A diver relocates thick-selled river mussels to safety ahead of tram bridge construction works
The Vantaa light rail route crosses the Keravanjoki River at Jokiniemenkatu. Construction of a new bridge is due to begin shortly, but first, we will carry out a relocation operation to protect the legally protected thick-shelled river mussels. Divers will collect the mussels from the riverbed and move them upstream to an area where the species is already established.
Diving is a team effort, both above and below the water. Photo: Sakari Manninen / City of Vantaa
The diver regularly checks the air pressure in their diving cylinders. Photo: Sakari Manninen / City of Vantaa
Diving requires specialist expertise
Diving is a highly precise operation, carried out by Alleco Ltd, a company specialising in aquatic environmental issues and underwater work. The divers are biologists who have also completed specialist scientific diving training. Juha Syväranta of Alleco explains what the work involves beneath the surface:
-We go over every square centimetre of the riverbed to find all the thick-shelled river mussels. The work requires careful planning and experience, as it is pitch-black even at a depth of just one metre. We have divided the riverbed into lanes approximately one metre wide, marked out with weighted ropes. Following these ropes allows the diver to systematically search the entire area. Typically, the diver follows the rope with one hand while feeling along the riverbed with the other.
Relocating the mussels is an effective conservation measure
The relocation operation aims to protect thick-shelled river mussels from the various risks associated with construction. The main threats include damage during the construction of the temporary work bridge and the piling work required for the new bridge foundations. Mussels play an important role in the wider ecosystem, including helping to filter and purify water. They can typically live for around 50 years.
The Keravanjoki river is home to a surprisingly diverse range of wildlife. Its clay-rich riverbed gives the water a dark appearance, while also contributing to its nutrient-rich conditions. The protected thick-shelled river mussel is just one of the many species living in the river. Otters and migratory fish can also be found here.
Thick-shelled river mussels share many characteristics with the much rarer freshwater pearl mussel. However, unlike the freshwater pearl mussel, they are adapted to living in darker, more turbid waters.
Thick-shelled river mussels typically measure around 5–10 centimetres in length. Photo: Juha Syväranta / Alleco ltd
Relocating thick-shelled river mussels and carrying out work in their protected habitat requires a special permit. Outi Hyttinen, Senior Specialist at Sitowise, explains the background to the project:
-Environmental surveys related to the Vantaa light rail project began several years ago. Particularly sensitive areas were identified early on, allowing them to be taken into account during the design phase and carefully monitored throughout construction. The project also initiated discussions on environmental matters with stakeholders well in advance. Construction can always bring unexpected challenges, but a solid knowledge base and a proactive approach to environmental issues provide a strong foundation for success.
Lifecycle impacts considered in light rail design
The Vantaa light rail has been designed with its entire lifecycle in mind. As the river was known to be a particularly sensitive environment, the new bridge has been designed to require as little maintenance as possible. Its structures and materials have also been selected to prevent anything unnecessary from entering the river, for example during maintenance work.
-We are building a dedicated bridge for the light rail alongside the existing road bridge. The new bridge will be approximately 50 metres long, with construction work getting fully underway in 2027. From the contractor’s perspective, it is reassuring to start work knowing that the environment has been thoroughly surveyed. We can begin building the bridge with confidence, knowing that the thick-shelled river mussels are safe in their new location, while remaining in the same familiar river, says Atte Heiskanen of Destia, the sub-project manager responsible for construction.
