Connectors as a Key to Efficient Storm Water Management System: An In-Situ Assessment of Residential Estates in Poland

Urbanization has worsened urban flooding to a great extent. Together with this was accomplished a rapid change in landscape from natural to man-made [1]. The development of impermeable surfaces blocks water from natural penetration and leads to the intensification of surface runoff. That increase of impermeable surfaces causes hydrological characteristic changes in urban catchments [1, 2]. To reduce the human impact on components of the hydrological cycle in urban areas, paradigm shifts have emerged in recent decades from drainage-based urban design towards nature-based solutions [3]. The term nature-based solutions (NbS) has been present since the late 2000s, however, the concept itself is not new, based on the traditional design, the alternative one to “conventional engineering” [4, 5]. While NbS have been influenced and can still learn from traditions, the conceptual framework of NbS is linked to 21^st century societal challenges. One of the challenges is climate change and the second is the human impact on biological diversity [6]. According to the United Nations definition, NbS for water are inspired and supported by nature and use, or mimic, natural processes to contribute to the improved management of water [7]. Such solutions introduce into cities, landscapes, and marine landscapes increasingly diverse natural and natural features and processes thanks to locally adapted, resource-efficient, and systemic interventions [8]. The definition, however, is not very detailed, but it’s commonly agreed that NbS must provide benefits for society, the economy, and nature at the same time. The opposite of the “traditional” water infrastructure, nature-based storm water control is characterized as a multi-functional system [9]. Those are the solutions that not only ensure safety against floods but also as an ecosystem service, to bring cultural and aesthetic values and have a positive impact on the health and well-being of the inhabitants [10, 11, 12, 13, 14].

In order to achieve sustainability goals to counteract these problems in Poland, there are prepared climate adaptation plans (MPA) that are strategic documents created on a local level. The goal of MPAs is to increase the adaptive capacity of the city in the face of possible threats of climate change [15]. Besides that, many cities take care of a rising awareness among citizens. Among other activities, they publish guides and catalogues that focus on easy-to-install retention and infiltration solutions.

However, there is a persistent gap between knowledge and practice. In the adaptation plan for Metropolis GZM (Poland) blue infrastructure solutions despite the declared systemic approach in reality propose individual elements [16]. Different cities started to publish informative catalogues and brochures that show just particular elements with their descriptions, advantages, and maintenance [17, 18, 19, 20, 21, 22]. Their purpose is not only to raise awareness about flush floods among residents but also among local authorities, architects, or town planners. One of the examples could be the catalogue prepared for the city of Bydgoszcz. It presents elements of blue-green infrastructure divided into categories of land development [22]. However, the information is mostly theoretical there is missing instruction about how to organize a system. Another publication prepared for the city of Gdańsk has information that presented solutions for rain gardens that could be combined together, but missing information on how to achieve it [21]. In reality, such publications have mainly educational purposes and are not useful for professionals. Blue and green infrastructure has developed mainly in an informal manner [23, 24] thus it is missing formal terminology that could develop methods, ideas, concepts, and techniques [23]. Formal publications about sustainable rainwater management appear as textbooks showing elements of their advantages and technical tools [25, 26, 27]. However, sustainability is a wide term used by the authors who represent a more universal approach toward sustainability that is missing common framing and differs in details. So, confusion can occur, with different authors using different terms to mean the same thing, or ascribing different meanings to a given term [24].

The research aims to identify possible rainwater collection systems around the studied housing estates. While the previous studies mainly focus on describing particular elements and catalogues, the innovation of the paper is to identify rainwater management systems (RMS) in a scale of residential housing. Respecting the nature of the blue infrastructure, which is a set of interconnected elements, needs to be managed within the catchment [28]. Analyses of the number and type of elements and their composition in the system helped to determine whether land development projects could be called water-sensitive. It also allowed to name typically missing elements and the efficiency of applied solutions and answer the question of which elements are critical for efficient functioning of RMS.

The investigation was based on desk research and in situ inventory. The research subjects were surface blue and green elements of land development, which were assessed considering their impact and role in rainwater management in residential estates.

To narrow down the area of choice of housing estates that were studied, I subjectively picked three localizations. The city of Warsaw, Wrocław, and Gdańsk. Those are three out of forty-four cities in Poland for which Urban Adaptation Plans were created. The plans, developed in cooperation with the Ministry of the Environment, aim to adapt to the forecasted climate changes. I used a systemic strategy for screening and finding relevant housing estates that are widely considered in media as “green”, “sustainable” or “water sensitive”. The selection criteria were location (Warsaw, Wrocław, Gdańsk), time of construction (between 2000 and 2020), and type of building (multifamily housing estate). The first step was to search for the projects from PLGBC (Eng. Polish Green Building Association) “Green Building” awards from 2010 to 2021. Two met the inclusion criteria of a total of ninety-five awarded projects. The second step was to search for examples in Google using the keywords “green housing estate”, “sustainable housing estate”, and “water management housing estate” in each localization. The resulting projects were screened for relevance and analysed in terms of the contents of their advertisements. This approach added eight housing estates to the research. In total, it gave eleven housing estates, including four estates in Gdańsk, two estates in Gdynia, one complex of estates in Wrocław, and four in Warsaw. The examples, due to the manner of selection, cannot be considered as representative ones.

The research concerned eleven housing estates in Poland that were built in the 21^st century. Part of the estate is still under construction, but the research concerned only selected, existing fragments of buildings and land development. The examined fragments can be divided in terms of size into three groups (Table 1) large with an area of 60,000–84,000 m², medium with an area of 34,000–48,000 m², and small with an area of 19,000–28,000 m². Wolne Miasto, Nowe Żerniki, 19 dzielnica, and Żoliborz Artystyczny are examples of large housing estates. The housing estate Nowe Żerniki in Wrocław consists of seven multifamily residential estates yet the structure of the complex was designed as a master plan. There are designed public spaces, green areas as well as a retention basin. So, in the tables as well they are visible as one. The medium estates are Zielony Południk, Nadmorski Dwór, Idea, and Beauforta estate. The group of small estates is Sokółka Zielenisz, Mickiewicza, and Jaśminowy Mokotów. Almost all housing estates, except Zielony Południk, have underground car parks. Underground car parks have an impact on green areas evaluation and may have an impact on shaping RMS.

The RMS was defined as a set of elements connected with the water flow where the rainwater is directed and captured for a specific purpose like an infiltration, or permanent or temporary retention. What is visualized in Fig. 1. The specific aspects of the research directly follow the presented definition on the local scale, and they were the origin of the rainwater (i), the general division of areas into use (ii), topography (iii), elements directing rainwater (iv), and identification of NbS elements (v). All of the chosen eleven housing estates were studied using those criteria.

Figure 1.

Each of the estates was analyzed to verify the composition of an individual system. Following specific aspects of the research, recognition started with the rainwater sources (i): precipitation, surface runoff, and roof runoff. At this stage, to evaluate if the water from the surface runoff is well used in the system, crucial are road and pavement materials and the sewage sump’s location. The second analyzed element was the overall evaluation of land cover (ii): the percentage of roads and pavements area, building area, and green area that were additionally divided into different types. Naturally, water flows from higher parts of the landscape to lower ones by gravity so the third investigated element is terrain profile (iii). For this purpose, geoportal.gov.pl was used which acts as the central node of the Spatial Information Infrastructure, mediating access to spatial data and related services. The characteristics were investigated on a numerical land cover model, and the height differences were taken from the terrain profile created for each example. The system is efficient when the water flow is planned and controlled, so the next investigated elements are those directing the rainwater. Water can be guided by underground pipes, open channels, or breaks in the curbs to the point where it could be used as a resource. The fifth element of the system is nature-based solutions (v). They are divided into three main groups divided in terms of the process of infiltration, temporary retention, and long-term retention.

The most valuable green elements for RMS are trees and green roofs. Trees are a key element shaping the microclimate of the estate because they lower the local temperature. They bring hydrological benefits by slowing down and limiting the flow of rainwater [29]. For example, they can reduce storm water runoff by intercepting 15% to 27% of annual rainfall [30]. Even better results in retaining the rainwater have extensive green roofs that are promising elements of reducing rainfall runoff [31]. They obtain different results according to the layers and vegetation types [32, 33]. According to the research in Opole, about 44% of the rainwater from torrential rains is retained in the layers of the green roof, and in the case of rains not exceeding 5 mm/day [34]. While the other research results showed up to 80% of retained rainfall volume [35]. In quantitative evaluation, roof runoff area has hydrological potential [36]. This makes it clear that overlooking the roof area in planning a retention system is a mistake and has a great impact on efficiency if not disqualifying a project from being water-sensitive.

A high percentage of green areas does not constitute efficient RMS but should be considered while evaluating overall efficiency. In the housing estate Mickiewicza where 70% of green area, the quality of those spaces was low because of a large area of trimmed lawn. There were four housing estates with high-quality green areas, but one of them (Sokółki Zielenisz) had missing elements of RMS. Nevertheless, even a slight inclusion of vegetation helps to create an urban environment with better sustainability parameters [37] Vegetation is one of the most effective mitigation strategies for reducing the global change impact. Even lawn areas immediately reduce the impact effect of rainwater through direct interception that delays the outflow [30]. Any actions that increase permeable areas are valuable, e.g., a 35% reduction in the impermeable surface due to tree planting can reduce runoff in a parking lot by almost 18% [30].

Previous and partly pervious surfaces are widely considered elements of storm water management [27, 38]. On open-graded aggregate, much of the pollution material can seep down into the pavement and soil, where the community of microorganisms living there begins a rapid breakdown process [39]. Whereas sealed surfaces cause the transport and accumulation of pollutants originating from traffic areas, building materials, and other impervious surfaces [38, 39, 40]. Hence, it is necessary to provide an effective solution to drain and capture rainwater runoff from the impermeable surface [41]. As was noticed at Zielony Południk and Beauforta estates impermeable surfaces could be a part of the RMS as a passage for rainwater into elements of nature-based solutions such as rain gardens. These are promising elements in controlling the water. Maximum capacity is a frequently used parameter for evaluating efficiency, however, it might be misleading. The important thing is if the water will end up there, so investigating the way of the rainwater. The crucial element in terms of the definition of the system is the connectors between the elements. Individual rainwater management elements not connected together do not always have an impact on increasing the area’s retention ability [42].

The research investigation focuses on the RMS in systemic review, not discussing its impact on land development in terms of the quality of spaces that make them attractive for visiting and, in turn, support health and wellbeing. A combination of those two should be an interesting topic for further discussion while the studies of the quality of green spaces [43, 44] and blue spaces [45, 46] have already started. Another point is that the definition and diagram focus on water as a resource to be useful for greenery. Rainwater can be used not only for gardening but as well for car washing, and laundry [47], which might be a promising extension for maintaining residential areas. The RMS diagram was presented as a conceptual framework for rainwater systems in housing estates where the forward research agenda would need to consider the user needs and expectations.