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Outline
Kehä I is a main traffic route for transverse traffic in the Helsinki metropolitan area. The normal daily traffic ranges from 45,000 to 50,000 vehicles. Vihdintie is one of the radial roads that convey traffic to and from the Helsinki metropolitan area. The Kehä I-Vihdintie interchange area was improved during the Kehä I improvement project in 2016-2017. The project was implemented in cooperation between the Finnish Transport Infrastructure Agency, the Centre for Economic Development, Transport and the Environment of Uusimaa and the City of Helsinki.
The Improvement project included ditch cleaning carried out to improve the drainage of Kehä I. On the east side of Vihdintie, where the stormwater from the area was discharged, flood issues were observed in the autumn of 2016 during heavy rain situations. During such heavy rainfall, it was even necessary to close lanes on Kehä I. To manage the stormwater and to improve the water quality of the discharge area of the ditches, stormwater billing ponds were designed.
Stormwater pond in 2022 (left) and 2023 (right)
Analysis of climate hazards
During the planning, a flood management study that considered the impact of climate change on heavy rainfall events was utilised, covering the entire drainage area.
Resilience assessment
In the project planning typical factors of the area were considered. The main methodologies were flood management and design for reducing sediment load. In the design of stormwater ponds and their outlets, as well as culverts and channels, flood risk for Vihdintie was considered together with drainage of Kehä I’s subsurface drainage.
How are adaptation solutions considered?
Towards the end of the planning phase, the design objectives were expanded upon the request of Helsinki Urban Environment Division’s regional planning to increase the flow attenuation capacity due to the needs of the Mätäjoki River basin. Based on the proposals, a flood area was designed in connection with the southern settling pond. It was not possible to expand the flood area in connection with the settling pond on the northern side of Kehä I, but a bottom dam with an outlet pipe was added to regulate the water level of the settling pond more efficiently (in addition, downstream, along Vihdintie, sections of the ditch were cleared), and a flood plain was extended in the northern side of Kehä I.
The solution was implemented a few years ago and it has been observed that the flooding has been significantly decreased. The stormwater ponds were designed so that natural vegetation could grow and the flood plains currently form a naturally grown forest. Nature-based solutions in the design of the stormwater ponds and stormwater retention were the initial conditions of the process.
How is the adaptation strategy implemented in practice?
During the construction phase, site monitoring was undertaken to ensure implementation according to the plan. The flooding in the area has been reduced since construction. The ditches are being monitored and they are maintained in a way that preserves vegetation.
Lessons learned?
The natural vegetation has been established strongly in the area, and there has been no need for additional planting. During the planning phase, it was not questioned if clearing of the ditches was necessary although ditch breaks were proposed. In the future, it will be necessary to define more clearly e.g. the type of biodegradable erosion control nets so that ultraviolet-degradable plastic material would not be used.
