A 20-hectare (50 acre) deposit of polluted dredging spoils, accumulated over 40 years near the city of Gothenburg in Sweden’s industrial heartland, is getting a new life as a wetland designed for waterfowl.
The Port of Gothenburg has had a permit since the 1970s to deposit materials dredged from the harbor floor to provide sufficient draft depth for the ships accessing the Port. Spoils are deposited in a small bay nearby called Torsviken, located on the Kattegat sea. The deposition permit expired in 2009 and current Swedish environmental legislation now requires the deposit to be covered to prevent leakage and dispersion of polluted materials.
Just outside the deposition area, the Torsviken area is known for its bird and wildlife rich habitat. The construction of a new wetland on top of the dredge spoils deposit was a natural step to cover the materials and reestablish the area to its original use as a wetland thriving with wildlife.
Designing a wetland
At an early stage in the project, Golder was commissioned by the Port of Gothenburg to perform the deposition design works in regards to the obtained environmental permits, and manage the geotechnical challenges related to designing the new wetland.
The polluted sediments, between five and seven metres (16 to 23 feet) deep, lie on top of glacial clay that extends between five and ten metres (16 to 30 feet) down. To encapsulate the impacted sediment, the entire deposit was covered with half a metre (1.6 feet) of clean clay. The new wetland was constructed on top of the clay cap.
The clay for this project comes from the construction of the massive Marieholm Tunnel in Gothenburg – helping to solve the question of what to do with the clay extracted from the tunnel.
Golder developed a computer-based hydraulic model to simulate possible locations for the water inlets and outlets, to ensure a constant flow, and avoid stagnant water, which hinders algae production. Inside the wetland several large islands were constructed in locations simulated and optimized based on recommendations from the Gothenburg Ornithological Society.
A major challenge the designers faced was low bearing capacity, low shear strength, and compressibility of the dredge spoils and sediments. To ensure the performance of the wetlands design would work in the long-term, the deformation and settlement properties were studied.
Golder performed both field and laboratory tests and constructed a test embankment to measure the settlement of the impacted spoils over time. The test embankment showed that while most of the settlement occurred during construction, it continued throughout the full test period. Golder concluded that the wetland will require maintenance over time to adapt to the various rates of settlement.
Settlement of the dredged spoil also meant a practical challenge for working at the site and distributing the clay after delivery by truck. To allow trucks and excavators to work safely, temporary work paths were created using crushed material on geotextile.
The deposit was fully covered during 2019, and Golder is now working on the construction documents and specifications related to the water inlets and outlets, due to be installed in 2020.
The desired result will be an area where both birds and other wildlife can thrive in the future while birdwatchers and wildlife enthusiasts can marvel with the results of an engineering solution to address an environmental challenge.
Photo courtesy of the Port of Gothenburg.