Bank of Actions
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Storm water wetland in Karisto
Actor: City of Lahti ● Year: 2015 ● Address:
A constructed storm water wetland system collects, retains and purifies the storm waters from a residential area in Lahti, Finland.
During 2008–2011, a large storm water wetland was built in a new residential area in Karisto in the city of Lahti. In the wetland, storm water is retained and purified before entering the eutrophicated Lake Kymijärvi. Furthermore, by using a wetland storm water is handled in an open system which also provides recreational benefits the local people. The wetland of Karisto consists of a large sedimentation pond and a wetland area. A meandering creek collects storm water from a 138 ha large catchment area, mainly used residential living.
Based on the available data of storm water quality, the wetland was estimated to reduce annual nitrogen load from the catchment area to Lake Kymijärvi by 38 % (61 kg N/y) and phosphorus load by 33 % (2 kg P/y). Because the City of Lahti is located in the Baltic Sea catchment area, partly these nutrient amounts are reduced also from Baltic Sea nutrient load. The wetland system provides also recreational benefits for citizen. As the number of residents will increase in future, the number of people enjoying the benefits of the wetland will also increase, and thus the value of annual recreational benefit were estimated to grow from 10 000 €/y to 20 000 €/y.
Comparison of investment and maintenance costs with benefits relating to recreation and an improved state of the Baltic Sea on a 50 year lifespan, resulted in a net present value of ca 100 000 €. This value did not include avoided costs from conventional underground storm water solutions such as drainage pipes and dwells. Furthermore, ecological benefits, such as the positive impact on local water systems relating to biodiversity and groundwater generation, were neither possible to include in the analysis. Thus, the storm water solutions seems to be worthwhile investments in a social perspective.
The impacts and the net present value of this action were assessed in the cost-benefit analysis study conducted in the EU Life+ funded project CITYWATER.
Name: Päivi Airas ● Email: paivi.airas ( at ) lahti.fi ● Web page: https://www.lahti.fi/