retention time for a Blue Spot previously identified.
  • Level 4: Evaluate Consequences uses traffic load data and forecasts to analyze the impact of flooding on road users. The results are placed into a risk matrix that ranks probability of the Blue Spot flooding, and the consequences of such flooding.

The Blue Spot method also allows for addition of sea-level rise and riverine flooding to the analysis. At Level 1, inundation at varying levels of sea-level rise is mapped (taking into account dike locations and levels), as well as added flood risk from varying increases in river water levels.

The DRD utilizes the Blue Spot Analysis results to inform climate adaptation projects and to prioritize locations for allocating funding. Under current climate conditions, the DRD has identified 14 Blue Spots nationwide.  The DRD is still conducting the Blue Spot Analysis to identify locations likely to become Blue Spots under future projections for increasing precipitation.

The DRD bases its climate projections on scenario A1B from the Intergovernmental Panel on Climate Change (IPCC), with local projections modified by the Danish Metrological Institute. One of the most severe anticipated effects of climate change in Denmark is increasing and changing patterns of precipitation.  Annual rainfall is projected to increase by over 20 percent by 2100, and severe storms are anticipated to increase in frequency.

The Danish Road Directorate, or, Vejdirektoratet, governs 3,800 km of roads in Denmark, which contain 50 percent of the country’s traffic volume.  In 2013, the DRD published the Strategy for Adapting to Climate Change to provide guidance for more efficient climate adaptation planning. The Blue Spot Analysis has also been used by the Netherlands Ministry of Infrastructure and the Environment to identify flooding vulnerabilities.

 

This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on March 23, 2016.

 

Publication Date: September 2014

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