Skien municipality, located about 130 km southwest of Oslo, Norway, is considering NBS as part of a potential reopening of the Kjørbekk stream. Currently 4 km of the stream is contained in an aging pipe infrastructure that is buried under two disused landfills. The landfills were constructed in the 1960's and thus do not have bottom liners according to todays' standards. In addition, the pipe infrastructure that some of the Kjørbekk stream runs through does not have the physical capacity to cope with an increase in rainfall caused by climate change. These factors in combination mean that excess surface water that can't be carried by the pipe has the potential to come into contact with waste in the disused landfill and then become contaminated. Furthermore, this water can then be transported with the stream course and reach the Skien River.
In order to try and improve the water quantity and water quality challenges at the site, the Kjørbekk stream is being considered for reopening, which is also referred to as "daylighting" or "deculverting". Daylighting is a climate change adaptation strategy that results in the creation of more water pathways and therefore contributes to an alleviation of flood event peaks. At the same time, daylighting provides social and economic benefits as it reduces the climatic risk to communities and infrastructure.
Within the reopening of Kjørbekk stream, the following types of interventions are being considered:
• Altering the landscape using landscape modelling in order to reduce the amount of water that is in contact with potentially polluted soil and waste. By leading the water away from the landfill areas, infiltration is minimised and subsequently so is the amount of contaminated leachate water.
• Concave topography, terraced ditches, ditches to collect surface water and dams which can pool water are possibilities. The density of natural vegetation in the area could be increased in order to reduce the amount and the speed of infiltration water as well as minimizing both airborne and waterborne particle spreading.
• Constructed wetlands which have been shown to efficiently reduce concentrations of total suspended solids, organic matter, nutrients, heavy metals and a range of organic contaminants such as pesticides, pharmaceuticals and contaminants of emerging concern are being considered.
• NBS that can remediate contaminated soil such as aeration, natural degradation, monitored natural attenuation and the addition of a sorbent that can bind the contaminants are also possible interventions for the site.
All of these NBS could contribute to a reduction in the amount of water in contact with potential contaminants from the landfill waste, a reduction in the amount of particle spreading, support remediation of contaminated water soil and overall could function as a catalyst for landscape transformation and subsequently an incremental city development at Skien.
This case study not only offers a demonstration of the multiple potentials of NBS for sustainability, but it can also serves as an example for other sites where the redevelopment of disused landfills needs to go hand in hand with a protection of water quality and quantity. The title photo shows the final outlet of Kjørbekk stream to Skien River where potential contaminants can move with ground water and be released.
Small scale model of the case study site with the Kjørbekk stream running to the Skien River
Kjørbekk stream is located in Skien municipality, Norway and is 4 km long. Kjørbekk stream is in the Skien watershed (10 780 sq.km.), which is the third largest watershed in Norway, and the stream discharges into Skien River. Kjørbekk stream runs between lake Hvitsteintjern and Skien River. The water in Kjørbekk stream is led into a pipe and then travels via the pipe system, constructed in the 1960's, approximately 4 km to Skien River. Kjørbekk stream can be roughly divided into six different areas according to the type of terrain. There are some areas that have a steep slope with a high run off rate and some that are gentler with lower run off rates. The land types include forest, farmland, industrial areas and housing.
The area around the second landfill
The piped system is buried up to 15 meters deep and in certain places is buried under two disused landfills. The first landfill (referred to as "landfill 1" throughout) was used between 1962 and 1976 and is approximately 85 000 ha and 5-7 m deep and does not have a bottom liner. The second landfill (referred to as "landfill 2") was used between 1975 and 1993, is 12 ha and 13 m deep and it is unclear whether this landfill has a bottom liner. Both landfills contain household waste, waste oil and a mixture of different special waste and have top covers of unknown material type and quality, varying from 0.3 to 1.5 m. This adds complexity to the site and the implementation of NBS.
The industrial development and housing built on top of the landfill 1 area
The area that was previously landfill 1 has been redeveloped and now includes light industry, factories and a residential area. Anecdotal evidence suggests that the area was previously subject to swelling under the surface which could be due to the settling of the material in the disused landfill or from gas production from the material.
Field sampling carried out by Skein municipality and the Norwegian Geotechnical Institute
In 2004 samples were taken from two wells to investigate potential contamination of the leachate water. The sampling locations were situated at the point at which water from the Kjørbekk stream comes out of the pipe, just before it discharges to the Skien River. Concentrations of organic and inorganic pollutants were analysed in addition to water parameters. The results showed that concentrations of certain organic pollutants were elevated, and certain water quality parameters were impaired. In 2020 a new sampling round was carried out and although lower pollutant concentrations were detected, the water was still classified as having a 'bad' chemical status according to the EU's Water Framework Directive water quality standards.
The upper part of the waterway before the Kjørbekk stream is diverted into the pipe
By reopening the part of Kjørbekk stream that is contained in the pipe, additional flood water pathways will be created contributing to increased infiltration and attenuation of surface water, thus combating the effect of increased rainfall and extreme rain events. The reopening or daylighting process is viewed as bringing a wide range of social, economic and environmental benefits. Daylighting can improve livelihoods by providing new recreational areas for the local community and can reduce flooding risk and associated economic and environmental problems as well as enhance community resilience. Thus, daylighting is inherently a NBS.
Concave topography (top left), terraced ditches (top right), ditches to collect surface water (bottom left), dams which can pool water (bottom right) (Figure adapted from E. Sjödahl, 2019, Kart og Plan, Landscape Measures for Improved Management of Stormwater and Leachate at Old Closed Landfills)
Transforming the landscape has a role to play in reducing the amount of water that is in contact with potentially polluted soil and waste. By leading the water away from the landfill areas, infiltration is minimised and subsequently, so is the amount of contaminated leachate water. Concave topography, terraced ditches, ditches to collect surface water and dams which can pool water could all be used. In addition to these changes in the landscape, the density of natural vegetation in the area could be increased in order to reduce the amount and the speed of infiltration water as well as minimizing both airborne and waterborne particle spreading.
Area downstream Kjørbekk stream prior to discharge to the Skien River
Constructed wetlands are a well known NBS whereby vegetation is used to improve water quality as it binds pollutants and results in a reduction in concentrations of total suspended solids, organic matter, nutrients, heavy metals and organic contaminants in the water. Using a constructed wetland to remediate any contaminated water at the site would have the most affect in the downstream area before discharge to the Skien River. The area downstream is the last point where the leachate water could impair water quality in a larger water body and where the installation of a constructed wetland could be technically feasible.
A site visit carried out to bring together key people from different institutes and involved in different aspects of the project
The work at the case study site is supported by the Norwegian Environment Agency who have funds that the public sector can apply for in order to implement climate adaptation strategies. Skien municipality was granted funding for this case study and thus own the project. Within Skien municipality, work at the site is carried out by various departments including the land use planning department, water and wastewater department and environment department. The site is used as a case study in research projects where several partners are involved (Norwegian Geotechnical Institute and Institute for Urbanism and Landscape at the Oslo School of Architecture and Design) to support municipal and national efforts. Progress at the site has been hampered by the lack of secure and long-term financing.
The Skien River and its surroundings
NBS inherently offer primary benefits and additional secondary co-benefits. Transforming the landscape provides the primary benefit of an improvement in water quality, while at the same time providing the co-benefit of mitigating flooding through the creation of alternative water ways. Reducing particle spreading also supports improved water quality and concurrently provides the potential co-benefit of improved ecological status and biodiversity in the area. Remediating contaminated water will remove pollutants from water in the Kjørbekk stream, but can also provide additional social, economic and environmental co-benefits. Social co-benefit is felt via an improvement in the aesthetics of the area, economic co-benefits are felt through an enhanced flood protection and environmental co-benefits are achieved via an improvement in the state of the native ecosystem. From a city development perspective, opening the river can potentially increase property value.
The case study is still in the planning stage and the NBS described above could be used as part of a reopening strategy for Kjørbekk stream as they are able to: create new waterways thus alleviating flooding and divert water away from the contaminated disused landfills, reduce the risk of pollutants spreading and remediate contaminated water and soil. In combination, these methods will ensure that water quality and water quantity is maintained.
When deciding which NBS to use at the site, the environmental targets to achieve as well as the additional social (aesthetics and health benefits), and economic (flood control) impacts of the intervention, will be considered. Following the selection and implementation of NBS for the Kjørbekk stream, subsequent monitoring will play an important role in determining the success of the applied interventions. Environmental monitoring focusing on water quality will show whether the NBS have been sucessful in alleviating water quality problems. However, as the co-benefits of the NBS are often long lasting, tools such as life cycle impact assessments are needed that allow a temporal factor to be considered.
In summary, the NBS at Skien offers opportunities as a best-case example of NBS to tackle both water quality and water quantity as well as functioning as a catalyst for city development. However, the constraints are a challenge – the landfill, contaminated ground and subsurface add complexity and costs; furthermore, landowners and businesses may be uncertain to the efficacy of the NBS with regard to flooding. That said, there is high potential to showcase NBS to address these multiple benefits given the necessary funding is secured and the relevant stakeholders are involved early."