Urban Stormwater Management: A Sustainable Approach

Abstract

Climate change is impacting urban areas, especially through extreme rainfall that stresses conventional water management systems. Rainwater resulting from impervious runoffs, stormwater leads to an increase in the amount of wastewater that requires treatment and an overflow of the combined sewer system. Therefore, untreated wastewater is released into the surroundings or, in some cases, causes damage to urban systems. Nevertheless, many cities in the world are in the process of establishing a sustainable approach to urban water management. Sustainable stormwater management practices are essential for overcoming various environmental challenges and promoting community sustainability and resilience. The purpose of this paper is to communicate Oslo’s success in implementing sustainable stormwater management in urban areas. By analyzing successful cases, researchers may be able to record patterns that offer potential answers to the stormwater management challenge. The present case study presents an approach that can enlighten urban planners and policymakers about the potential outcomes of sustainable stormwater management under the conditions presented.

1. Introduction

Climate change is impacting urban areas, especially through extreme rainfall that stresses conventional water management systems [1,2]. Rainwater resulting from impervious runoffs, stormwater leads to an increase in the amount of wastewater that requires treatment and an overflow of the combined sewer system [1,2]. Therefore, untreated wastewater is released into the surroundings or, in some cases, causes damage to urban systems. Nevertheless, many countries in the world, such as Norway, Denmark, and Germany, are in the process of establishing a sustainable approach to urban water management. Such countries include green structures, which are climate-adaptive as well as resistant to flooding [3,4].

Sustainable stormwater management practices are essential for overcoming various environmental challenges and promoting community sustainability and resilience [3,4,5]. The local management of stormwater helps to reduce dependence on centralized systems, including water supply and treatment, leading to the efficient conservation of water resources and reducing freshwater sourcing demand [4,5]. Similarly, flooding management, erosion, and the deterioration of water quality, among other factors, are addressed, helping to build resistance against water scarcity that is synonymous with climate change. Retrofitting stormwater ponds, reinstating wetlands, and extending stormwater retention improve water quality and reduce peak flows and flood risks [5,6]. On the other hand, public acts, offering focused professional workshops, and including urban stormwater management are viable efforts that enable efficient, sustainable, and more diligent stormwater management from local to federal levels [6,7]. Sustainable stormwater management treats stormwater as a valued resource and promotes its conservation, including water management among other interacting water bodies [4,5,6,7].

Sustainable stormwater management practices have multiple benefits for urban areas that cut across environmental, economic, and health concerns. As indicated earlier, water availability is one of the most crucial issues for urban areas given the increasing pressure on the resource [5,6,8]. Sustainable stormwater management strategies are thus critical in reducing the volume of stormwater discharged, further minimizing water stress [8]. For example, green infrastructure allows far less polluted stormwater to run off [9]. Such strategies capture pollution from stormwater runoff, such as heavy metals, oil, and bacteria, and then filter it, reducing pollution levels. In addition, the use of green infrastructure in urban areas is crucial as it captures stormwater to reduce flooding and keep waterways clean and healthy [10]. Water quality is also improved through numerous processes, including the adsorption, filtration, plant uptake, and decomposition of organic matter [11,12]. Moreover, sustainable stormwater management strategies enable urban areas to address the issues of climate change, aging infrastructure systems, and increasing imperviousness [13].

The challenges associated with implementing sustainable stormwater management practices in urban areas are diverse and require new and innovative approaches to address them [5,14]. As urban areas have long been characterized by retrofitting sites into sustainable stormwater management, this option is less likely to be effective in locations with extensive amounts of impervious pavement, including parking lots, and sidewalks that do not create enough space for green infrastructure to be introduced. Furthermore, for decades, traditional engineering solutions have also been a standard approach in the field of stormwater collection systems, consisting of areas on-site where stormwater is regulated and released into waterways [14]. However, new and innovative measures and practices are desperately needed due to non-point source pollution, incidental flooding, and sickly waterways’ current conditions, all of which undermine sustainable stormwater management in urban areas [15]. Moreover, since much stormwater infrastructure has been designed to move water off-site as quickly as possible, achieving sustainable objectives necessitates altering traditional engineering approaches [14,16]. As urban water resources face increasing pressures, the need to manage stormwater sustainably in a variety of contexts is evident, leading to the pressing need for effective and efficient stormwater management [14,15,16].

Oslo, a city committed to combating climate change, is determined to become a hub of sustainability by eliminating all greenhouse gas emissions. In May 2020, the City Council embraced a comprehensive climate strategy that outlines the roadmap to achieve this ambitious goal by 2030. The Paris Agreement, a global commitment to limit temperature rise to below 2 degrees Celsius, serves as the foundation for Oslo’s climate strategy. With the world’s population increasingly gravitating towards urban areas, cities play a crucial role in driving climate action. Oslo recognizes this responsibility and aims to lead by example. While the task of transforming into a zero-emission city within a decade is undeniably daunting, Oslo has made significant strides in technology and policy advancements. However, achieving this vision necessitates not only technological innovation but also societal shifts and collective efforts. By aligning with the Paris Agreement, Oslo is poised to spearhead the transformation towards a sustainable future [17].

By the year 2030, Oslo is set to become a city that emits virtually no greenhouse gases. This transformation will not only create a healthier and more enjoyable city, but also one that is well managed and characterized by reduced social inequality, as well as cleaner air and water. Oslo serves as a platform for innovation, where new climate change solutions are tested, refined, and brought to market. The city’s climate and business policies work hand in hand, reinforcing each other’s impact. Oslo is leading the charge in environmental and climate policies, both within Norway and on the international stage. By sharing our solutions and experiences, we are influencing other cities and countries to accelerate their efforts in reducing emissions. Our goal is for Oslo to become a “carbon-negative city” by 2030, actively working to remove greenhouse gases from the atmosphere through the implementation of biological and industrial carbon capture and storage technologies [17].

The purpose of this paper is to present Oslo’s success in implementing sustainable stormwater management in urban areas. By analyzing successful case studies, researchers may be able to record patterns that offer potential answers to the stormwater management challenge. The present case study grants knowledge to urban planners and policymakers regarding different scenarios and their patterns of sustainable stormwater management.

2. Materials and Methods

2.1. The Case Study of Oslo

Oslo is among the fastest growing cities in Europe. Oslo municipality makes a projection of the population in Oslo and its districts every year. This projection is used for planning investments, service offerings, and planning work. As of 1 January 2023, Oslo municipality had 709,037 inhabitants. Given the assumptions in this year’s projection (the intermediate alternative), the population will grow by more than 100,000 until 2050, to just under 813,000 [18].

This urbanization increases dense surfaces and makes the city more compact. Additionally, the drainage system now includes many of the city’s original streams. A good example is Hovinbekken. The stream closure took place in stages. In 1955, the municipality began to lay Hovinbekken in large concrete pipes, because the streams functioned as recipients for sewage and waste. They appeared to be unlovable and a danger to children. There was no opposition and the area was in the process of being transformed into business and housing.

A combination of closed surfaces, stream closures, and limited capacity in the pipeline network to handle stormwater during intense rainfall leads to back-up in basins, overflows, and the pollution of waterways, stormwater flooding in streets and buildings, as well as the infiltration of water into basements. Against the background of the above as well as the climate change the city faces, Oslo municipality launched its strategy on stormwater management in 2014. The purpose is to deal with excess water resulting from urban growth and climate change. More torrential rain creates more stormwater and possible flooding. The water must be given its place back in the city, which means opening closed streams. Opening the streams has an important additional function; nature returns to the city, to the delight of its citizens.

Oslo municipality has been innovative in giving private businesses and residents their own responsibility. This responsibility involves managing stormwater in one’s own property without damaging neighboring plots as well as setting aside plenty of space for one’s own management when planning. In the case of new buildings, the builders have to apply for a permit to discharge stormwater into the public sewer network [2]. The applications must include stormwater calculations and provide the discharge quantity in liters per second (L/s).

The strategy for stormwater management reduces flood risk and includes measures for safe flood management when extreme weather hits the city. The Oslo Fjord is a large recipient to which the water must be directed safely. Safety involves the control of both the quantity and the quality of the water that is discharged, e.g., in the form of E. coli [2,19].

Moreover, removing snow from Norwegian roads is considered a necessary task in order to be able to maintain the agreed road standards during the winter season. The snow is driven away to a large extent to ensure traffic safety and meet requirements and expectations for mobility and accessibility. Snow that is driven away from the roads is often contaminated, to a greater or lesser extent, with environmental toxins, particles, microplastics, sand, gravel, salt, and rubbish and must be handled properly. The amount of snow that is driven away during a season may vary depending on the snow conditions. A great amount of snow can fall in a short time. Analyses show that Oslo municipality needs a solution that can handle a total of at least 500,000 m³ over a 12-week period and a continuous volume flow of at least 1000 m³/h. At the same time, cities can also have winters with little snow, where the need is not necessarily as great [20].

2.2. The Stormwater 3-Step Strategy (S3SS)

S3SS is a method which, by means of three steps, defines how rainfall of varying intensity (low, moderate, and heavy) should be handled in a sustainable way (Figure 1). S3SS lays down that rainfall from small rainfall events should be able to be infiltrated into permeable (penetrable) surfaces (Step 1), runoff from large rainfall events should be able to be delayed before it is channeled further (Step 2), and runoff from extreme rainfall events should be diverted onto the surface via safe floodways (Step 3) [21]. More specifically:

Step 1: A total of 10 mm of rainfall from the measured area is diverted to permeable surfaces and infiltrated or collected and reused within the measured area. This is achieved by using green areas, such as trees, rain beds, and green roofs and permeable surfaces instead of asphalt. Stormwater from roofs, roads, and open spaces is directed to green areas, open ditches, and canals or to a body of water. This leads to a greener city with multifunctional solutions. The idea is to give nature its place back. Landscape architecture meets hydraulics and together they create a more attractive city. Processes that take place during stage 1 are infiltration, evaporation, and purification. A map of green roofs in Oslo is shown in Figure 2.

Step 2: Runoff from a climate-adjusted 5-year rainfall must be retained in the measured area. Amounts of water calculated for stage 1 cannot be subtracted when dimensioning the needed retention for stage 2. This is achieved by dedicating areas to drainage and ensuring that the areas are equipped to deal with stormwater. We must reduce the risk where stormwater often collects and leads to flooding and damage, and where a flood is critical. Stage 2 involves collecting and diverting stormwater to achieve an improvement in the water quality of the watercourse by reducing overflow operation. Oslo municipality has 751 stormwater discharges into waterways and 218 overflows [22]. In addition to digestion, less extraneous water and a more sustainable wastewater treatment process are achieved by saving energy. The separation of common sewer lines is a costly measure in itself. Considering that Oslo has many quick clay areas, there is often a need for geotechnical investigations as well as the piling of trenches, which increases costs even more. Project managers in Oslo municipality encounter this challenge very often. Sometimes sedimentation tanks can be a good solution, but the use of shallow stormwater pipes has increased recently. These are used a lot by Stavanger municipality due to less snow.

Step 3: Runoff from a climate-adjusted 100-year rainfall must be diverted in safe flood plains and must not cause flood damage to the measured area. This is achieved by arranging dedicated floodways, i.e., certain roads and waterways, and ensuring that the stormwater is channeled safely to the floodways and onward via these towards the fjord (the result of the submergence of a glacial valley characterized as a narrow, deep, and long inlet of the sea between high rocks). In some places, it must also be supplemented with systems below the ground.

3. Implementation

3.1. Blue-Green Roofs

Blue-green roofs combine the water-retention benefits of blue roofs with the vegetation and insulation benefits of green roofs. They can store significant amounts of rainwater and can strengthen biodiversity with native plants [23]. Blue-green roofs handle stormwater in two ways. It can happen by runoff, also called detention, which is the natural delay that occurs when water flows through various layers and materials, and slowly drains out from the roof within hours or days after a rain event. The other way is by retention, which is water that the roof consumes through evaporation and transpiration from vegetation that is part of the roof solution. Blue-green roofs can contribute significantly to keeping stormwater away.

According to Bent C. Braskerud, chief engineer in the Water and Wastewater Agency in Oslo municipality, the structure of the roof is a key factor. Vega Scene on Grünerløkka in Oslo is a reference project for blue-green roofs in Norway. The project was completed in 2019 and won Oslo City’s architecture award. The roof consists of a thick (15–40 cm), vegetated cover that absorbs and evaporates precipitation. Precipitation is absorbed in the growing medium on the roof, which is lightweight and shallow, often made of a mix of crushed brick and expanded shale with a small amount of organic matter, and choked downspouts control runoff from the roof. The construction ensures that rain (first step) is delayed and evaporates, while rain (as indicated in the second step) is released in a controlled manner into the public sewerage network. The plants on Vega Scene are planted out with different thicknesses of growth mass to create progressively more demanding vegetation on the thickest layers, according to Hans Martin Hanslin, researcher at the Norwegian Institute of Bioeconomy Research (NIBIO). The research shows that more than 130 species have moved onto the roof and that the roof handles extreme rain events [24].

3.2. Rain Bed

A rain bed is a planted depression in the terrain where water is temporarily stored on the surface and infiltrates to the ground or the drainage network (Figure 3). The surface of the rain bed should be flat and horizontal so that water can reach and infiltrate the entire area. The depth of the drainage volume on the surface can be advantageously 20–30 cm. Increased depth results in a smaller area requirement, and the entire surface is moistened more often, to the delight of the vegetation. Good maintenance is important for good function and for rain beds to be perceived as a relevant alternative for stormwater management. Failure to empty sludge traps and replace dead plants has caused some rain beds to fall into disrepair. An advantage of measures on the surface is that we can more easily see if they work and need maintenance than buried solutions. Correcting errors is also easier and cheaper.

Deichmans gate in Oslo was originally a dilapidated street garden that was mainly used as a parking space. The water and drainage lines needed upgrading and on that occasion there was a desire to use the surface water as a resource rather than directing it into the municipality’s drainage lines and treatment plant. The rain beds were sized to handle the 20-year rain with a climate factor of 1.2. An infiltration capacity of 30 cm/h in the rain beds was assumed. The rain beds made up approx. 3.5% of the total catchment area [25].

Figure 3. Rain bed: roof water leading to rain bed [26].

Figure 3. Rain bed: roof water leading to rain bed [26].

3.3. Shallow Storm Water Pipes

In a typical ditch, the storm water pipe is at the bottom. What characterizes Norwegian trenches is covering pipes with a protection layer due to frost. For downspouts in Oslo, the cover should be at least 1.8 m (Figure 4). This is considered a sufficient frost-free depth. In contrast, shallow storm water pipes are located much higher. They can be used in areas with stable cold temperatures.

The dimensions vary depending on the diameter of the pipes. For the bottom width (

Table 1. Bottom width (nm) [27].

Dim (mm)	Bottom Width (mm) Approximately
<250	>600
250–400	650–800
400–800	900–1300
800–1200	1600–2000
>1200	>2000

):

3.4. Open Streams

Hovinbyen is Oslo’s largest urban development area. The reopening of parts of the Hovinbekken has been part of the transformation area. The plan is to reopen from Marka to the fjord with walkways that create life along the brook. Reopening and planting will help reduce dust in the air. In addition, Hovinbekken will contribute as a safe floodway in the event of heavy rainfall (Planning program with VPOR for Hasle and Valle Hovin, Planning and Building Agency).

3.5. Safe Floodways in Roads and Streets

A premise for the planning and engineering of safe floodways in streets and roads is to define the dimensioning flow of water. An assessment of a suitable method for calculation (manual calculations or hydraulic modelling) of dimensioning water flow and any capacity check of the planned floodway will be situational, and must be carried out in each individual case.

The choice of cross profile when designing the road/street can be decisive in achieving the desired drainage capacity. Often the cross section alone will not have enough capacity and must therefore be combined with other elements to achieve sufficient capacity. To illustrate the possible drainage capacity of the cross-sections, three typical street cross-sections in Oslo are presented, based on three road widths and curbs of varying height (Figure 5) [28].

4. Discussion

The street profile with the highest drainage capacity is V-shaped. However, it is little used in Norway so far. Plowing snow in winter can be a challenge. However, it can be an applicable transverse profile to use when establishing a floodway. Roof drop is one of the most used cross-sections, where the surface water is directed out to the sides either in an open ditch or in a drain along the curb. One-sided drop is a widely used cross profile in road and street design, where excess water is led out to one side in a drain along a curb or open ditch. The cross-section must include elements such as curbs, ditches, and the like to be able to function as a safe floodway [29].

One of Oslo’s notable achievements in stormwater management is its emphasis on green infrastructure. Green roofs, permeable pavements, raingardens, and vegetated swales are among the green infrastructure components, which can absorb and filter stormwater, thus reducing the load on the city’s drainage systems. In addition to alleviating flooding, these components enhance biodiversity, reduce air pollutants, and add aesthetic value. Additionally, Oslo has invested in the restoration and conservation of natural water bodies, such as rivers, streams, and wetlands. By enhancing the capacity of the ecosystems to absorb and attenuate stormwater, the city reduces the risk of flooding downstream. This is consistent with the nature-based solution for managing stormwater, which has been effective in many places globally, with co-benefits.

When it comes to implementing Oslo’s environmentally friendly stormwater management techniques in other cities, the following principles will be crucial: tailoring to the local geography or the climate, engaging community participation, integration into urban planning, investing in innovation, supporting policies, and financing. In other words, while the Oslo approach can be an inspiring model for other urban areas, every city should analyze its unique geographical, climatic, and infrastructural characteristics.

Green roofs and sustainable buildings can make large and dense cities more attractive. Small rainfall can be handled locally without putting the pipe system under strain, and energy can be saved in the hottest months. Vegetation provides shadow and transpiration from the foliage cools. Buildings with vegetation on the roof and/or on the walls will act as air conditioning with minimal power consumption, only a small pump that distributes the water as needed. If rainfall is collected and used for irrigation, the indoor temperature will improve and the urban heat island effect will be reduced. It is unreasonable that we use energy to cool buildings inside for high temperatures outside, by sending the heat out of the building to streets that become even hotter. Despite the fact that other countries may not have such deep ditches, digging can be minimized with innovative stormwater solutions, so that greenhouse gas emissions are reduced.

Furthermore, taking responsibility and accepting consequences according to rules can thus give more back to the city and its citizens. Oslo municipality has been active on social media and has shared its campaigns in a short, simple, and understandable way. The “Do it for Oslo” campaign, which among other things asked people to reduce water consumption in dry periods, had a great effect. People have to understand and accept that they affect the climate and quality of life with their own actions, such as, e.g., by directing the roof water into the terrain rather than into the public network.

Community engagement and collaboration with all stakeholders are vital in ensuring that the public takes an active part in the efforts and has a sense of ownership. Integration with urban development curbs future infrastructure challenges and mainstream sustainable practices. Investment in research is critical in developing the existing approaches and the creation of new technologies and practices.

The government should also play a central role in supporting policies and financial incentives. Sustainable urban stormwater management can be achieved through the collaborative participation of different stakeholders operating in diverse but coordinated roles. Policy formulation, financial incentives, infrastructure development, and public outreach are the responsibility of the government at every level; city management, implementation, readiness checks, monitoring data collections, and maintenance. It is also the private sector that develops and innovates, which carries out enforcement that engages in corporate social responsibility. Nonprofits and community organizations contribute by running education activities around outreach as well as guiding the local projects. Researchers and academia carry out research, analyze the data, or educate the professionals of tomorrow. Municipal residents and property owners incorporate best practices, such as participating in community initiatives or maintaining stormwater management features. Moreover, partnerships and integrated planning are essential to prevent or protect the quality of water, reduce flooding, and enhance urban environments.

Finally, while the S3SS method from Norway offers a promising approach to sustainable urban stormwater management, its replication in other countries requires careful adaptation to local conditions, infrastructure, climate, regulatory environments, and social contexts. Conversely, it should be emphasized that comparing Oslo’s green infrastructure efforts with those in other regions, such as Denmark, does come with certain risks. These risks primarily stem from differences in geographic, climatic, socio-economic, and regulatory contexts.

5. Conclusions

Oslo’s efforts in sustainable stormwater management from a holistic point of view becomes a representative example to the rest of the worlds’ cities due to the importance of blue-green infrastructure, citizens’ involvement, and innovation in developing sustainable urban spaces. City authorities’ commitment to sustainability and creativity in developing feasible solutions can reduce the negative impacts of residential growth on the future and keep the city’s residents prosperous. The present case study presents an approach that can enlighten urban planners and policymakers about the potential outcomes of sustainable stormwater management under the conditions presented.