Urban Oases: Shaping a Sustainable Future through Environmentally Functional Landscape Features


Wetlands are joyful to visit year round!

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Dr. Outi Wahlroos
outi.wahlroos (at) helsinki.fi

Prof. Harri Vasander
harri.vasander (at) helsinki.fi


Welcome to explore the Life+ Urban Oases-project!

Our project has ended. Since the project resulted in continuing benefits to the environment, we are still updating information e.g. on the "Show&Tell" pages. Please also visit and enjoy our pilot sites in Nummela, Vihti and Viikki, Helsinki to observe nature based stormwater management, as well as ecosystem services and critically endangered clay stream habitats established in urban areas. While the following summary is text only, please visit the link pages at the top of this page for many demonstrative graphs and pictures.

Project Number and short name: Life+11 ENV/FI/911 Urban Oases

Project duration: 2012–2017

Project budget: 3,411,690 eur

EU contribution: 1,702,770 eur

Urban Oases project brochure (pdf, 12,7 MB)
(Print: "flip on short edge")

Layman Urban Oases report (pdf 2,81 MB)

Urban Oases final report (pdf 1,0 MB)


Outi Wahlroos, project coordinator, March 2018


The aim of the Urban Oases project has been to develop designs for landscapes that would introduce water-environment protecting elements, ecosystem services and habitats to urban areas. The functional landscape types that we explored for design, establishment, and function were stormwater wetlands and vegetated stormwater swales. We also looked at the establishment of clay stream habitats, which are critically endangered in Southern Finland, in urban areas. For our design basis we wanted to stress the importance of both connecting site scale designs with watershed scale considerations and respecting and learning from local nature.

Our overall aim for the water environment mitigation landscapes has been to support the goals of the Water Framework Directive on good status of surface waters. While ecosystem services are benefits that natural systems provide to humans, using only the sun´s energy, they need to be established or enhanced in the urban settings. Our aim was to look for replicable designs, construction methods, and maintenance procedures to cost efficiently establish long-term ecosystem services, that would especially provide for the water environments in urban areas. Our aim was also to monitor the implemented pilot landscapes so that we would demonstrate the impacts of design choices of such structures. By gaining accurate understanding of achievable benefits of a given landscape structure, we can better understand the efforts needed and accomplishments achievable for preserved, enhanced and newly constructed landscapes.

With the piloted water environment sensitive designs in this project, we aimed at establishing flood control, water purification, erosion control, carbon binding, rich and connected habitats, and recreational amenities. We aimed to accomplish these goals in the pilot stormwater wetlands and vegetated stormwater swales by mimicking natural water-dry land interphase systems. The intended benefits included prevention and mitigation of adverse impacts of urbanization such as: habitat destruction, contamination and fragmentation; loss of biodiversity; as well as harmful algal blooms and eutrophication in the receiving urban streams, rivers and lakes, and ultimately in the Baltic Sea.


In this project we worked as a team of a Municipality (Vihti), Academic (University of Helsinki), a Governance agency (Uusimaa Centre for Economic Development, Transport and the Environment), and a Watershed protection association (Water Protection Association of the River Vantaa and Helsinki Region) as well as with external assistance and collaborating companies (Luode Consulting Oy, Vapo Clean Waters Oy, and Suvilumi). We also collaborated with the City of Helsinki Life+ City Water-project, and other Life+ projects. Our team consisted of engineers, landscape architects and scientists, as well as practitioners and theorists. We also involved local people through conducting landscape construction volunteer events and environmental education events. Our common goal was to turn stormwater from waste to joy and to promote sustainable development. In our work we respected the natural and cultural characteristics of the pilot sites. A sign of respect to local people was naming the project major pilot stormwater wetland park “Niittu”, which means Meadow in local dialect. The project pilot was created to strengthen the place-identity of the landscape in the long term, and locals were involved in design and implementation.
We piloted stormwater wetlands design, establishment and monitoring in two Southern Finland urban sites, one in Nummela, Vihti and the other in Viikki, Helsinki. We also piloted stormwater management swale designs and examined the difficulties in retrofitting better design in dense urban settings in Viikki, Helsinki. We implemented pilots and monitored the sites for water quantity (flood control), water quality, greenhouse gases (sink or source), and biodiversity (vegetation, amphibian, avian, and invertebrates). We also distributed a questionnaire to interview urban dwellers on their attitudes toward, and willingness to pay, for sustainable stormwater management landscaping.

In order to promote mainstreaming of functional and nature-based stormwater management we demonstrated our pilots and monitoring results. To do this we established educational trail boards at the pilots. The boards include a wealth of knowledge on local nature and natural processes in the parks, as well as information on how to prevent pollution of the water environment. We held several events and excursions at the sites for local, national and international audiences, published both scientific and citizen science papers, and gave many national and international presentations on the project. Audiences ranged from professionals, for whom also courses were held, to elementary school children who attended our four one-week wetland science schools in Nummela, Vihti. Also, we sought cost-efficiency in the implementation and maintenance of the functional landscaping to facilitate mainstreaming of the designs.


We examined the establishment of functional landscapes providing stormwater management services in urban areas. The many functions which urban landscapes provide can be described as various ecosystem services. We targeted ecosystem services relating to stormwater management landscapes, such as: flood control, water pollution control, increasing biodiversity, and providing recreational amenities. We piloted, monitored and demonstrated achievable functions in created wetlands and vegetated swales. We were able to show that ecosystem services can be established in urban areas to a higher extent than expected, and such services can be measured quantitatively to help planning, design and decision making. We were surprised by the degree of success of our pilot stormwater wetlands based on all the monitored parameters. With success comes responsibility: one needs to assure that the rich habitats created by stormwater landscaping are safe for the fauna.
While our stormwater management wetland pilots that were created in parks were thoroughly successful, the planning and design of retrofit stormwater swales in dense streetscapes, on the other hand, revealed a lack of consideration for sustainable stormwater management in our cities: space available for green infrastructure is scarce above ground, and underground unnecessary complex criss-crossing pipelines almost fully take up the potential root zone and space for water retention soil. Land ownership/governance borders are limiting holistic landscape design above ground, and below ground, restrictions, responsibilities and opportunities are unclear. Furthermore, the lack of availability and accuracy of maps of above and below ground structures does not support design for sustainable stormwater management.

We piloted three stormwater wetlands. In Nummela, Vihti we piloted two following one another upstream from Lake Enäjärvi. The wetland names, construction years, and proportion of mean water level inundated area of the contributing watershed area were: Gateway Wetland, 2010, and 0.1%; and Niittu Wetland, 2013-2015, and 0.2%. In Viikki, Helsinki we constructed three parallel inundated stormwater swales or shallow wetlands in 2015 each were 0.1% the area of the contributing watershed. The contributing watershed sizes and levels of imperviousness were 550 hectares and 35% in Nummela, and 120 hectares and 45% in Viikki.

We stressed two aspects of wetland design to enhance water quality mitigation: 1) dissipation of flow energy of incoming water and increasing retention time and 2) facilitating the establishment of densely vegetated and diverse wetland area. A constructed wetland typically consists of three compartments: an inflow pond for solids settling, an emergent wetland vegetation covered shallow area for dissolved pollutants control, and an outflow pond for settling plant debris from the vegetated area. A bottom dam at the end of the pond - shallow wetland - pond sequence defines the water level fluctuation in, and retention capacity of the structure. Our designs followed this basic design which proved to work. The shallow wetland area is often recommended to be enhanced by berms to extend the flow path. We tried both a berm structure, and a simple widened flow path structure. We observed that in the berm structure water flowed the shortest path allowed by the berms resulting in retention time that was shorter than theoretically expected. The simple wide and shallow water drop shaped wetland structure worked well. We observed that the wetlands provided water retention well for flood management. However, the impact of urbanization on increasing runoff volume is so high that the size of our wetlands were only able to partially compensate for flooding induced by urbanization.

The two ponds at the beginning and end of each stormwater wetland collected two kinds of solids, as was intended: coarse sandy sediments at the inflow and organic sediments from the wetland at the outflow. However, in Southern Finland clay particles and phosphorus bound to clay is a major concern in runoff as phosphorus is the limiting nutrient to blue-green algae in the receiving lakes and the Baltic Sea. Clay is very difficult to settle, and ponds are not successful in clay removal. In our wetlands clay and phosphorus was removed by wetland vegetation. As our wetlands grew lusher over the duration of the project, they removed clay and phosphorus better, and thus increasingly prevented algal blooms in the receiving waters. This demonstrated the crucial importance of wetlands design with shallow areas supporting vegetation establishment, and the importance of not cutting or removing vegetation. This result also applies to agricultural wetlands and stream buffer zones. Similarly, our recommendation for vegetated swales in dense urban settings where perennial meadow plants are maintained due to adjacent hardscapes, is also to only cut the withered plant stems in late fall, leaving circa 10 cm of the stems standing.

We monitored the implemented pilot wetlands continuously for water quality. The detection frequency was set to ten minutes. At this interval each change in pollutant concentration could be detected. We then used the continuous monitoring data to calculate event based, monthly and annual removal rates for pollutants by our wetlands. Again looking at phosphorus we learnt that while relative phosphorus removal rates (percentage of the inflowing concentration removed before the outflow) were at the highest during the growing season in summer (when precipitation is low), the highest absolute phosphorus removal rates (kilograms of phosphorus removed from water and retained in the wetland) occurred during fall rainy season and spring snowmelt season. In other words, in regard to algal blooms, the most pollution potential occurred and quantitatively the best water treatment was achieved in our cold climate region wetlands outside the growing season when the perennial herbaceous vegetation is dormant yet present. We observed both the relative and absolute pollutant retention rates increase each year after wetland construction, following the increase in vegetation density, species diversity, and biomass in the wetlands. Similarly, the maturing vegetation also contributed to increase in hydraulic retention time in the wetlands.

Our wetland designs included pond area depths of 1.5 m and shallow area depths of 30-50 cm. The shallow area depth was chosen based on what we see in nature where our desired dense wetland vegetation communities exist. Vegetation was very fast to self-establish in the intended areas with mostly native species. At the Nummela Gateway wetland the number of species increased from the initial 50 to 150 five years after construction. While the native perennial vegetation was found to be very important to pollutant control during fall and winter months our recommendation is not to cut or remove native herbaceous plants from constructed wetlands. In contrast, out of the four invasive species found in Nummela, the Himalayan balsam at the Niittu Wetland was found to need control by weeding. This annual plant suffocates the native perennials. If the perennial plants were replaced by the annual balsam, the important water pollution and erosion control during fall rainy season and spring snowmelt season would be lost. The Himalayan balsam spread by floating seeds to the Niittu Wetland from the stormwater sewershed. We found that it was thus important to control this invasive plant in the contributing watershed.

We followed solids accumulation in the pilot wetlands and concluded that while any management to a constructed wetland will cause a disturbance to the system, it is important that the wetland is designed for the ponds to be dredged every ten years, or more rarely, and for the vegetated area even less frequently. If possible, the wetland should be established at a site where the bottom dam at the outflow could be raised, once the wetland becomes sediment filled. However it would be even better if people´s awareness of environmental issues would rise through the familiarity with the wetland, and take action within the watershed to prevent contamination of runoff, or treat it as close to the source as possible. This way the wetlands would increase in habitat and recreational value as water treatment needs decreased. The need for prevention of pollution of water environment at all sources should be stressed regardless the treatment services offered by constructed wetlands.

In addition to flood and water quality control services the increase in plant and faunal diversity in the established stormwater management parks/landscapes was high. In parts of the parks with flooding areas we established amphibian ponds without fish access. These areas were inhabited by amphibians right after construction, and the numbers of amphibians increased throughout the project. Our monitoring showed an increase of frog egg clusters from zero to 350 in three years following wetland establishment at the Niittu wetland alone. We learnt that the frogs began to spawn when water temperatures reached 5 oC in spring and that they preferred pockets of still, fishless and clear water surrounded by emergent vegetation. The wetlands also invited various nesting birds (some of them rare), fish, crawfish, and even otter. The invertebrate counts showed successful succession towards balanced natural wetland communities. At least three EC habitats directive species inhabited the Nummela pilot wetlands: the moor frog, the northern bat and at least one species of mouse-eared bat. The project included implementing safe street crossings from the parks all the way to the upstream forested areas, which with the high increase in fauna proved to be very important.

In construction of the wetland parks we used natural and local materials, which excluded materials as geotextiles and resulted in the establishment of wide clay stream habitat areas which is a critically endangered habitat type in Southern Finland. This success brings to light that nature-based solutions and sustainable stormwater management need to be extended throughout the urban watersheds, to make the created habitats in parks be safe for fauna. Reducing stormwater formation and scattering vegetated stormwater swale structures throughout urban watersheds where stormwater is formed and snow needs to be piled would help to treat water quality and restore water balance before runoff reaches stormwater wetlands.

Our monitored wetlands were quite young for assessing their impact on climate change mitigation. Even so we learnt that a flood meadow and shallow wetland emergent vegetation carbon binding capacity was about tenfold higher than a lawn; and on an annual level our monitored stormwater wetland was a slight source of carbon dioxide and methane, with methane dominating the global warming potential. The Gateway wetland showed roughly one third of methane emissions of a natural fen, while the total carbon budget assessed for the still young wetland indicated close to carbon neutrality.

Establishing vegetated swales at densely urbanized settings revealed challenges of fractioned land ownership, lack of space below ground and the difficulty of obtaining accurate and up-to-date above and below ground maps to facilitate design. Even so, we were able to pilot designs for retrofit stormwater management swales for the management of runoff from small urban subwatersheds in Viikki, Helsinki. For dense urban area swales ease of maintenance is an issue for longevity and mainstreaming. Our swale designs include area for solids removal, area for pollutant trapping, vegetated area on structural soils, and an overflow connection to an existing stormsewer network. The soils need to be bearing, include voids, and be low in nutrients to prevent nutrient leaching. The structure is made simple and easy to maintain for cost efficiency. This structure provides many ecosystem services and would also work well for managing the increases in rain intensity. Thus, instead of increasing the size of stormwater pipes to meet the challenges of intensification of urban structures and climate change, one could install specific vegetated swales in the streetscapes to increase rainwater retention in urbanized watersheds. The swales would protect water environments downstream, and provide sites for long-living street trees and urban meadows with their associated ecosystem services.

We also administered a questionnaire about local perception of and willingness to pay for stormwater wetland parks. We learnt that the wetland parks were successful in making nature accessible to locals and creating identity to the place. The respondents’ attitudes towards stormwater wetland parks and the biodiversity in them was very positive. Willingness to pay for stormwater management by nature-based solutions resulted in a higher sum than the municipality was currently paying for conventional stormwater management. Willingness to pay for constructed stormwater wetlands was higher among people who had visited a constructed wetland. The attribute, or ecosystem service, most valued by the respondents was the ability of the wetland to improve water quality. We organized four children´s one-week summer schools and many excursions and events at the establishing wetland parks. These events were very well received and proved successful in increasing awareness of local nature and the use of nature-based solutions in mitigating challenges of urbanization.

Wetlands are very dynamic in growth and change over time. The designs need to take their development (site succession) in mind and design time scale should be some fifty years. Since maintenance is a disturbance, one should design for solids excavation and plant material removal at a frequency of every ten years or longer if at all. Mowing and/or removal the plants should not be done except for such invasive species as the Himalayan balsam found at the Niittu Wetland and weeded there. A wetland’s many ecosystem services are due to plants. Plant removal and disturbance causes loss of the ecosystem services. Plants also support diverse micro-organisms which are important in dissolved pollutants degradation. Plants are also the basis for rich food webs, and contribute to the constantly changing views and recreational values appreciated by urban dwellers.

Our pilot wetlands covered only 0.1 to 0.2% the area of their contributing urban watersheds. Even so they were in their respective park landscapes visually very large. A current rule of thumb for water treatment wetlands recommends 1 to 5% surface area of the contributing watershed. With continuous monitoring we were able to demonstrate that water quality treatment by the only 0.1% of watershed area Gateway Wetland reached 21% annual removal rate of phosphorus by the fifth year after construction. There was no maintenance or plant removal carried out to the wetland, which could thus work to its highest potential in water pollutant control. Stormwater inflow varied between 2 and 1800 l/s at this wetland. Over our project time there was one “climate change” (extreme) year with almost no snow in winter and heavy rain events in December. This was seen as a disappearance of the snowmelt event in spring and high absolute phosphorus removal in the wetland in December when it was raining heavily instead of snowing. Our vegetated swales were designed to treat and retain all inflow water – thus identical to wetlands continuous monitoring for water treatment at inflow and outflow will not be possible in them.

The level at which our pilot wetlands were removing phosphorus reduced algal blooms in the receiving lake at a detectable level. However, the wetlands were not sufficient to address the problem of algal blooms in the receiving lake due to the large lake watershed. All the lake sub-watersheds would need to have the minimum of circa 1% area of treatment wetlands to reach the natural background level of nutrient loads to the lake. Learning and acknowledging this, the municipality began to develop and implement stormwater wetlands upstream of our project sites, even while the project was still going on.

The Gateway wetland takes up circa 40 kg of total phosphorus a year. According to the HELCOM-PLC6 an estimated 22 million kg of riverine total phosphorus reaches the Baltic Sea on an annual basis out of which 30% is estimated to consist of natural background.  Referring to this HELCOM-PLC6 estimate (load estimates vary), and expecting constructed wetlands to be the only water mitigation effort within the subwatersheds, to restore natural conditions of the Baltic Sea total phosphorus load you would need 385,000 Gateway wetlands within the Baltic Sea watershed. For the lake Enäjärvi the number is 70. Before human impact in early 1600s, Finland used to have 33% wetland cover. Current wetland coverage is circa 13%.


We established stormwater management wetland parks and landscapes as places of urban oases that people want to have near their home and are willing to pay for. We monitored our piloted stormwater management landscapes for actual impact on water, climate, species diversity and peoples´ attitudes. We found all of these to be more positive than anticipated. However, we also found out that instead of piloting, mainstreaming functional stormwater management landscaping is necessary to reach the Water Framework Directive goals of good status of surface waters. Furthermore, we learnt that achieving mainstreaming sustainable stormwater management with functional landscaping in dense cityscapes is difficult due to fractioned division in land ownership, governance and responsibilities both above and below ground. Also, site design background materials are mostly lacking and/or inaccurate. We put much effort in design of our pilots to achieve optimally functioning and cost-efficient structures. Our designs, construction guidelines and monitoring can be copied and transferred to sites similar to Southern Finnish conditions. Communicating our results has already resulted in replication. All protective measures count, and many kinds of measures need and should be used throughout watersheds starting from preventive measures to runoff formation.

With the detailed and long-term monitoring of our project implemented pilots we could demonstrate that even small stormwater wetlands provide significant water treatment, as long as they are designed and maintained to have an undisturbed and densely vegetated area. The size of our piloted wetlands, up to one hectare in mean water level surface area, appears large to a wetland park visitors but, is very small relative to the contributing watershed areas. What we could show is that it is important that there were multiple nature-based solutions within urban watersheds. It appears that in the case of stormwater wetlands one would need to aim at the minimum of 1% of the watershed area as wetlands in the Southern Finland conditions to treat runoff. Then upstream of such wetlands one would need multiple vegetated swales to trap severe pollutants before they reach the wetlands, which attracted much fauna. Furthermore, safe passages in traffic crossings for fauna are needed as well. Maintenance plans respecting succession of plants in the established landscapes are needed to assure that the sites vegetation will continue providing ecosystem services optimally and long-term.

We recommend that both urban and agricultural constructed wetlands include shallow emergent vegetation areas in which plants are allowed to grow without disturbance. The absolute removal rate of phosphorus, the limiting nutrient of blue-green algae, was by far the highest in our wetlands during fall and winter, when the removal was due to density of dormant vegetation stems. Continuous monitoring was essential for accurately assessing pilot landscapes´ water environment mitigation and climate impact capacities.

Our project was made possible by the length and breadth of the EU Life+ project funding which allowed the successful integrated approach carried out and the creation of lasting water protection urban landscapes in Nummela, Vihti and Viikki, Helsinki. We also benefitted from the many communications organized by the Life+ organization across the European Life+ projects. Our project successes were also owing to the diversity of our project participants and the synergism of collaborating practitioners and theorists. We owe sincere thanks also to the local people and the many participating students, as well as the contributing funders Maa ja vesitekniikan tuki ry Foundation, and the Ministry of the Environment.

Last modified: OW 22.02.2019