Nature-based solutions for stormwater management in the Helsinki Metropolitan Area, Finland – Prerequisites and good practices

The term stormwater management implies different types of measures that impact the accumulation of stormwater, or are related to the conveying or treating of stormwater. There are various stormwater management methods that are essentially structural solutions. A distinction can be drawn between so called "grey" and "green" stormwater management solutions (EEA 2021, Kuntaliitto 2012, US EPA 2021):

• Grey solutions: conventional engineered infrastructure designed to move stormwater away from the built environment; collect and convey stormwater from impermeable surfaces (roads, parking lots, rooftops) into pipe system that ultimately discharges untreated stormwater into a local water body.
• Green solutions: designed to mimic natural processes and capture rainwater where it falls; reduce and treat stormwater at the source; vegetation plays an important role; provide multiple (co-) benefits.

This division of solutions can be considered as part of the stormwater management systems presented in the next section.

Stormwater management systems

There are slightly varying ways to categorise stormwater management systems – and these are often combined in practice – but in general terms, they can be divided into three categories according to their operating principle (Fig. 3) (Kuntaliitto 2012, Lähde and Ariluoma n.d.):

• On source management, i.e. systems avoiding or decreasing stormwater;
• Conveying, i.e. systems conveying stormwater; and
• Detention and filtration systems, i.e. systems slowing down stormwater.

On source management systems concentrate on avoiding or decreasing stormwaters by limiting the amount of impermeable surfaces, filtrating stormwaters that have formed, and evaporating them with help of vegetation. These include permeable surfaces (like porous asphalt), green – or vegetated – walls and roofs, and maintaining the existing vegetation. On source management is the primary focus of stormwater management as only through these types of measures can the hydrological cycle be restored to the state before construction.

There are two types of conveying systems that convey stormwater, above- and underground systems. These include open ditches and streams as well as canals, channels and rills; also (bio)swales can be considered as conveying systems.

Systems slowing down stormwaters include various types of structures that detain, retain, filtrate and/or infiltrate stormwaters. These can be detention basins or ponds that have a permanent water surface area; or wetlands that have a lower water level but stay moist throughout the year and are typically covered by water and wetland vegetation. Vegetation covered basins, pits or depressions can be called for instance biofiltration or bioretention areas, or rain gardens. There are also depressions (aboveground) or pits (underground) that are not covered by vegetation, and therefore are not built with an infiltration or storage layers.

The scale of these solutions varies from local or small scale to regional or large scale solutions. The aim of the local (plot- or neighbourhood-scale) solutions is often to reduce the amount of stormwater, level off flow peaks and remove the impurities carried by the stormwater as near as possible to the source. Regional-scale solutions in turn aim to reduce and level off flood risk caused by stormwater. (Kuntaliitto 2012).

Chapters 6-9 present more in detail some of these systems through the selected green or nature-based stormwater management solutions in the Helsinki Metropolitan Area cities.

From quantity to quality management

With climate change advancing, it is anticipated that the amount of stormwater will increase and their purification need considerably grows (Kuoppala 2021). The quality of stormwaters is generally weakened as the cities grow denser (Valtanen 2015) and it can greatly vary in time and between locations in particular in urban areas (Eriksson et al. 2007 cited in Oral et al. 2020). The most common harmful substances in stormwater are solid matter, nutrients, heavy metals, chloride, oils and fats and other organic compounds such as polycyclic aromatic hydrocarbons (PAH compounds) and pesticides. They can also contain pathogens (such as E.coli) and bacteria. (Kuntaliitto 2012).

Land use has a great impact on the harmful substances in stormwater. In residential areas’ stormwater, there are usually high amounts of bacteria and nutrients whereas there are more metals in industrial and traffic areas’ stormwater. (Kuntaliitto 2012, Vahtera and Lahti 2016). Kuoppamäki et al. (2014) and Jokela (2008) have shown that the more heavily trafficked streets are, the more there are impurities in the stormwater.

The need to manage the quality of stormwater in Finnish city centre areas have been indicated at least in terms of solid matter, phosphorus, heavy metals like zinc and copper, and nitrogen (Helsingin kaupunki 2021a, Sillanpää 2020); in Helsinki also oil hydrocarbons and chloride have been considered to need more investigation (Airola et al. 2014).

Nature-based solutions for stormwater management

Urban sewage systems are designed for rains with certain intensity, and water from heavier rains, as a result of the impact of climate change, is directed to flood routes. In other words, the sewage system is not designed to handle all that water. Therefore, there is a need for decentralised and integrated water management.

Nature-based solutions (NBS) can act as an umbrella concept to various other concepts regarding sustainable urban water management (European Commission 2021a, see Fig.4). The concept of nature-based solutions was originally coined by the International Union for Conservation Nature (IUCN) (Cohen-Shacham et al. 2016; for the discussion on the evolution of the concepts, see for instance Ramírez-Agudelo et al. (2020) and Hanson et al. (2020)). However, the first use of the concept can be dated to the late 2000’s in the context of seeking solutions that connect climate change mitigation and adaptation, biodiversity protection and sustainable livelihoods (Eggermont et al. 2015).

The NBS are defined by the European Commission (2015, p. 4-5) as follows:

“Nature-based solutions aim to help societies address a variety of environmental, social and economic challenges in sustainable ways. They are actions which are inspired by, supported by or copied from nature….They have tremendous potential to be energy and resource-efficient and resilient to change, but to be successful they must be adapted to local conditions….Many nature-based solutions result in multiple co-benefits for health, the economy, society and the environment, and thus they can represent more efficient and cost-effective solutions than more traditional approaches.”

Alternative responses implementing nature-based solutions in the water management context are: “Low Impact Development” (LID) in North America, “Water Sensitive Urban Design” (WSUD) in Australia, “Sponge City” in China, “Sustainable Urban Drainage Systems” (SUDS), “Integrated Urban Water Management” (IUWM), and “Edible Cities” (Ramírez-Agudelo et al. 2020).

According to Ruangpan et al. (2019), the most common nature-based solutions applied in urban areas seem to be (intensive or extensive) green roofs, rain gardens, rainwater harvesting, dry detention ponds, permeable pavements, biofiltration (or retention), swales with vegetation as well as trees. Stormwater management systems mentioned in Section 3.1 can in many cases be considered nature-based systems as they take inspiration from or imitate nature and natural processes, offer benefits in terms of and beyond stormwater quantity and quality management, and can be cost-effective solutions especially in less extreme hazard scenarios and when co-benefits are considered in the long term (Seddon et al. 2020, see also Le Coent et al. 2021). Often these systems are in fact a combination of green and grey solutions, as the stormwater is directed to sewer systems after treatment with nature-based systems (EEA 2021, see also Wendling and Holt 2019). These can also be called “the most natural technological solutions” or eco-engineering (Oral et al. 2020). In fact, Seddon et al. (2020) argue that there is a need to focus on finding synergies among various solutions instead of framing NBS as an alternative approach to engineered solutions.

NBS have been addressed in the urban water management context recently e.g. by EEA (2021) and OECD (2020). In addition to introducing systems and structures mentioned in Section 3.1, removing excess asphalt and concrete in urban spaces can offer opportunities to implement NBS by reopening channelised watercourses and restoring riverbanks. Such solutions are more likely to be large-scale NBS, realised across landscapes, intersecting different ecosystems, and designed to reduce flood risk. Small-scale NBS in turn are typically implemented within a specific place, such as a single building or street (EEA 2021). These small-scale solutions have been found to reduce urban run-off by 30-65 % for porous pavements, up to 100 % for rain gardens, and up to 56 % for infiltration trenches (Ruangpan et al. 2020). Combination of NBS measures can lead to an improved performance in terms of runoff volume and peak flow reduction. For instance, combining detention ponds and rain garden, urban runoff volume reduction can be up to 71% (Goncalves et al. 2018 cited in Ruangpan et al. 2020). One of the most known – and successful – international examples of combining a number of NBS measures at the city scale is the Sponge City Programme in China (Ruangpan et al. 2020).

A nature-based solution aimed at a specific societal challenge is likely to produce co-benefits in other challenges (Raymond et al. 2017). In the urban water management context, this can mean benefits either beyond stormwater quantity management, or both quantity and quality management. When latter co-benefits of nature-based solutions for water management are considered especially in small-scale, urban context, they include supporting biodiversity, improving aesthetic and recreational value, or promoting social cohesion and inclusion as well as strengthening a sense of place within urban areas (Song et al. 2019 cited in EEA 2021). They can, for instance, help reduce noise pollution and the urban heat island effect, and hence contribute to enhancing quality of life. Solutions like vegetated roofs or facades can promote food production and provide insulation and therefore reduce the need for cooling or heating, and thus energy consumption. (Frantzeskaki, 2019, UNaLab, 2019 cited in EEA 2021, Oral et al. 2020). NBS can also contribute to carbon sequestration, and enhancing mental and physical health (Jessup et al. 2021). NBS can act as a “no-regrets” measure to adapt to the effects of climate change because they can provide benefits even without climate change (Hallegatte 2009 cited in OECD 2020, p.9). Furthermore, Hankonen et al. (2018) point out that by applying nature-based solutions, cities can find new ways to use city spaces and raise the appreciation of residential areas as well as respond simultaneously to several obligations, such as the development of water protection, leisure services, green infrastructure and biodiversity.

OECD (2020) provides international examples of the realised potential of the NBS to manage urban flooding. For example, investing EUR 22 million to retrofitting of drainage systems to include nature-based systems in Augustenborg in Malmö, Sweden resulted in a reduction of urban runoff by 50 % and a substantial increase in biodiversity (European Commission 2015). In Portland, Oregon, USD 250 million were estimated to have been saved in stormwater infrastructure costs when green alleys and tree planting were implemented with an USD 8 million investment (Foster et al. 2011 cited in OECD 2020).

For these benefits to be realised, nature-based solutions need to be carefully planned and designed. The effectiveness of NBS depends on the type and design of the solution, scale and purpose of implementation as well the local conditions and cultural setting (EEA 2021, Ruangpan et al. 2020). Participatory approaches and the inclusion of stakeholders’ perspectives from early stages of the design are said to be essential to ensure the effectiveness in realising multiple benefits and public acceptance as well as justifying investments in such options (EEA 2021, see also Le Coent et al. 2021). Moreover, siting of NBS can be vital: Jessup et al. (2021) found that the largest social and public health benefits may be delivered when NBS are implemented in and around heavily developed areas; whereas greatest water quality benefits can be accrued when NBS are sited in areas with a high density of commercial and industrial land uses. In addition, the implementation of NBS is argued to demand continuous monitoring (Gałecka-Drozda et al. 2021).

Orta-Ortiz and Geneletti (2021) investigated the performance of various NBS types to manage stormwater in urban areas, reviewing research from the last 10 years in the field. They found that effectiveness to meet stormwater management objectives highly varies – even when considering the same type of solution – due to the characteristics and dimensions of NBS (e.g. soil porosity, plant species and substrate depth) and the local conditions (e.g. rainfall patterns, temperature and maintenance practices). They conclude that there remains a knowledge gap with regard to the relationship between these factors and the performance of nature-based solutions (Orta-Ortiz & Geneletti 2021). The vital role of local context in NBS application is emphasised also by Rehunen et al. (2021).

However, NBS can also have trade-offs, and face various barriers or limitations. For instance, the construction work related to their implementation may negatively impact the water quality downstream, or they can create conditions for unwanted organisms to thrive (e.g. mosquitoes) (EEA 2021). Limitations stem largely from the lack of knowledge with regard to the effectiveness of NBS and insufficient planning or design of the solutions. Nature-based solutions often require more space than traditional grey structures, which can lead to high opportunity costs due to land values being high (EEA 2021); limited space available is a major shortcoming especially in densely built urban areas and protected historical city centres (Oral et al. 2020). Therefore, including the economic and social values of various benefits in cost assessment of NBS is central (EEA 2021, Le Coent et al. 2021).

It is evident that nature-based solutions are complex and require the consideration of multiple benefits and limitations (Luxton 2021). Adding to the complexity, stormwater management is increasingly regarded a multidimensional and multidisciplinary issue (Oral et al. 2020). A lack of tools to assist identifying the impacts of NBS in a holistic manner to support decision-making has been brought forward e.g. by Luxton (2021) and Paloniemi (2019). However, guidance is increasingly available: for instance, European Commission (2021a) has recently published a handbook for practitioners on evaluating the impact of NBS. It offers an overall evaluation framework for NBS with a protocol for selection of key indicators of NBS impact and methods for their assessment. Furthermore, Raymond et al. (2017) introduce a framework for assessing and implementing the con-benefits of nature-based solutions in cities. Le Coent et al. (2021) present a methodological framework for the economic assessment of NBS for water related risks. In addition, the interactive mapping tool of the University of Oxford (n.d.) helps to link nature-based solutions to climate change adaptation outcomes.

In the Finnish context, a project called TASAPELI (“Efficient and effective nature-based solutions as tools for climate change adaptation”, 2018-2019) developed an approach that aims to advance comprehensive recognition, evaluation and consideration of the NBS’ benefits in stormwater management related planning. Paloniemi (2019) called for comprehensive planning and wide-ranging cooperation between different actors, and suggested that researched knowledge, good practices and past experiences will help implement nature-based solutions. In addition, Luxton (2021) proposed a methodology to prioritise large-scale nature-based solutions in the City of Helsinki to assist municipal decision-makers with strategic implementation of NBS for stormwater management.

Due to the NBS being characterised by multiple benefits, they essentially require seamless and sustained cooperation of actors not only in planning and implementation but also in terms of funding of such solutions (Vikström et al. 2019). Rehunen et al. (2021) argue that main barriers to NBS applications in the urban water management context are often found in governance, regulation, organisational interaction, and planning practices. In addition, they consider the biggest opportunities for development to be found in the political arena, institutional collaboration and knowledge production (Rehunen et al. 2021). Bohman et al. (2020, p. 2) presents that the biggest challenge in transforming the stormwater management sector to become more sustainable is about “developing new working procedures and planning routines that involve wider actor collaborations” rather than advancing technology. In their study on Swedish municipalities, Bohman et al. (2020) recommend networks on vertical collaboration to ensure stormwater related planning ambitions are maintained from the planning to the implementation stage, and further, to facilitate mutual learning and dialogue.

Nature-based solutions in stormwater management were found to be driven in Malmö and/or Copenhagen by a collaborative culture, establishment of official steering groups in addition to existing climate change adaptation plans and previous experience in implementing NBS (Udomcharoenchaikit 2016). Good examples and best practices on nature-based solutions, also in the urban water management context, are provided, for instance, by the Urban Nature Atlas (2021) database and Oppla (2021) platform. Furthermore, various ongoing EU-funded Horizon 2020 projects are shedding light on the barriers and success factors of nature-based solutions. In the urban water management context, the projects include UNaLab, Grow Green, Urban GreenUP and Connecting Nature (European Commission 2021b). Atenas is one of the ongoing projects (funded within the EU Water JPI) that has studied critical factors for replication and upscaling of NBS in urban water management context (Rehunen et al. 2021). In addition, a Finnish project titled Baltic Sea Cooperation for Climate Resilience – Flood and Drought Risk Management has provided insights into nature-based and natural flood risk management solutions also in the stormwater management context (Parjanne and Marttunen 2021).

It is noteworthy that the idea of taking inspiration from nature or replicating natural processes is older than the concept of nature-based solutions that became more common in the late 2010’s. Identification and utilisation of nature-based solutions in urban stormwater management has become established worldwide. However, in urban planning NBS are stated to be still in the experimentation phase due to the challenges in connecting their benefits to urban planning. (Hankonen et al. 2018 cited in Vikström et al. 2019). In the Finnish water management context, it was reported that "natural" stormwater management was arriving to Finland and tested in projects in the late 2000’s; and that there was only a little experience at the time on such "ecological" management methods compared to the other Nordic countries (Pihlajamaa 2010). Fast forward 10 years, nature-based solutions are swiftly becoming more common (Valtanen 2021).

Currently, Finnish municipalities appear to be well aware of the stormwater related challenges and many of them have developed a stormwater management plan or a program. While stormwater has earlier been connected to draining and underground structures, it is nowadays increasingly seen as a resource and an opportunity to create attractive and interesting living spaces for city dwellers. Also, natural or nature-based management solutions are emphasised, and stormwater directed as part of the natural water cycle in stormwater management planning. The quality treatment is becoming more and more important in the future. (Ala-Prinkkilä 2019).