Report on the impact of demolishing the Vanhankaupunginkoski dam completed

The demolition of the dam in the western branch of the Vanhankaupunginkoski rapids has been extensively reviewed in several stages and from various perspectives. Numerous reviews carried out over the years form the basis for a recently completed comparative analysis of alternatives, which examined the impacts of demolishing the western branch dam and restoring the eastern branch in relation to the aquatic ecosystem, fish stocks, biodiversity, the landscape and cultural environment, water management, recreational use, technical risks and costs. The conclusion is that restoring the eastern channel of Vanhankaupunginkoski rapids would be a more appropriate solution than demolishing the dam in the western channel.

In many locations, removing a dam and restoring rapids to a more natural state can improve the passage of migratory fish and enhance the river’s ecological diversity. The situation is different in terms of Vanhankaupunginkoski, however, as Vantaa River splits into two branches, of which the eastern branch already serves as a fish passage. According to the reports, even during periods of low water, there should be sufficient water in both branches for them both to function effectively as fish migration routes throughout the year.

Removing the dam would leave one river branch dry for part of the year

As part of the dam removal options, natural rapids would be created in the western branch and the water level upstream of the dam would be maintained close to its current level by constructing one or more bottom sills on the riverbed. At the same time, the eastern branch would be renovated in accordance with a previously drawn up plan.

In these scenarios, either the western branch would remain low in water or dry up for a long period, or the eastern branch would be dry for as many as 200–275 days out of the year. Restoring the western branch to a more natural rapids environment could enhance the landscape and ecological diversity, but the low water level would limit the benefits. Under several of the options, fish would have even fewer opportunities to migrate than at present.

A fall in the water level and periodic drying out would also alter the water balance and vegetation of the riverbed and banks. The existing shoreline trees could die off due to drought, and new pioneer vegetation could take over the exposed riverbank. Furthermore, there would be uncertainties regarding the foundations and load-bearing capacity of the existing buildings and structures in the context of further planning.

Demolishing the dam would require detailed further planning, a separate permit procedure under the Water Act, an amendment to the local detailed plan, and extensive impact assessments. The costs of further planning are estimated at between 600,000 and 900,000 euros. The cost of demolishing the dam is estimated at between two and three million euros. The cost estimates will be refined as the planning progresses.

Renovating the eastern branch would improve fish migration up Vantaa River

In the comparison, the best overall solution proved to be the renovation of the eastern branch and the preservation of the dam in the western branch. The renovation would increase the water flow in the eastern branch, facilitate the migration of various fish species, and improve recreational use and safety. At the same time, the historic dam and the area’s landscape and cultural heritage values would be preserved. This option is also clearly the most cost-effective, as the project plan estimates the cost of the renovation at around 650,000 euros. The plans are also already finalised and a permit in accordance with the Water Act has been granted for them.

As part of the renovation of the eastern branch, the fish migration route would be made less steep than it is at present. Resting areas would be created on the riverbed to help fish swim upstream; natural stones would be placed on the riverbed and spawning beds would be built at the top of the rapids. The increasing water flow would guide the fish more effectively towards the eastern branch and improve the chances of several fish species migrating up Vantaa River.