Water in the city – rainwater and its in situ management in blue-green infrastructure

There is no more glaring evidence of climate change than that associated with changes in the nature of precipitation. Everyone is familiar with examples of flooded tunnels and basements, accumulated sand and sediment all over the surface of sidewalks, paths and roads, or knocked-out sewer well lids. What was once a rarity is now an everyday occurrence.

Challenges of rainwater in cities

The main cause of infrastructure problems is the inefficiency of sewer systems, which were not designed to handle such extreme events. In extreme cases, excess rainwater leads to costly damage, obstruction of urban traffic, and runoff of pollutants into the water resources we use, such as river systems and lakes.

Another challenge is that large cities have fewer and fewer permeable surfaces, with concrete, asphalt and other paved surfaces preventing the natural absorption of water by the ground. As a result, rainwater quickly drains into drains, causing overflows. The problems multiply in regions struggling with drought, and exacerbate the phenomenon. We’ve barely touched on the topic of the effects of climate change, and already the solution is quite clearly in the form of the need to retain water wherever possible, with the simultaneous use of stormwater drains to intercept heavy rainfall and discharge the excess to receiving bodies.

One more important point – when talking about retention, we must remember that behind this word there is a whole scale of solutions with a span from a few meters (a rain garden on a housing estate, about 5^m3) to many millions of cubic meters (the Racibórz Dolny Reservoir, about 180 million^m3). In everyday communication, this term is used quite arbitrarily, which can cause problems in mutual understanding and substantive discussions. For the purposes of this text, let’s assume that we are talking about many small solutions, such as a rain garden or a rain barrel, which by their widespread use will add up to millions of cubic meters of capacity.

To underscore the need to focus on small-scale solutions, it’s worth leaning into the way our cities are growing – the fastest to be built is residential development. It is it, fueled by private money, that buys up available land and carries out the investment process without undue delay (commonly at the expense of overtime on projects and construction sites). Investment in large-scale retention facilities usually takes at least as long as two consecutive housing developments.

Securing the availability of land for such developments is also often a problem – competition for space with private developers. These issues, however, are not disconnected, and increasingly often water problems arise only after the construction of a housing development. This is not surprising – if these processes were swapped places and a drainage system was implemented to accommodate runoff from future development, most of the problems could be eliminated. It’s easy to write, it’s harder to convince everyone of this – despite the fact that everyone intuitively feels that such an order makes much more sense.

Cities face a challenge – climate change is evident, and precipitation will cause more and more problems, so a change in approach is needed, both in terms of development strategies and comprehensive urban planning, which will take into account not only infrastructure development, but also water conservation and sustainable rainwater management. Not taking action and not requiring investors to plan for retention makes it a matter of time before the situation in southern Poland is repeated in other cities.

Blue-green infrastructure: definition and meaning

A response to the needs and a common solution that can be used complementary to drainage infrastructure is blue-green infrastructure. That is, an approach to urban space planning that combines rainwater management with greenery integrated into the landscape. Its main goal is to create places that can quickly fill with water during rainfall and allow it to soak in. This means that rainwater is not discharged into the sewer system, but is retained and used on site to support vegetation and improve microclimates, biodiversity and affect other negative effects of climate change.

During dry weather, the space should be able to withstand the phenomenon of drought, such as through natural shading. It is worth paying attention to the planting of such species that can well withstand changing conditions of water availability. Nothing prevents the facility from collecting rainwater permanently, but an emergency overflow should be kept in mind. On a day-to-day basis, the facilities create a cohesive space that is friendly to residents – including by introducing more greenery in their surroundings, promoting biodiversity, improving air quality and reducing the heat island effect.

Examples of such solutions include rain gardens, ponds, drainage ditches, absorption swales, green lanes, green roofs, ecotone zones at the banks of standing and flowing waters, and many other solutions based on the principles of ecohydrology that allow water to seep naturally into the ground. These should be accompanied by a shift in thinking about rainwater management to an approach of finding a place for water wherever possible. The easiest way to understand the need for these solutions is to compensate for the loss of green space by an appropriate amount of cubic meters of retention for every square meter of built-up, impervious surface.

In situ rainwater: a key role in sustainable water management

Managing rainwater in situ, that is, directly on the site where it falls, is the most sensible way to compensate for impervious development with retention. It is also the most popular solution used in cities around the world. The idea is to reduce as much as possible the rapid runoff of water into the sewer system and keep it where it fell, allow it to be naturally absorbed and used by the local ecosystem. The difference from the typical urban drainage approach: gully, sewer, outlet to receiver is drastic – we don’t get rid of water, but try to retain and use it in the catchment area.

One of the simplest and most effective solutions that can easily achieve these goals are rain gardens. That is, specially designed areas in the form of depressions that collect rainwater, if only from nearby roofs, sidewalks and parking lots. A stone or gravel baffle can be placed at the garden’s inlet to trap fine mineral debris, and with the right vegetation and permeable soils, water can seep in there naturally. Plant species for planting in rain gardens are numerous.

It all depends on how much water there will be in it – consider using hydrophytic plants and those that tolerate changing water conditions well. If we do not know whether the soil is permeable, then it is enough to perform a simple percolation test (Retencjapl, Kalisz Supports Nature and Collects Rainwater: Collect and use – how to practically save rainwater, 2022):

1. Dig a hole similar in depth to the one where infiltration will actually occur and watch for 24 hours to see if the water seeps through – if it does, this may indicate a high groundwater level, making infiltration difficult.
2. Check the permeability of the soil by digging a deeper hole (30x30x30-50 cm) and moisten it. If the water soaks in within 24 hours, the site is suitable for creating an absorption basin or rain garden.
3. If water soaks in slowly or not at all, consider other solutions, such as building a pond or sealed tank.

No one needs to be convinced of the positive effects of such solutions – set up a garden, and in time it will be populated by amphibians from a nearby pond, bees from beehives, which are more and more numerous in urban centers (and bring a bountiful harvest of honey!) and other insects and creatures that will create their own micro-world around the object.

Within cities, green roofs are commonly used as part of biologically active areas, which also serve to manage water in situ. Roof surfaces are second only to roadways in generating rapid surface runoff and an easy way to overload sewers. Green (or brown!) roofs, thanks to the layers of substrate (not soil!) and vegetation that cover the top of buildings, allow rainwater to be partially retained, gradually draining away excess water and evaporating. Such solutions not only reduce the amount of water flowing into the sewer system, but also improve the thermal insulation of buildings and introduce greenery into dense urban spaces. They also make up a significant portion of all the green spaces we see from the high floors of some buildings in downtown Warsaw! Bravo!

However, let’s return to brown roofs for a moment, because it is a concept, for the time being, not very popular in Poland, and it is definitely quicker to find a tile in this color than to find a definition of the phenomenon and examples. A brown roof (London, 2016) is an unfinished green roof – we prepare the structure and the ground, and leave its settlement in the hands (or pecks) of nature. It’s a simple way to employ nature (mainly birds and wind) to do the work of recreating the local flora by freely distributing seeds. In this case, over time, plants that grow on nearby lawns will appear on the roof.

In cities, where compact and tight buildings don’t allow much, it’s worth considering rain gardens in containers (under a gutter, with an emergency overflow to a nearby grating) or permeable pavements that allow partial infiltration of water into the soil. Unlike asphalt and concrete pavements, which completely block the flow of water, permeable pavements are made of materials (such as a mixture of epoxy resin and gravel) that allow water to soak into the ground. They are ideal for parking lots, sidewalks or city squares.

Another method is the use of retention tanks or sealed rain gardens, which allow rainwater to be collected in areas where its direct absorption into the ground is difficult. The tanks can be placed underground or on the surface, but it is worth remembering to use the collected water, such as for watering vegetation, washing streets or other municipal purposes, and in the case of sealed rain gardens, plant vegetation that will survive in it. Such systems are a valuable solution in cities where there is a lack of natural permeable areas or space for surface solutions.

There are many more ideas, and how they are integrated into the space depends on the designers, who will be motivated to do so by the requirements of the network managers. In Gdansk, any project involving the construction of a drainage system must pass an agreement and provide for retention – without this, no building permit will be issued (Gdansk Water, 2020)! Many other cities are developing procedures that we are helping to implement as Retencjapl (Retencjapl, 2024).

In-situ solutions are a key component of blue-green infrastructure and should form the permanent, essential core of planned development. When undertaking any new development, provision should be made for places to soak up rainwater. The wrong approach is to collect precipitation through sewers and dispose of it – by doing so, we will only end up shifting the problem elsewhere, while getting rid of a valuable resource that may be in short supply during a drought. Climate change forces adaptation, and the most sensible methods are those that occur naturally – it is foolish not to take advantage of them.

As Retencjapl, we put our hand to making a difference and have been helping cities manage stormwater effectively for years, implementing strategies based on blue-green infrastructure, among other things. Our approach focuses on gathering information to map sewer systems and catchments with the highest possible probability when building hydrodynamic models and calibrating them. Then, by performing a series of analyses for different rainfall scenarios, we adjust the conceptual solutions to enhance the attractiveness of cities. Our completed projects are an ideal starting point for building sustainable solutions with climate change adaptation in mind. We also help obtain financing, which in recent years we have successfully translated into investments made in cities such as Bydgoszcz, Lodz, Gdansk, Slupsk, Pruszcz Gdanski and Starogard Gdanski.

Challenges and prospects for the development of blue-green retention systems

Despite its many advantages, the implementation of blue-green retention systems faces a number of challenges. One of the most important is the lack of adequate legal regulations and complicated administrative procedures, such as water permits. In Poland, despite the growing awareness of sustainable water management, there are still many legal barriers that hinder the implementation of nature-friendly projects.

Another problem may be financial constraints. Investments in blue-green infrastructure, despite the long-term benefits, involve high outlays, which discourages some local governments and developers. The cost of installing green roofs, underground infiltration basins or permeable pavement can be high, especially in the context of upgrading existing infrastructure. However, we must not look only through the prism of finances, and should look at the balance of costs and benefits. By investing in rainwater conservation now, we are buying future generations better access to it. If we get rid of water from cities through pipes and outlets – in the near future, urbanized areas will become unfriendly and too polluted to live in.

The prospects for the development of this sector are promising, especially with EU funds earmarked for investment in sustainable water management. The European Union has been supporting environmental protection and climate change adaptation projects for years, and blue-green infrastructure fits perfectly with these goals. The availability of funds from such programs as OPI&E, FEnIKS or LIFE is a great opportunity for Poland to accelerate the implementation of current and future projects, and some of them have already been established and are operating alongside us!

Many cities are taking advantage of the funds, making investments that not only improve stormwater management, but also improve the quality of life for residents – this year about PLN 1 billion was allocated for climate change adaptation investments in large and smaller cities (Ministry of Funds and Regional Policy, 2024). Programs such as FEnIKS are not only money for investment, but also promotion and education so that further construction does not involve inconveniencing residents. They are meant to encourage involvement in and anticipation of investments – one of the few opportunities to use EU funds to raise the attractiveness of our close, everyday surroundings.

The future of blue-green retention is therefore closely linked to the approach of network managers to requiring the use of these solutions, but also to the funding sources available for adapting cities to changing climate conditions. Combined with increasing pressure to protect the environment, these systems are a key element in building the cities of the future – more resilient, greener and more livable.

Summary

Climate change is bringing more and more challenges with alternating problems of excess and shortage of rainwater in cities. Such phenomena as flooding of tunnels or knocking out sewer well manholes are an everyday occurrence in summer, only the place and time differ. An enthusiast can spend the entire holiday season reading articles about extreme phenomena that occur every other day in Poland and every day in the world. Unfortunately, the reality that we experience on our own has its causes in our attitude and neglect of certain issues in the past.

In order for change to occur, a shock is needed (we are able to unite in critical situations), but effective solutions to problems on the climate scale are already a matter of years of commitment and a change of approach to things that we are accustomed to doing under the old rules. In this case, the revolution needs to happen on several levels – formal-legal, administrative, mental and practical. Nevertheless, we have more and more tools (Retencjapl, WaterFolder – Platform for Designers, 2023) and growing awareness, and EU funds, such as the FEnIKS program, represent an opportunity to materialize the dream of the universality of blue-green infrastructure integrated into our cities.

These solutions are essential for adapting cities to changing climate conditions and building more resilient, green urban spaces. The key is to use in situ solutions, which means stopping rainwater directly where it falls. Each successive element using simple nature-inspired methods can do much to manage water in a small catchment area, bringing a range of benefits. Observing the greening of our cities already confirms that we are heading in the right direction.

In the article, I used, among others. z:

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[3] Ministry of Funds and Regional Policy. (2024). European Funds for Infrastructure, Climate, Environment 2021-2027. retrieved from https://www.feniks.gov.pl/

[4] Retencjapl (2022). Kalisz Supports Nature and Collects Rainwater: Collect and use – how to practically save rainwater. Kalisz.

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