Investigating the demolition of the Vanhankaupunginkoski dam continues

The City of Helsinki has commissioned a preliminary study on the technical feasibility and costs of demolishing the Vanhankaupunginkoski dam. The recently completed technical and economic preliminary study provides an idea about the effects that the demolition of the dam would have on the local environment.

The area studied includes Vanhankaupunginkoski rapids and the upstream section up to Ruutinkoski rapids in the Vantaanjoki river and up to Kirkonkylänkoski rapids in the Keravanjoki river.

Demolishing the dam would have repercussions

Completed in 1873, the Vanhankaupunginkoski dam is an arch dam whose stones have been laid such that they are supported against each other. According to the study, it is not possible to partially demolish the dam due to the construction method.

Demolishing the dam completely would, in turn, lead to a drop in the water level of the Vantaanjoki river. However, the impact could be mitigated by building new bottom sills in the Vantaanjoki river that would make it possible for fish to travel up the stream.

The preliminary study has found two possible options for the safe demolition of the dam. In the first option, the dam would be completely demolished and a rapids-like bottom sill would be built at the northern end of Kuninkaankartanonsaari island. The area south of it would be shaped like a rapids. In the second option, two bottom sills would also be built on the north side of the dam area.

Both of the above options would improve the possibilities for the sea trout, eel and migratory whitefish to travel up the Vantaanjoki river. In all the demolition options studied, facilitating the upstream travel of fish requires the reconstruction of the rapids area by means of waterway and landscape construction.

According to the study, the demolition of the dam without bottom sill construction would involve significant risks. In this option, the water level of the Vantaanjoki river would drop the most, over a distance of nearly 10 kilometres. At worst, the drop in water level would affect the stability of the soil in the riverbanks in such a way that the bridges, buildings and other structures in the area would be in danger of being damaged.

Cultural-historical value is also considered

The technical and economic preliminary study was commissioned by the City of Helsinki and prepared by Sitowise Oy. The preliminary study was carried out after the City Board decided during the processing of a council initiative that Helsinki would initiate the necessary measures to demolish the Vanhankaupunginkoski dam and would carry out the relevant studies. The City Council discussed the relevant council initiative on 1 June 2022.

The technical and economic preliminary study helps to assess what further planning and investments would be needed to demolish the dam. In addition to a more detailed environmental impact assessment, it must be investigated how the demolition can be carried out considering the values of the cultural environment. In the current local detailed plan, the dam and bridge structures have been protected or ordered to be replaced in such a way that the historical value of the environment is preserved.

The City of Helsinki Urban Environment Committee will discuss the technical and economic preliminary study at its meeting on Tuesday, 28 February. According to the decision proposal, the Urban Environment Division will complete the environmental history survey of the Vanhankaupunginkoski dam and continue to investigate the demolition of the dam in more detail. The demolition of the dam requires the preparation of a new local detailed plan for the area.

The Vanhankaupunginkoski dam is located in the western branch of the river. In the further planning, the impact of the demolition on the eastern branch must also be examined in more detail. A plan has been drawn up for the restoration of the eastern branch. Its implementation is awaiting a decision on the possible demolition of the dam, since the flow and water level of the eastern and western branches affect each other.