Nature-based solutions – Green Space Index and stormwater strategy
Nature will do the work where nature-based solutions manage stormwater, create recreational value, and enhance biodiversity. This involves new ways of thinking and planning for how plants and water are used in urban environments that can withstand a changing climate and handle increased and more intense rainfall, as well as drier periods. The Green Space Index and the stormwater strategy are important tools for climate adaptation in Stockholm Royal Seaport.
Stormwater Strategy in Stockholm Royal Seaport
Stormwater consists of rainwater and meltwater from roofs and other hard surfaces. In Stockholm Royal Seaport, stormwater is viewed as a resource, and a tiered management approach is employed to reduce the risks of flooding and drought. The goal of the strategy is to adapt the neighbourhood to withstand future high sea levels, to dimension the stormwater systems to cope with heavy rain and intense precipitation periods without flooding buildings and courtyards, and to retain stormwater on site for later use in irrigation. The stormwater system consists of an integrated system with green roofs and roof gardens that interact with water flow in courtyards, streets, grassy areas, rain gardens, ponds, and stormwater pipes. Vegetation in courtyards, on walls and on roofs reinforces the ecosystem in the parks and contributes to a robust ecosystem.
Stormwater as a resource, landscape architect Gösta Olsson explains. (Swedish only) The area is designed so that stormwater can drain on the surface when the pipe system is overloaded and primarily be directed to plant beds for retention and purification. Heavier rains and downpours require that area elevations be designed to allow stormwater to be surface-drained to the nearest recipient in Husarviken and Värtan. In one of the later phases, Kolkajen, the local elevation will be shaped to primarily channel stormwater into plant beds for delay and purification. The goal is for at least 75 per cent of stormwater on streets and squares to be directed to plant beds.
A rain garden is a combination of a planting bed and a stormwater facility. It is expected to both delay and purify stormwater and function as a viable plant bed for trees, perennials, and shrubs. The design can vary widely and utilise different substrates. In Stockholm Royal Seaport, the rain gardens are set below street level, aligned with the outflow pipe of the storm drain. The pipe from the drain is directed over the top of the planting bed, ensuring a robust structure for operation and maintenance, and allowing unimpeded water flow with a margin for delaying water during heavy rains. The substrate in the rain gardens consists of charcoal gravel, which facilitates rapid infiltration and a relatively dry growing environment. To minimise the risk of the plants receiving insufficient water, dams are used to retain some water at the bottom of the planting bed. Plant selection must also be adapted to this unique growing environment, and nutrients are often needed to ensure the plants thrive and develop.
Gasverket and Värtahamnen have been identified as areas where stormwater management is a challenge. This is because both areas are flat, low-lying, and already have existing buildings. Extensive investigations have been carried out to avoid water accumulation, such as elevation, plant beds, wetlands, and ditches. In Southern Värtahamnen, for example, plans are being made for a skate park that can also handle large amounts of rain.
In the stormwater strategy, stormwater is viewed as a resource. It consists of rain and meltwater from roofs and other impervious surfaces in urban environments. The goal of the strategy is to adapt the neighborhood to handle future high sea levels, to size the stormwater systems to cope with heavy rains and intense periods of precipitation without flooding buildings and courtyards, and to delay stormwater in the area to later be used for irrigation.
Green Space Index
The aims to support system solutions where urban greenery and stormwater are used in various ways to strengthen ecosystems, mitigate negative effects of climate change, and at the same time shape attractive courtyards and outdoor environments. Climate change entails altered conditions for both the city's green structure and stormwater management. There will be more rain during autumn and winter and more frequent and more persistent heatwaves during summer, which puts pressure on plants and greenery. The Green Space Index is a planning tool that has been further developed in Stockholm Royal Seaport. The tool was previously developed in Germany and further in Malmö. The Green Space Index describes the eco-efficient area - that is, the area that contributes positively to the site's ecosystem and local climate, as well as social values related to greenery and water. The purpose of the Green Space Index is to strengthen ecosystem services that enhance biodiversity, manage stormwater to reduce flooding, increase pollination, and lower temperatures during heatwaves. It also contributes to people's well-being through pleasant environments. The Green Space Index achieved should be balanced between the various functions pursued. Green structure is used as an active component in planning. The Green Space Index is applied at the block level, and all developers need to co-operate to jointly achieve the Green Space Index.
Effects and results development site
From the outset, developers were positive towards the Green Space Index, even though it was a new tool that required a different type of construction to be designed. The City of Stockholm's contribution to capacity building and a shared vision were important factors that contributed to a successful outcome. The Green Space Index served as a source of inspiration in the landscape architect's work. The clear scoring of different measures and plant choices provided by the tool made it easy for developers to relate to various implementation alternatives.
As part of the innovation project C/O City, which was completed in 2018, NCC, together with RISE, conducted moisture measurements on the green roofs in Stora Sjöfallet. Results showed that the green roofs did not negatively affect the underlying structure of buildings, nor contribute to mould growth on tongue-in-groove boards under the waterproofing layer during the time the measurements were carried out. Within the framework of C/O City, calculations were made of the benefits of a 55 mm thick sedum roof, equivalent to those in Stora Sjöfallet. The conclusion was that the sedum roof creates benefits, ecosystem service benefits, which are often not noticed until they have disappeared. This makes it especially important to highlight the value qualitatively, quantitatively, and monetarily.
A thicker biotope roof strengthens the ecological dispersal routes between green areas. Calculations also show that the sedum roof reduces stormwater flow by 40 per cent. This means that stormwater is stored on the roofs, resulting in reduced load on the stormwater network. Additionally, the roof contributes positively to the noise environment by absorbing noise, reducing noise levels by approximately 2 dBA. Green roofs reflect light, meaning that heat is not stored to the same extent as on a metal roof. During a heatwave, the roofs lower ambient temperatures by up to five degrees Celcius. Green roofs with solar panels create a positive synergy effect, as green roofs have a cooling effect and thus improve the efficiency of the solar cells.
Surveys such as resident questionnaires show that the design of the courtyards and greenery is appreciated by both residents and visitors. To gain a better understanding and increased knowledge of how outdoor environments, such as residential courtyards and roof terraces, are perceived, a special survey on vegetation was conducted among residents and housing associations in the autumn of 2020. The purpose was also to gain further knowledge on how the Green Space Index can be further developed. The results showed that 80 per cent of respondents feel that the courtyard works well. More than 60 per cent responded that it meets the household's needs. The study also showed that the courtyards work best for younger children and less so for teenagers. Over half are aware that the courtyard's vegetation is part of the climate adaptation to reduce flooding in the area.
Results for public open space
In Hjorthagen, several parks, streets, and blocks have been completed. A system of wet strips and ponds has been established in Hästhagsparken. These handle large volumes of water in the hilly terrain and form part of a dispersal area for aquatic animals and plants. Opportunities for frog habitats have been enhanced with a newly constructed frog tunnel beneath the main street in the new district.
Lawns have been designed as retention areas for runoff water from blocks and streets.
Connected green corridors in the form of so-called ditches have been constructed on local streets with recessed planting areas that receive all the water from streets and sidewalks. Walkways and car crossings to garages are elevated over the ditch to prevent standing water during heavy rain. The water is managed, delayed, and ensures the growth of trees and other vegetation in these green corridors.
On the main street, Bobergsgatan, trees are traditionally planted in tree pits with grates. These are designed as concrete boxes with a specially designed drain. All water from the street is led to the trees through a drain for each tree.
To handle large amounts of water, retain and purify it, and ensure good growth conditions, biochar soil has been used. This is a mixture of gravel and charcoal that provides very good growth conditions while carbon dioxide is bound in the charcoal.
Extreme downpours with more than 50 millimeters of rain an hour are still relatively rare, but are expected to increase in future climate challenges. The video shows how one of the plant beds on Madängsgatan in Stockholm Royal Seaport handled similar amounts of rain in June 2021. The major challenge now is periods of no rain, where the need for irrigation increases.
Video by Fredrik Ohls, Sweco.
Good examples
The examples described below are from Hjorthagen, the first development phase of Stockholm Royal Seaport. Here, some parks, streets, and blocks have been completed, and a large number of residents have moved into their homes.
- A system of moisture corridors and ponds has been created in Hjorthagsparken. These corridors receive large amounts of water in the area's hilly terrain and are part of a dispersal area for aquatic animals and plants. Options for expanding habitats for frogs have been strengthened with a newly constructed frog tunnel under the main street in the new district.
- Lawns have been designed as retention areas for runoff water from blocks and streets.
- Continuous green corridors in the form of so-called ditches have been built on local streets with lowered planting surfaces that receive all water from the street and sidewalk. Footbridges and car crossings to garages are positioned at a height above the ditch to avoid standing water during heavy rain. Water is retained and used to irrigate trees and other vegetation in these green corridors.
- On the main street, Bobergsgatan, the trees are planted in a conventional avenue style in grated tree pits. These are designed as concrete boxes with a specially designed well. All water from the street is channelled to trees with a well for each tree.
- To make it possible to handle large amounts of water, retain and clean it, and ensure good growth conditions, biochar soil has been used. This is a mixture of gravel and charcoal that provides conditions for excellent growth while carbon dioxide is bound in the charcoal.
Related links:
Interactive map: Green roofs, dispersal paths and planted trees
Article Published: 17/04/2022 Updated: 07/06/2024