Ensuring environmental protection and compliance for the world’s largest immersed tunnel – from planning to construction

The Fehmarnbelt Fixed Link is a major infrastructure project connecting Denmark and Germany via an 18-kilometre immersed tunnel. Set for completion in 2029, it will be the world’s longest immersed tunnel, reducing travel time and enhancing regional connectivity. Femern A/S, appointed by the Danish Ministry of Transport, is responsible for the planning, construction and future operation. The project must meet strict environmental obligations, ensure sustainable construction and report monitoring results to authorities. DHI has contributed from the early stages, supporting pre-investigations and EIA work, and providing environmental monitoring and consultancy throughout the approval and construction phases. The aim is to build essential transport infrastructure while safeguarding the sensitive marine and coastal environment of the Fehmarnbelt region.

Challenge

The main overall challenges for Femern A/S include efficiently managing the technical complexity of constructing the tunnel and obtaining necessary authority approvals, all while safeguarding the environment.

In the planning and approval phase, key tasks included assessing potential environmental consequences, selecting and defining the optimal project solution and securing the necessary approvals in both Denmark and Germany. During construction, continuous monitoring was required to document environmental impacts, such as on marine ecosystems like benthic flora and fauna and to provide contractors with live data on e.g. onsite local waves and current speeds. Further challenges included defining and complying with conditions related to environmental impact, such as acceptable sediment spill limits during dredging. Managing stakeholders and contractors, as well as maintaining transparency with the public, were also crucial aspects of the project. To address this, the ÆGIR portal was developed to facilitate real-time sharing of environmental data. 

Additionally, as a large project developer, Femern A/S aimed to improve knowledge on monitoring techniques, environmental impacts and potential nature-enhancing solutions for future projects.

Solution

In close collaboration, DHI and Femern A/S have worked together since the early days of the Fehmarnbelt Fixed Link project, developing a comprehensive range of environmental solutions beginning with the initial feasibility assessments in the mid-1990s. These services span from advanced modelling and environmental monitoring to the development of innovative tools and strategies that ensure the project's environmental compliance and sustainability throughout its planning, approval and construction phases.

Solution highlights

• Consultancy for design, e.g. making beneficial use of dredged material
  Incorporating nature-based solutions, 18 million m³ of dredged material was used to create a large land reclamation area, designed to provide new habitats and recreational opportunities. Through its consultancy services, DHI assisted in shaping and assessing the natural elements to ensure success in the dynamic marine environment.
  
• Advanced modelling and consultancy for EIA
  DHI used a combination of hydrodynamics, sediment transport, water quality and ecological modelling to reproduce environmental conditions before, during and after the tunnel construction, which in combination with field data, provided strong tools to predict and assess during the Environmental Impact Assessment (EIA) phase and to follow-up and document environmental impacts during and after construction.  
• Field surveys and environmental monitoring
  Comprehensive field surveys were conducted, including hydrography, water quality, turbidity and benthic flora and fauna assessments. The introduction of environmental DNA (eDNA) techniques provided broader biological information and assessments, supporting ongoing monitoring efforts.
• Defining and managing sediment spill budgets
  Working closely with the project team, DHI defined acceptable sediment spill budgets during dredging. This approach reduced environmental risks and ensured compliance with local ecosystem protection standards while providing operational spill requirements for the project to manage project timelines and costs.
• Development of the ÆGIR environmental data portal
  DHI developed the ÆGIR portal with Femern A/S to communicate environmental data including real-time monitoring data, ensuring transparency and facilitating efficient sharing of environmental information with authorities, contractors and stakeholders. Learn more about our work here.
• Innovative sediment spill monitoring using AUV technology
  An innovative Autonomous Underwater Vehicle (AUV) approach was implemented to monitor sediment spills close to the spill source. This method reduces interference with construction activities and minimises HSE risks while providing accurate data.

Results

Over the span of almost 30 years, DHI’s environmental monitoring and consultancy efforts have supported Femern A/S in meeting regulatory requirements and safeguarding the marine environment in both Denmark and Germany. Throughout different stages of the project, these ongoing efforts have helped manage environmental risks, ensure compliance with environmental conditions and facilitate communication with stakeholders, contractors and the public. The collaboration has also provided practical insights into environmental management and is expected to inform the development of improved environmental management for future infrastructure projects.