Assessment of Blue and Green Infrastructure Solutions in Shaping Urban Public Spaces—Spatial and Functional, Environmental, and Social Aspects

Abstract

Blue and Green Infrastructure (BGI) provide one of the key Nature Based Solution (NBS) approaches for sustainable stormwater management in cities, in conjunction with extending the scope of Ecosystem Services (ES). In both the process of planning and designing highly urbanized areas, the implementation of BGI is important for the improvement of living conditions and counteracting the negative effects of climate change. Based on the literature review, 19 BGI solutions were identified and then valorized in relation to the following three key aspects: spatial and functional, environmental, and social. The results of the assessment were derived using the scoring method and allowed for the identification of BGI solutions with a high, medium or low value for shaping sustainable urban public spaces. Using the potential of analyzed BGI solutions to improve the functioning and attractiveness of urban areas requires a comprehensive approach. Conscious planning and designing should use the knowledge presented to make the implementation of BGI solutions as effective as possible in relation to the above-mentioned aspects of shaping urban public spaces.

1. Introduction

Water is a basic good that people have been using for centuries. Understanding the value of water and the natural environment is fundamental to achieving the Sustainable Development Goals (SDGs) of the United Nations 2030 Agenda for Sustainable Development and a high quality of urban life. Access to clean water is a human right, as stated in SDG6: “Clean water and sanitation” which requires the availability and sustainable management of water and sanitation for all [1,2]. Actions in the field of water management in cities should be implemented jointly with other SDGs, such as SDG3, requiring ‘Good health and well-being’, SDG11, that outlines the need for ‘Sustainable cities and communities’, and SDG13 relating to ‘Climate action’, thereby contributing to a complex network of connections. However, meeting them is difficult as many cities around the world face serious problems in terms of water management. Urbanization processes (intensification of buildings, domination of impermeable surfaces, reduction of biologically vital areas, etc.) cause a decrease in natural capital and pose a threat to ecosystem functions in cities [3,4,5,6]. Urbanization processes have also led to the disappearance of many forms of natural water infrastructure, and the dangerous effects of storms and floods are exacerbated by climate change. Commonly used forms of water discharge into stormwater drainage systems lead to an increase in the water deficit in cities [7]. The degradation of sewage systems [8,9,10] and their inefficiency make them ineffective in the event of torrential rains, and they also disrupt hydrological cycles, which is detrimental to the shaping of urban environments and the people living in them [11,12].

1.1. Sustainable Water Management

Managing water resources is one of the greatest challenges of modern cities [13], requiring integrated planning and design activities at various levels. In view of growing environmental threats, new solutions are sought to support sustainable development. Many leading practices are focused on managing stormwater decentrally to relieve pressure on centralized infrastructure [14,15] and to restore the natural water balance [16].

Many new concepts such as the Sustainable Urban Drainage System (SUDS) [17], stormwater Best Management Practices (BMPs) [18], Blue and Green Infrastructure (BGI) [19,20], Water Sensitive Urban Design (WSUD) [21], and Low Impact Development (LID) [22] have already altered the management methodologies of drainage systems from more conventional applications [18,23,24]. The implementation of all available Nature Based Solutions (NBSs) at the planning stage is crucial for contemporary urban development [25,26,27,28]. All these concepts have a common goal of achieving sustainable stormwater management to reduce stormwater runoff (collecting water for utilization or storage) and increase infiltration and evaporation by treating as close to the source as possible. These concepts aim to bring water back to a nature-oriented water cycle within the city [29].

1.2. Blue and Green Infrastructure

Blue and Green Infrastructure (BGI) as part of NBSs is considered to be one of the most nature-friendly measures to manage urban flood risk. Such infrastructure can combine water management and green infrastructure to maintain natural water cycles and to enhance environmental and urban renewal [30]. The term ‘blue-green’ or ‘green-blue’ infrastructure appeared at the turn of the last decade as a result of a growing awareness of the need for a more integrated-system approach, to incorporate environmental elements to cities [19,20,31,32]. BGI is ‘an interconnected network of natural and designed landscape components, including water bodies and green and open spaces (…) which provide multiple functions such as water storage, flood control, water treatment and many others’ [33]. BGI solutions provide a multidimensional increase in the resilience of cities to urban pressures, and for counteracting the negative effects of climate change [33,34,35,36,37,38,39].

BGI solutions can be considered for multiple aspects of designing resilient urban areas. Categorization by function refers to the contribution of BGI solutions in reducing stormwater runoff by retaining and storing stormwater during and after extreme rainfall, followed by its gradual infiltration into the ground or drainage into sewers. The storage and retention elements of these solutions comprise reservoirs with a low infiltration capacity, that are always filled with water. The infiltration and retention components directly infiltrate the water without trapping it. BGI components can be categorized based on where they are positioned, including above ground (e.g., blue and green roofs, green walls collecting stormwater and limiting its runoff), on the ground (e.g., swales, trenches, water reservoirs, and other vegetated areas of a significant impact on a site’s livability), or under the ground (e.g., reservoirs and systems collecting and infiltrating stormwater) under public spaces and structures. Their categorization based on scale includes solutions implemented on the regional/urban level (agriculture, parks, protected areas, public spaces, wetlands, and retention and detention ponds), through private scale components, (blue and green roofs, private gardens, and rainwater containers) or through block scale components (planters, permeable pavement, water squares, and subsurface storage) [33].

BGI has gained popularity in recent years, and a positive change in the design and management of urban drainage has been observed [40]. There is a generally growing need to implement pro-environmental solutions, and to restore natural stormwater absorption and retention mechanisms closer to its place of origin [30]. The applicability of BGI solutions in various types of spaces is also increasing [21,29,41], and there is a continuous increase in documented evidence for the various environmental, social and health benefits of blue and green elements in cities [42]. As a result, BGI solutions are increasingly seen as an effective way of managing flood risk and improving the public realm in urban spaces [29,43,44]. Urban public spaces refer to those areas or places located in cities that are opened and accessible to all people regardless of gender, race, ethnicity, age or socio-economic status. They include different types of spaces such as plazas, squares, parks as well as connecting spaces such as sidewalks and streets. They are characterized by their different sizes and the diversity of individual features related to their spatial arrangement and functioning that are focused on public life, activities and events [45]. Good public spaces are perceived not only as responsive and inclusive for their users, but are also created in a sustainable way, allowing for the implementation of different environmental solutions, including those related to water management [46], It is therefore crucial to recognize BGI solutions in relation to spatial and functional, environmental, and social aspects in the context of planning and designing urban public spaces.

1.2.1. The Spatial and Functional Aspect

There are various types of public spaces in cities, including streets and car parks, squares, green areas, buildings and their surroundings, etc. They occupy areas of various sizes and fulfill many functions. In highly urbanized areas, there are a deficit of public spaces, and many of the available spaces are dominated by impermeable surfaces and lack natural elements, which intensifies the occurrence of negative phenomena, i.e., the Urban Heat Island (UHI) effect. It is therefore suggested that BGI solutions should be implemented to minimize the effects of these phenomena. However, there are numerous technical and biophysical limitations [12,47,48]. The physical dimension of space in a city is determined by its morphology, i.e., the arrangement of the basic elements of the urban structure [49], and the physical features and components of the place [50], including the slope of the land [51] or the type of ground [52]. Barriers in this group include potentially negative interdependencies cascading through the urban system under both flood conditions and non-flood conditions [48]. Unfortunately, the physical elements typically used for protection against flooding, (dams, dikes and infrastructure serving as storage and transport of water) [53] tend to limit the availability of valuable space for rest and recreation, greenery, and prevent the improvement of biodiversity, etc. [54,55]. Further, the lack of space for the implementation of new BGI elements [49,56], even those interfering less with the city’s structures, is a factor that inhibits and reduces the effectiveness of the application, operation, and development of sustainable solutions. The problem of a lack of space is therefore a key issue to overcome, as BGI infrastructure typically requires more land cover than traditional drainage methods [57,58].

It is important to consider the design challenges related to the urban environment and solutions that can be applied, such as integration with existing spatial elements [59,60], and the need for above-ground or subsurface flood protection solutions (subsurface infrastructure) [61]. There may be difficulties associated with selecting the most appropriate engineering methods when planning and designing BGI components and systems, as well as construction and implementation challenges [6,47,48,50]. BGI solutions also include limitations such as the need to obtain funds and the difficulty of implementing complex technologies for monitoring, and also the difficulty in the maintenance of extensive BGI systems after their implementation [50,62,63,64,65,66].

1.2.2. The Environmental Aspect

BGI solutions play an important role in shaping the natural capital of cities by providing numerous benefits [20,33,47,67]. The most important benefits relate to the enhancement of water-related ecosystem services [68], including a positive impact on the regulation of the disturbed urban hydrological cycle by capturing, retaining, infiltrating and reusing stormwater within catchment areas, closer to the source of runoff while reducing the risk and effects of torrential rains and floods [18,36,69]. It is also important to support stormwater treatment processes, especially in those BGI solutions that rely on the participation of vegetation, thus contributing to the improvement of water quality in urban catchments and reducing the cost of removing impurities [70,71,72].

Another significant benefit is supporting biodiversity, by shaping extensive BGI systems in highly urbanized spaces [68,70,73]. Solutions of different scales and impact can be one of the means of enlarging areas for the development of various plant and animal species in cities [74], enriching biotopes and at the same time helping to shape landscape connections. They can also support the reduction of the biophysical and biochemical effects of land-cover changes in favor of the occurrence of natural ecological processes [27,29,75]. In the wider context of their positive impact, comprehensive BGI systems are becoming a very effective instrument for creating, restoring and protecting aquatic and terrestrial habitats. However, it should be kept in mind that the impact of BGI on biodiversity can vary, and its actual scope depends on the given BGI’s individual characteristics, especially in its local integration with hydrological urban conditions and wider ecological activities. The negative consequences of BGI for biodiversity relate to the creation of conditions for the uncontrolled spread of invasive species [33].

The most desirable positive effect of BGI as a sustainable approach is the modulation of the urban climate by mitigating negative changes. It is important to reduce the Urban Heat Island (UHI) effect and regulate temperatures by providing green and blue cooling elements for the city [76,77,78]. This significantly improves the adaptability and resistance of cities to extreme weather conditions [34], including their resistance to drought, and flooding during heavy rainfall [79]. The key to improving living conditions in cities is the improvement of air quality [80], including the provision of water for plant development, and then using their purification and oxygen production capacities [81].

1.2.3. The Social Aspect

Many elements contribute to the broader social context of the functions and perception of BGI. Its contribution to the improvement of climatic conditions translates directly into the improvement of living conditions in the city [82,83], the health of its inhabitants [5,84,85,86], as well as ensuring individuals’ well-being [87,88,89,90]. BGI solutions play a part in the organization of recreational spaces, providing city dwellers contact with nature and with outdoor activities. The integration of people with the natural environment takes place through the combination of both water and greenery in urban public spaces, which as a result become more attractive.

At the same time, more and more frequent extreme phenomena such as torrential rains and flooding has aroused social fears and created a sense of threat [91,92], especially in highly urbanized areas. There is a growing need to counteract the effects of these phenomena, but the awareness of the role that BGI can play in this regard is still insufficient. BGI’s sustainable functioning and its potential benefits depend on the behavior of those who use it [20]. At the management level, it is crucial to develop attitudes aimed at understanding and accepting the comprehensive implementation of BGI solutions in relation to other NBS solutions affecting human-derived capital in cities [5]. Institutional and social barriers usually appear due to mismanagement and the lack of a conscious policy, including the slow implementation of sustainable solutions [6,47] or the lack of a long-term vision regarding their functioning and monitoring [6,93]. Applying sustainable solutions on a large scale continues to be a political, economic [94], scientific and social challenge. Therefore, in order to achieve success, an interdisciplinary approach to adaptive planning and design is crucial, necessarily involving all potential stakeholders in this process, so that the actions taken are socially understood and considered acceptable [95,96,97,98].

It is important to involve the local community in the planning and management process [99] for the success of BGI initiatives. Creating positive changes in social behavior requires raising general awareness of the potential benefits of NBS [100] to encourage city dwellers to appreciate the role and value of BGI as well. Its benefits are numerous [101], i.e., those related to the positive long-term impacts of BGI solutions on public health and well-being [102]. It is also crucial to encourage sustainable social behavior. An understanding of and recognizing the direct and potential benefits of pro-environmental solutions can be realized through various practices, including community volunteering and civic participation [101,103], which constitute different forms of inclusive “bottom-up” community participation [30]. Social engagement usually results from the identification of relevant communities, [102,103,104,105] and is based on a multi-directional approach to the identified communities’ integration. Meikle and Jones [106] propose a broad typology of five ‘community’ forms: Interest (people sharing similar interests or passions), Action (people trying to enact change), Place (people associated due to geographical features or boundaries), Practice (people representing similar activities or sharing a profession), and Circumstance (people brought together by external events or situations). Combining these aspects will prove relevant for the social acceptance of initiatives related to the implementation of BGI solutions in urban public spaces [20]. Education in the field of social responsibility for the environment and the promotion of environmentally friendly behavior are also important [107].

The aesthetic value of BGI solutions constitutes another significant aspect in their social evaluation, because their appearance influences positive or negative reception and acceptance [108]. However, too few sustainable stormwater management systems have been applied in a way that is appreciated by the public. Their negative perception derives from the use of quite commonly implemented solutions that are little differentiated, ineffective, and at the same time are visually unattractive. They are often poorly implemented and maintained.

It should be emphasized that the implementation of BGI is crucial for improving stormwater management in cities and counteracting the negative effects of climate change. The knowledge of ecosystem services can be used to shape resilient areas, and there are many ways to overcome potential barriers that prevent the full functioning of BGI. Nevertheless, a broader understanding of the possibilities and limitations in the implementation of BGI solutions in urban space as an alternative to conventional systems, and the development of sustainable urban drainage is needed [109]. Such an understanding is important for supporting the process of spatial planning [21,110], especially in designing urban public spaces adapted in terms of scale, in order to increase their value as sustainable areas [95,111]. For these reasons, this study aims to broaden the knowledge of BGI solutions in cities, by focusing on the identification of available stormwater retention solutions and providing a multi-faceted assessment. This new approach links the spatial and functional, environmental, and social aspects of BGI to develop a comprehensive assessment of the benefits of various BGI solutions in the context of their implementation in urban public spaces as part of planning and designing processes.

2. Materials and Methods

2.1. Selection of Cases

The subjects of the study are 19 BGI solutions that act as representatives of a wide group of Urban Landscape Objects (ULOs) [112]–the diversity of stormwater retention structures and systems introduced in cities that can be used in shaping urban public spaces. The selection of cases was made on the basis of a literature review. They include 12 BGI solutions applied on the surface, 4 applied underground and 3 applied above the surface (see

Table 1. Division of BGI solutions related to their location [108,113,114,115,116] (elaborated by authors).

Location	Type of BGI Solution
on the surface	runoff troughs grassed swales infiltration trenches vegetated swales (street-side) bioretention basins grassed retention and infiltration basins rain gardens wetland ponds surface water reservoirs retention and infiltration water reservoirs water squares permeable/pervious pavements
underground	infiltration wells infiltration boxes structural tree root cells underground water reservoirs
above the surface	blue roofs green roofs green walls

).

2.2. Methods

The 19 BGI solutions selected for the study were discussed following an established common framework, consisting of 2 main stages.

The first stage concerned the identification of individual BGI solutions based on the literature review, including data from expert opinions, and practical information collected from catalogues and design guides [113,114,115,116,117,118,119] supported by data from scientific publications [120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136]. The general characteristics of the selected BGI solutions in this study, including their main features, are presented in

Table 2. General characteristics of BGI solutions (elaborated by authors).

BGI Solution	General Characteristics	Main Functions	Sources
runoff troughs	- linear open channels with tight bottom and walls - made of various materials (concrete casts, stone cladding, pebbles, etc.)	- collection of stormwater and slowing down surface runoff - discharge of excess water into sewage systems and/or larger retention facilities - exposition of water in the urban space	[113,114,122,123]
grassed swales	- small, wide and shallow linear open channels with trapezoid or parabolic shape, covered with grass (low vegetation) and supplemented with partitions dividing them into sections - require a large area	- collection and infiltration of stormwater - slowing down the flow and keeping stormwater in place - stormwater treatment - discharge of excess water into other collecting facilities (reservoirs, basins, wetlands, etc.)	[113,114,115,119,122,123]
infiltration trenches	- linear excavations with a perforated drainage pipe laid on the bottom in a gravel cover, require a small slope of the terrain - the surroundings of the trench planted with vegetation to support water retention	- collection of stormwater and delaying surface runoff - water purification (thanks to the process of filtration, adsorption and the action of microorganisms) - water infiltration into the ground - raising the groundwater level	[113,114,115,116,119,124]
vegetated swales	- linear, fairly shallow depressions − pots or surfaces − with sealed bottom, filled with fertile soil and densely planted with hydrophilic vegetation - occupy a small area	- collection of water and delaying runoff - discharge of stormwater into designated places - stormwater treatment - decorative functions - promoting biodiversity	[113,114,125]
(street-side) bioretention basins	- small linear or point depressions with underground drainage, located next to a pavement or road, planted with multi-species plants resistant to periodic flooding − water should be drained in 48 h from the end of rainfall	- retention of stormwater from impermeable surfaces - stormwater treatment - discharge of excess water into the sewage system - decorative functions - facilitating maintenance of street-side greenery	[113,114,126,127,133,134]
grassed retention and infiltration basins	- small and shallow grass-shaped depressions in the form of a basin, containing a drainage layer - a dry basin should be completely drained in 48 h from the end of rainfall and remain dry between storm events	- stormwater retention and infiltration - significant purification of stormwater by seeping through successive layers of the substrate; a process aided by vegetation	[113,114,115,116,119]
rain gardens	- small-sized depressions and/or containers planted with vegetation resistant to periodic flooding - do not require a large area	- collection of stormwater for re-use or for infiltration into soil - slowing down the flow and keeping stormwater in place - treatment of stormwater through the plant root system and/or gravel layers - decorative functions - promoting biodiversity	[108,113,114,115,116,128,129,133]
wetland ponds	- artificial reservoirs of various sizes, with a sealed bottom covered with vegetation adapted to the water and marsh environment − solutions similar to natural marsh systems	- stormwater retention - water purification due to microorganisms - landscape values	[113,114,119,121]
surface water reservoirs	- reservoirs of various sizes with sealed walls and bottom	- collection of stormwater from impermeable surfaces − slowing down surface runoff - distribution of water to other reservoirs (without infiltration into the ground) - possible re-use of water - possible development of aquatic vegetation	[113,114,115,116,117,118,119,121]
retention and infiltration water reservoirs	- open surface reservoirs of various sizes - with a bottom ensuring infiltration of water into the ground - planting with vegetation promotes water purification	- leveling and slowing down the outflow of stormwater - collection, treatment and infiltration of stormwater into the ground, as well as discharge of a small amount of stormwater to other receivers - stormwater treatment - ensuring the development of aquatic vegetation	[113,114,115,116,117,118,119,121,133]
water squares	- surfaces of various sizes with a recessed form and a profiled bottom enabling water drainage - they fill up with water during heavy rainfall	- stormwater retention−storage for a certain period of time, and then draining the excess to other receivers or sewage systems; water evaporation - provision of exceptional hydrologic performance in reducing the peak runoff - recreation place during dry periods - decorative functions	[113,114]
permeable/pervious pavements	- walking, driving or walking-driving flat surfaces with a low slope - made of various permeable materials: gravel, stones, grass, slabs with gaps, eco-grids filled with grass or gravel, porous asphalt or concrete poured on permeable substructures, etc.	- ensuring water seepage into the ground while slowing down the runoff of excess water to the sewage system - provision of exceptional hydrologic performance in reducing the peak runoff - communication functions (pedestrian, road traffic)	[113,114,115,116,119,130]
infiltration wells	- underground vertical devices with permeable bottom and walls−used for concentrated, point collection of stormwater - require no above-ground space	- stormwater retention - infiltration and facilitation of water infiltration into deeper layers of the soil - possible re-use of water for household purposes	[113,114,115,120]
infiltration boxes	- a compact system of openwork boxes with a light plastic structure, placed under the ground (mainly under impermeable surfaces) - they do not limit the space on ground for urban functions	- collection of stormwater from surface runoff - storage of excess stormwater - stormwater infiltration into the ground	[113,114,115]
structural tree root cells	- an underground system of connected cuboidal plastic structures, creating a scaffolding for trees in street-side plantings - a system filled with soil and substrate with optimal water-air properties for the development of tree roots, directing their development	- collection of water - infiltration of part of the stormwater into the ground and discharge of excess to the sewage system - enabling tree planting in compact building area and roots protection - preventing soil compaction and ensuring adequate retention capacity (system that transfers the loads of road communication) - preventing damage to the surface above and the underground infrastructure by tree roots	[113,114,115]
underground water reservoirs	- underground tanks of various sizes and shapes, made of various materials (plastic, concrete, prefabricated elements, etc.) - they can be part of the stormwater drainage network	- storage and temporary holding of stormwater to slow down surface runoff - distribution of stormwater to other places and/or re-use (watering urban green areas, cleaning roads and squares, etc.)	[113,114]
blue roofs	- solutions for the development of horizontal roof surfaces as water reservoirs, also with the participation of vegetation	- stormwater retention, usually temporary - stormwater re-use - visual qualities depending on the presence of water	[113,114,131]
green roofs	- a form of development of roof surfaces, ceiling covering consisting of several layers of substrate enabling vegetation - solutions of various sizes and intensity of plant participation (extensive or intensive roofs)	- stormwater retention, discharge of excess water to sewage or other receivers - stormwater treatment - stormwater re-use - recreational and decorative functions - increasing biologically vital areas and biodiversity - oxygen production - improving the functioning and reducing the operating costs of buildings - counteracting the phenomenon of UHI	[113,114,119,131,132,133,135,136]
green walls	- the form of developing vertical elements with vegetation (building facades, walls, fences, etc.), incl. vines or plants in containers, creating a complex system	- retaining a small amount of stormwater on the surface of plants - improving the functioning and reducing the operating costs of buildings - decorative functions - increasing biologically vital areas and biodiversity	[113,114,119,136]

.

The second stage of the study involved performing a comparison of 19 BGI solutions in order to identify similarities and differences among them. The assessment was carried out in relation to the following 3 aspects characterizing BGI: spatial and functional, environmental, and social. Based on data from the above-mentioned literature, these aspects have been further developed to identify key factors as well as the main criteria used to define the limitations and possibilities related to the implementation of water retention solutions in planning and designing urban public spaces.

The presented studies are quantitative and use a scoring method, assigning a specific number of points for each factor resulting from the characteristics of the relevant criteria. The authors assigned points by adapting methods used for assessing the valorization of ULOs, linking subjective and objective approaches [137,138]. A 2-, 3- and 4-level rating scale has been developed as follows:

• The 0–1 scale refers to the presence of the factor, where 0 means its absence, and 1 − its presence;
• The 1–2 scale refers to the degree of participation in activities, where 1 is low and 2 is high;
• The 0–2 scale refers to the intensity of the influence of a factor, where 0 means none, 1 − partial, and 2 − significant influence of the factor;
• The 0–3 scale results from a detailed valorization conditioned by the presence of a large number of features characterizing the factor, where 0 means none, 1−low degree, 2−medium degree, and 3−significant degree of occurrence or influence of the factor.

It was assumed that the more points a given BGI solution scores, the higher its potential for implementation in urban public spaces is. In the spatial and functional aspect, 9 factors characterizing BGI solutions were taken into account, in the environmental aspect 11 were taken into account, and in the social aspect 3 were taken into account. A detailed list of factors and their main criteria as well as the rating scale assigned to them is presented in

Table 3. Rating scale assigned to factors and their criteria characterizing BGI solutions (elaborated by authors).

Aspect	Factor	Criteria	Rating Scale
Functional and Spatial	Limitations on implementation/execution	Design documentation and building permit needed	high	low	-	-
			0 pts	1 pts	-	-
	Costs of implementation	Size, diversity of elements and complexity of a BGI solution	high	low	-	-
			0 pts	1 pts	-	-
	Maintenance costs	Size, diversity of elements and complexity of a BGI solution	high	low	-	-
			0 pts	1 pts	-	-
	Surface requirements	Size of space needed for the implementation of the BGI solution, including minimum dimensions: 10m2 for surface objects, 30 cm width for linear objects	> min. space size	≤ min. space size	-	-
			0 pts	1 pts	-	-
	Maintaining in good technical condition	Difficulty resulting from technical aspects	high difficulty	medium difficulty	low difficulty	-
			0 pts	1 pts	2 pts	-
	Preservation despite the passage of time	Susceptibility to destruction resulting from location and the use of the BGI solution, and external conditions	high difficulty	low difficulty	-	-
			0 pts	1 pts	-	-
	Facility rank	Size and scale of the BGI solution in spatial terms	-	local	supralocal	-
			-	1 pts	2 pts	-
	Additional functions	Other special functions related to the presence and use of the BGI solution in public spaces (e.g., social, aesthetic, recreational, sports, educational, scientific, etc.)	none	1–2 additional functions	3 or more additional functions	-
			0 pts	1 pts	2 pts	-
	Ability to combine with other BGI solutions	Possibility of connection resulting from spatial form and/or location of BGI solution	none	possible connection	-	-
			0 pts	1 pts	-	-
Environmental	Air temperature	Impact of the presence of plants and water	none	possible	-	-
			0 pts	1 pts	-	-
	Elimination of air pollution	Impact of the presence of plants	none	possible	-	-
			0 pts	1 pts	-	-
	Removal of pollutants from rainwater	Impact of plant functioning, chemical interactions and/or use of technical devices	none	possible	-	-
			0 pts	1 pts	-	-
	Fulfillment of ecosystem services	Number of ecosystem services (including categories: food, raw materials, water, climate, extreme events, wastewater treatment, pollination, biological control, habitats for species, recreation, tourism, aesthetic values, spiritual experiences, etc.)	1–3 (not related to environmental functions)	4–6	7–9	10 and more
			0 pts	1 pts	2 pts	3 pts
	Diversity of plant species	Number of plant species	no vegetation	1 plant species	2 plant species	3 and more plant species
			0 pts	1 pts	2 pts	3 pts
	Diversity of plant structures	Number of plant structures (e.g., single tree, group of trees/shrubs, perennial beds, vertical plants, etc.)	no vegetation	1 plant structure	2 plant structures	3 and more plant structures
			0 pts	1 pts	2 pts	3 pts
	Shaping biologically vital areas	Impact of the presence of greenery and permeable surface	none	low	significant	-
			0 pts	1 pts	2 pts	-
	Reduction of surface water runoff	Degree of slowing down water runoff	-	low	significant	-
			-	1 pts	2 pts	-
	Rainwater retention	Ability to collect and retain water	none	low	significant	-
			0 pts	1 pts	2 pts	-
	Stormwater infiltration into the ground	Implementation of permeable surfaces	none	low	significant	-
			0 pts	1 pts	2 pts	-
	Use of low-emission materials, recycling	Implementation of low-emission materials (e.g., concrete, cement) or recycled materials (e.g., metal, glass, aggregate)	none	possible	-	-
			0 pts	1 pts	-	-
Social	Recognition of visual values by the community	Number of points obtained from the visual assessment of BGI solutions using the SBE method (The study was carried out in 2020 on a test sample of N = 267 (M = 79; F = 188) on 16 BGI solutions visible in urban public spaces - except underground water reservoirs, infiltration boxes and structural tree root cells)	0 pts	1–3 pts	4–7 pts	8–10 pts
			0 pts	1 pts	2 pts	3 pts
	Possibility to foster social inclusion	Availability for social use	none	low	high	-
			0 pts	1 pts	2 pts	-
	Possibility to participate in the implementation and care	Limitations resulting from location, structure and technical requirements, and/or lack of plants	none	high	-	-
			0 pts	1 pts	-	-

.

The first level of assessment concerned the ranking of BGI solutions for each of the 3 aspects individually. The points obtained for each factor were added together to form a total overall score for each BGI solution, for a maximum of 12 points in the spatial and functional aspect, 20 points in the environmental aspect, and 6 points in the social aspect. The results allowed for the prioritization of individual BGI solutions according to the decreasing number of points and their classification into three groups: high (75–100% of the maximum score), medium (50–74% of the maximum score) and low (0–49% of the maximum score) values in their implementation in urban public spaces.

The second level of assessment included a comprehensive ranking of BGI solutions based on the sum of points obtained in the three listed aspects. Each BGI solution could score a maximum of 38 points. This allowed for the development of a final ranking of the solutions, using the decreasing number of points and re-classifying them into the following three groups (using the above-mentioned percentage scale): high (75–100% of the maximum score), medium (50–74% of the maximum score) or low (0–49% of the maximum score) potential for implementation in urban public spaces.

3. Results

3.1. The Spatial and Functional Aspect

The BGI solutions assessed for the spatial and functional aspect, with regard to their value in shaping urban public spaces, could score up to a maximum of 12 points for 9 different factors. The results are presented in

Table 4. Assessment of BGI solutions in terms of the spatial and functional aspect (elaborated by authors).

BGI Solution	Limitations on Implementation/Execution (0–1 pts)	Maintaining in Good Technical Condition (0–2 pts)	Preservation Despite the Passage of Time (0–1 pts)	Facility Rank (1–2 pts)	Additional Functions (0–2 pts)	Ability to Combine with other BGI Solutions (0–1 pts)	Costs of Implementation (0–1 pts)	Maintenance Costs (0–1 pts)	Surface Requirements (0–1 pts)	Sum (1–12 pts)
(street-side) bioretention basins	1	2	1	1	2	1	1	1	1	11
vegetated swales	1	2	1	1	2	1	1	1	1	11
grassed swales	1	2	1	1	2	1	1	1	0	10
permeable/pervious pavements	1	2	1	1	2	0	1	1	1	10
rain gardens	0	2	1	1	2	1	1	1	1	10
grassed retention and infiltration basins	1	2	1	1	2	1	0	1	1	10
infiltration trenches	1	2	1	1	2	1	1	1	0	10
green walls	1	1	1	1	2	1	1	1	1	10
wetland ponds	0	2	1	2	2	1	0	1	0	9
runoff troughs	0	2	1	1	1	1	1	1	1	9
retention and infiltration water reservoirs	0	1	1	2	2	1	0	1	0	8
surface water reservoirs	0	1	1	2	2	1	0	1	0	8
infiltration wells	0	1	1	1	1	1	1	1	1	8
structural tree root cells	0	2	1	1	1	1	0	1	1	8
water squares	0	1	1	2	2	1	0	0	0	7
infiltration boxes	0	1	1	1	1	1	0	1	1	7
green roofs	0	1	1	2	2	0	0	0	0	6
blue roofs	0	0	1	2	2	0	0	0	0	5
underground water reservoirs	0	1	1	1	1	1	0	0	0	5

and Figure 1. The division of points into groups is as follows:

• 10–12 points−high-value solutions;
• 6–9 points−medium-value solutions;
• 1–5 points–low-value solutions.

None of the assessed BGI solutions received the maximum (12) or minimum (1) number of points. With regard to shaping urban public spaces, the number of solutions that were classified into each of the three groups of values in the spatial and functional aspect, varies.

Only 2 BGI solutions (10.52%), blue roofs and underground water reservoirs, obtained the lowest number of points (5), falling into the low-value spatial and functional group. They were therefore classified as the least valuable in terms of shaping urban public spaces. Both were poorly rated due to the large space requirements, numerous limitations concerning their location (roof, under the ground), as well as high construction and maintenance costs. In the case of blue roofs, there are also certain constraints on combining them with other BGI solutions.

The following 8 BGI solutions (42.11%) obtained the highest number of points (10 and 11) in the spatial and functional aspect: (street-side) bioretention basins, vegetated swales, grassed swales, permeable/pervious pavements, rain gardens, grassed retention and infiltration basins, infiltration trenches and green walls. Most of these are linear or spot solutions that do not require significant space for implementation and can accompany various urban infrastructure solutions, i.e., roads, squares. They scored the maximum points for their low construction and maintenance costs, ease of maintenance after completion, and participation in additional functions beyond water retention.

The group with a medium spatial and functional value included 9 BGI solutions (47.37%): wetland ponds, runoff troughs, retention and infiltration water reservoirs, surface water reservoirs, infiltration wells, structural tree root cells, water squares, infiltration boxes, and green roofs. Most of these require a large area for implementation, generate high construction costs, and there are further factors that prevent their introduction into selected types of urban space. However, these limitations are compensated for when recognizing their significance in shaping the space, performing additional functions apart from stormwater retention, and relatively slow degradation over time, due to which these solutions obtained an average number of points (6 to 9).

3.2. The Environmental Aspect

The BGI solutions assessed for the environmental aspect of shaping urban public spaces could score up to a maximum of 20 points across 11 factors. The results are presented in

Table 5. Assessment of BGI solutions in terms of the environmental aspect (elaborated by authors).

BGI Solution	Air Temperature (0–1 pts)	Elimination of Air Pollution (0–1 pts)	Removal of Pollutants from Rainwater (0–1 pts)	Shaping Biologically Vital Areas (0–2 pts)	Fulfillment of Ecosystem Services (0–3 pts)	Diversity of Plant Species (0–3 pts)	Diversity of Plant Structures (0–3 pts)	Reduction of Surface Water Runoff (1–2 pts)	Rainwater Retention (0–2 pts)	Stormwater Infiltration into the Ground (0–2 pts)	Use of Low-Emission Materials, Recycling (0–1 pts)	Sum (1–20 pts)
rain gardens	1	1	1	1	3	3	1	2	2	2	1	18
vegetated swales	1	1	2	2	3	3	3	1	1	0	1	18
retention and infiltration water reservoirs	1	1	1	1	2	3	2	2	2	2	0	17
green roofs	1	1	1	1	3	3	3	1	2	0	1	17
(street-side) bioretention basins	1	1	2	2	2	3	1	2	1	1	0	16
wetland ponds	1	1	1	1	3	3	2	2	2	0	0	16
infiltration trenches	1	1	1	1	2	1	3	2	1	1	1	15
grassed swales	1	1	1	2	2	1	1	2	1	2	0	14
grassed retention and infiltration basins	1	1	1	1	2	1	1	2	2	2	0	14
permeable/pervious pavements	1	1	1	0	1	1	1	2	1	2	1	12
green walls	1	1	0	1	2	3	2	1	0	0	1	12
blue roofs	1	1	1	0	1	0	0	2	2	0	1	9
infiltration wells	0	0	1	0	1	0	0	2	2	2	1	9
structural tree root cells	0	0	0	0	1	3	1	1	1	1	1	9
water squares	0	0	0	0	2	0	0	2	2	0	1	7
surface water reservoirs	0	0	1	0	1	0	0	2	2	0	1	7
infiltration boxes	0	0	0	0	1	0	0	1	2	2	1	7
underground water reservoirs	0	0	0	0	1	0	0	2	2	0	1	6
runoff troughs	0	0	0	0	1	0	0	2	1	0	1	5

and Figure 2. The division of points into individual groups is as follows:

• 16–20 points–high-value solutions;
• 10–15 points−medium-value solutions;
• 1–9 points–low-value solutions.

None of the assessed BGI solutions received the maximum (20) or minimum (1) number of points. In the environmental context of shaping urban public spaces, the number of individual solutions classified into each of the three defined groups varies.

The following 6 (31.58%) BGI solutions are the most valuable in the environmental context: rain gardens, vegetated swales, retention and infiltration water reservoirs, green roofs, (street-side) bioretention basins, and wetland ponds, all of which scored a high number of points (16 to 18). Most of them are surface solutions. They obtained the maximum or average number of points in relation to the following factors: the fulfillment of many ecosystem services, a large diversity of structures and plant species, as well as a significant share in the retention of stormwater. Some, such as vegetated swales, green roofs and wetland ponds, did not receive any points in terms of infiltration of stormwater into the ground, resultant of their tight construction. BGI solutions such as retention and infiltration water reservoirs, (street-side) bioretention basins and wetland ponds did not receive any points due to the limited use or the inability to use low-emission or recycled materials in their construction. These limitations are compensated for by a large number of points obtained in terms of counteracting climate change or improving environmental conditions which results from a high share of vegetation supporting biodiversity.

Only 5 BGI solutions (26.31%) were assigned to the medium-value group in the environmental context of shaping urban public spaces, of which 4 are surface solutions (infiltration trenches, grassed swales, grassed retention and infiltration basins and permeable/pervious pavements) and 1 is an above-ground solution (green walls). They all scored between 12 and 15 points. They obtained an average or low number of points for most other factors.

In the environmental context of shaping urban public spaces the lowest-value group included the following 8 BGI solutions (42.11%): blue roofs, infiltration wells, structural tree root cells, water squares, surface water reservoirs, infiltration boxes, underground water reservoirs, and runoff troughs. They obtained the lowest number of points (5 to 9). Half of them are located underground. In most cases, the BGI solutions from this group did not receive points for factors such as the possibility of shaping biologically vital areas and the presence of diverse structures and species of plants, which at the same time prevents the improvement of biodiversity. Due to their small size or/and location below the ground, they also do not have a significant impact on the improvement of air quality and its temperature, and only some of them allow for the treatment of stormwater.

3.3. The Social Aspect

The BGI solutions assessed for the social aspect, in the context of their value for shaping urban public spaces, scored a maximum of 6 points in 3 factors. The results are presented in

Table 6. Assessment of BGI solutions in terms of the social aspect (elaborated by authors).

BGI Solution	Recognition of Visual Values by the Community (0–3 pts)	Possibility to Foster Social Inclusion (0–2 pts)	Possibility to Participate in the Implementation and Care (0–1 pts)	Sum (0–6 pts)
rain gardens	3	1	1	5
vegetated swales	3	1	1	5
retention and infiltration water reservoirs	3	1	1	5
green walls	3	1	1	5
permeable/pervious pavements	3	1	1	5
water squares	3	2	0	5
green roofs	3	2	0	5
runoff troughs	3	0	1	4
grassed retention and infiltration basins	2	1	1	4
infiltration trenches	3	0	1	4
(street-side) bioretention basins	2	0	1	3
grassed swales	2	0	1	3
blue roofs	2	1	0	3
surface water reservoirs	2	0	1	3
wetland ponds	2	1	0	3
infiltration wells	2	0	0	2
underground water reservoirs	0	0	0	0
structural tree root cells	0	0	0	0
infiltration boxes	0	0	0	0

and Figure 3. The division of points into individual groups is as follows:

• 5–6 points–high-value solutions;
• 3–4 points−medium-value solutions;
• 0–2 points–low-value solution.

None of the assessed BGI solutions obtained the maximum (6) number of points, while three of them obtained 0 points (a minimum). The share of individual solutions when classified into the three values groups varies in the social context of shaping urban public spaces.

The least-valuable group included the lowest number of BGI solutions, and all 4 of them (21.05%) are located underground. One of them—infiltration wells—was included in the assessment due to a structural element visible on the surface, and it obtained 2 points. The remaining three solutions in this group: underground water reservoirs, structural tree root cells, and infiltration boxes were not included in the assessment, scoring 0 points, as they were classified as invisible in the public space.

Further, the following 8 BGI solutions (42.11%) were assessed as having an average value in the social aspect (3 and 4 points): runoff troughs, grassed retention and filtration basins, infiltration trenches, (street-side) bioretention basins, grassed swales, blue roofs, surface water reservoirs and wetland ponds. Most of them are characterized by high or medium visual qualities, which predisposes them for use in urban public spaces. However, some of them obtained a low score in terms of the possibility of being used for social integration, and due to their limited availability and complicated construction process, and in the case of blue roofs a lack of social participation in the process of their implementation and care.

The second largest group was the high-value group, consisting of 7 BGI solutions (36.84%) that scored 5 points. These include the ones placed on the ground: rain gardens, vegetated swales, retention and infiltration water reservoirs, permeable/pervious pavements, water squares, and above-ground ones: green roofs and green walls. They obtained the highest number of points in terms of visual qualities. The remaining solutions, apart from water squares and green roofs, were rated medium or high in terms of supporting social integration, as well for the possibility of social participation in their implementation and care.

3.4. Valorization of BGI Solutions

The summary shows the comprehensive assessment of BGI solutions in relation to the total number of points obtained for all three aspects: spatial and functional, environmental, and social, as presented in

Table 7. Collective assessment of BGI solutions (elaborated by authors).

BGI Solution	The Spatial and Functional Aspect (1–12 pts)	The Environmental Aspect (1–20 pts)	The Social Aspect (0–6 pts)	Sum (2–38 pts)
vegetated swales	11	18	5	34
rain gardens	10	18	5	33
(street-side) bioretention basins	11	16	3	30
retention and infiltration water reservoirs	8	17	5	30
infiltration trenches	10	15	4	29
grassed retention and infiltration basins	10	14	4	28
wetland ponds	9	16	3	28
green roofs	6	17	5	28
green walls	10	12	5	27
grassed swales	10	14	3	27
permeable/pervious pavements	10	12	5	27
infiltration wells	8	9	2	19
water squares	7	7	5	19
runoff troughs	9	5	4	18
surface water reservoirs	8	7	3	18
blue roofs	5	9	3	17
structural tree root cells	8	9	0	17
infiltration boxes	7	7	0	14
underground water reservoirs	5	6	0	11

and Figure 4. The division of points into individual groups is as follows:

• 29–34 points–high-value solutions;
• 19–28 points−medium-value solutions;
• 2–18 points–low-value solutions.

The maximum number of points available in this study was 38. None of the analyzed solutions obtained the maximum (38) or the minimum (2) number of points.

The valorization results show that the following 5 (26.31%) BGI solutions were in the most-valuable group in their implementation in urban public spaces: vegetated swales, rain gardens, (street-side) bioretention basins, retention and infiltration water reservoirs and infiltration trenches, which scored between 29 and 34 points. The highest rated solutions include vegetated swales and rain gardens, which, despite or due to their small size, offer many possibilities for their use in various types of space, and additionally their functioning for water retention is associated with a significant share of vegetation. In most cases, BGI solutions from this group obtained a number of points in each of the three aspects, allowing them to be classified as those with the highest value in all aspects of shaping urban public spaces. In the case of classification them as medium-value for the selected aspect, they obtained high points, and even the maximum number of points at times.

The following 8 BGI solutions classified as medium value in the context of their implementation in urban public spaces, thereby comprising the largest group (42.11%): grassed retention and infiltration basins, wetland ponds, green roofs, green walls, grassed swales, permeable/pervious pavements, infiltration wells and water squares. Most of these BGI solutions obtained a high, or the maximum number of points assigned to the group in question (27 or 28), and also obtained, in one of the three aspects, the number of points that allowed them to be classified as the most valuable solutions. Against this background, however, two BGI solutions stand out, infiltration wells and water squares, which obtained the lowest number of points in this group (only 19).

The third group, consisting of the least valuable BGI solutions for shaping sustainable urban public spaces included 6 BGI solutions (31.58%). Half of these are underground solutions; structural tree root cells obtained a significant number of points (17), infiltration boxes received an average number of points (14), and underground water reservoirs received the lowest score among all analyzed BGI solutions (only 11 points). The blue roofs also obtained 17 points. The remaining 2 surface solutions, runoff troughs and surface water reservoirs, obtained 18 points each, which is the highest number of points obtained in this group. Half of the BGI solutions discussed, from this group, obtained a number of points that classify them as the least valuable for sustainable stormwater management in two aspects, the rest obtained a number of points that classify them as the least valuable in one aspect.

4. Discussion and Conclusions

This paper addresses the value that BGI solutions can bring to urban areas. The study contributes to a discussion on the need to implement certain approaches, based on sustainable stormwater management, at the level of spatial planning and especially in the context of designing specific places such as public spaces. The factors discussed may be helpful in understanding how significant a role BGI solutions can play, even those solutions that are small and inconspicuous. Therefore, paying attention to the spatial and functional, environmental and social aspects important for the shaping of urban public spaces is justified and is intended to broaden the knowledge about the values of individual BGI solutions. The conducted assessment confirms their potential for a multifaceted positive impact on the urban environment and improving its functioning.

Sustainable stormwater management in urban areas should be as comprehensive as possible, because in the face of growing threats and the intensification of negative weather phenomena, it is crucial to be aware of all available solutions and approaches [29,43,44,111,117,118] and to apply them as widely as possible in an appropriate context. The BGI solutions assessed as the most valuable in this study, of course, have the greatest impact on shaping urban public spaces focused on sustainable development, and at the same time allow for the provision of many ecosystem services. Such studies are important for the functioning of modern cities, for enhancing their attractiveness, and increasing the social awareness of the need to eliminate threats which are constantly growing in number. Their potential should be consciously integrated into processes both at the level of planning [21,110] and the level of designing urban spaces [95,111]. According to Matos Silva and Costa [139], the multi-directional implementation of BGI solutions in these processes supports the new emerging tendency of managing urban flood management in an intelligent way. It also improves the quality of urban design practices. The information presented in this paper can therefore contribute to supporting the contemporary planning and designing of public spaces.

It is also worth noting that the majority of BGI solutions assessed as having an average value for shaping sustainable public spaces obtained a high number of points. This proves that they also possess a considerable potential for positively influencing the urban space for the three aspects discussed. The variety of their individual features and their different possibilities in influencing individual aspects of the functioning of urban public spaces emphasizes their role as alternatives to those classified as the most valuable and comprehensive. Greater availability of solutions with a diversified, yet high degree of positive impact on the urban environment should be perceived as an advantage facilitating the complicated process of their selection for areas with numerous constraints.

Further, the lowest rated BGI solutions also have a number of valuable aspects and can therefore be used in shaping urban public spaces. This is the case for underground BGI solutions, the impacts of which are not as significant as other solutions assessed, but their location allows for the expansion of many urban functions on the surface. They should be considered when other options are exhausted. Even small scale solutions implemented comprehensively, and fully adapted to the limitations and possibilities of a space, may consequently contribute to the formation of a complex system permeating the city, increasing its effectiveness in managing stormwater. The results presented in this paper can therefore significantly support decision-making processes in the selection of specific BGI solutions and provide directions for their implementation in urban public spaces.

In terms of their diversity and the possibility of influencing a number of factors, the identified BGI solutions confirm that water retention in cities does not have to, or cannot be limited to classic large-scale engineering solutions that significantly interfere with the landscape and separate different areas from each other, while preventing residents from coming into contact with water [139]. Such activities result in the accumulation of negative social feelings related to the perception of water only as a threat, as reported by Echols [140]. Raising public awareness of the role and importance of BGI solutions, both in the case of city dwellers and representatives of authorities, may become a tool used for increasing their common acceptance and may accelerate the implementation of sustainable solutions [12,30,66,95,141,142,143]. The implementation of BGI solutions should be carried out with the participation of all interested parties, including the knowledge contributed through scientists and the experience of urban planners and designers. Community participation in decision-making processes on the direction of sustainable development of urban spaces, as well as its role in the acceptance of BGI solutions proposed by decision-makers, are also of key importance [20,30,102,103,105,106,107]. Without social agreement, and with a low level of information exchange, the functioning of modern cities and their spaces for people who use them will remain significantly limited [139].

In order to understand the value of BGI solutions as well as potential benefits resulting from their implementation in urban areas, it is crucial to understand their advantages in the context of the diversity of factors which characterize them. Urban public spaces, which function at the intersection of spatial, environmental, and social aspects, provide an excellent place for the implementation of the goals of sustainable stormwater management, and for obtaining tangible evidence of the positive impact of BGI solutions on cities and their inhabitants. The adaptation of urban areas to climate change is increasingly dependent on well-designed public spaces and their flexibility towards possibilities for implementing sustainable design solutions [139,144,145]. Research in this area should therefore be further developed.

In conclusion, the approach presented in this paper intends to highlight that BGI solutions require a comprehensive assessment to understand their limitations, and the possibilities following their implementation in urban public spaces. The presented study provides a comparison of 19 types of BGI solutions, based on various factors that make up their multi-threaded characteristics, which allowed for their ranking in terms of associated value. The use of an uncomplicated quantitative evaluation method, based on the knowledge of experts, may facilitate its public understanding, as indicated by Sowińska-Świerkosz et al. [112], and thus increase the popularity of BGI solutions [146,147]. The proposed ranking is intended as a method of educating policy makers about the possibilities of evaluating the choice between different BGI solutions. It can be used as a tool that covers many dimensions of sustainability. Furthermore, it can facilitate the decision-making processes through the selection of optimal BGI solutions for practitioners–urban planners and designers who directly influence the final shape of urban spaces. It may also help professionals to strengthen their role in the implementation of BGI solutions in urbanized areas in order to provide opportunities for improving water drainage. This study therefore makes an important contribution in both demonstrating the value of different BGI solutions and contributing to the knowledge related to the possibilities of a more conscious use of the listed solutions, supporting the creation of urban forms that benefit ecological processes within cities.