Figure: Bathymetric Survey of the area
A new fixed link across Roskilde Fjord in Denmark is proposed to relieve the severe congestion in the city of Frederikssund for traffic across the existing bridge that was built before the Second World War. The Danish Road Directorate undertakes a major EIA study in order to develop and evaluate new alternative alignments for a fixed link. DHI has been commissioned the hydraulic investigations for this study.
The Fjord of Roskilde is designated as a NATURA 2000 area as it is an important refugee for migrating and resident water fowls. NATURA 2000 is an ecological network of protected areas in the territory of the European Union designed to protect the most seriously threatened habitats and species across Europe under the Habitats Directive and Birds Directive.
The EIA poses a number of challenges:
- Plans or projects that potentially can impact a NATURA 2000 area must be evaluated taking into consideration the best available scientific knowledge with the field.
- The hydraulic conditions of Roskilde Fjord are governed by a number of natural and man-made restrictions that controls the flow to the inner part of the Fjord. This challenges the hydrodynamic models that must be able to resolve both the area around existing and new marine structures as well as the narrow navigation channel along the Fjord.
- The water quality of the inner part of Roskilde Fjord is sensitive to changes in the water exchange with the outer part and the intrusion of salt water. Thus 3D effects must be included in model.
- Eelgras formations along the fjord are important for feeding of water fowls. The eelgrass formations are sensitive to increased turbidity due to either sediment spills during dredging works or enhanced biological productivity from release of nutrients.
Figure: View of Roskilde Fjord in Denmark, Island of Zeeland and possible corridors for a new link. The photo inserts shows view in the proposed corridors, one along the existing bridge and one further south.
The EIA process evolves around establishment of baseline information and development of viable tunnel or bridge options.
The success criteria for the EIA study defines that the preferred option shall improve the environment or be as neutral as possible and that during construction the impact is minimized with respect to the Fjord.
About 50 different options for the fixed link have been considered including long and short bridges and submerged tunnels. A decision process has been established by Road Directorate where all the aspects of the EIA have been evaluated in order to select a limited number of options that shall be assessed in more details.
DHI has contributed in the selection process with assessment of hydraulic and water quality issues. The selection has been based on an understanding of both the detailed flow and impact of marine structures on the hydraulic resistance as well as the Fjord as a whole.
In the next phase of the EIA study the impact of the selected link options shall be determined and their impact minimized. For this purpose a MIKE 3 FM model has been set-up that is under calibration by comparison with both data from the national Danish Marine Environmental Monitoring Program and hydraulic data collected by DHI specifically for this study (bathymetry, water levels, currents, salinity, temperature and sediment sampling).
Figure: Zoom of computed current speeds around the existing bridge.
In order to minimize the impact of the marine structures of the fixed links the layout is evaluated and developed in close collaboration with the technical consultant in order to achieve a design with least hydrodynamic resistance and if required achieve a so-called Zero Solution. The water quality aspects are evaluated in collaboration with the environmental consultant.
The study continues in 2009 with a final recommendation at the end of the year.
Figure: Example of representation in MIKE 3 FM of one of the many fixed link options in the area close to the existing bridge.