Blue–Green Infrastructure Effectiveness for Urban Stormwater Management: A Multi-Scale Residential Case Study

Abstract

Climate change, urbanization, and extreme weather events such as heavy rainfall and drought present major challenges for urban water management. This paper proposes a framework to evaluate the effectiveness of blue–green infrastructure (BGI) as a sustainable stormwater management solution across different residential development scales. Two contrasting case studies are examined: a small terraced housing catchment and a large housing estate. A multi-criteria analysis (MCA) supports a structured comparison of BGI effectiveness, while a complementary SWOT analysis informs strategic implementation approaches. The results demonstrate the practical applicability of the framework and underscore that successful stormwater management requires both innovative technologies and reform in urban planning governance. This study offers valuable insights into building climate-resilient cities.

1. Introduction

In the face of growing challenges related to climate change and progressing urbanization, cities are increasingly seeking sustainable solutions to support adaptation to extreme weather events and improve residents’ quality of life. One of the key approaches gaining importance in urban planning and management is blue–green infrastructure (BGI) [1]. It represents a set of nature-based solutions that combine water systems (blue) and vegetation (green), contributing to multifunctional land use and increasing cities’ resilience to the impacts of climate change [2,3].

In recent years, BGI has evolved from traditional forms such as parks, green spaces, and retention reservoirs toward more advanced and integrated solutions, including next-generation green roofs, urban wetland systems, air-purifying green walls, and infiltration–retention systems implemented in a decentralized layout [4,5,6]. Increasingly, blue–green elements are being integrated with smart city technologies—for example, through the use of sensors monitoring the soil moisture and water levels in real time [7]. In the face of growing urban challenges, there is a growing emphasis on designing not just sustainable but regenerative cities. According to Girardet [8], the future of urbanization lies in cities where humans interact with nature to restore local ecosystems. Other scientists add that sustainability alone is insufficient—what is needed is active environmental regeneration [9].

Regenerative cities are based on holistic, long-term thinking, promoting the co-evolution of human and natural systems and the renewal of natural resources. However, the accelerating climate crisis demands rapid adaptation measures, creating tension between quick results and long-term planning [10]. Public sector implementation faces additional barriers—legal, financial, and organizational—especially in densely built areas with complex ownership structures [11]. In this context, blue–green infrastructure (BGI) offers potential solutions, though in practice its implementation is often fragmented and narrowly focused. Researchers stress the need for more strategic and integrated BGI planning [12,13].

Current research indicates that properly designed BGI not only mitigates the risk of urban flooding and improves the quality of stormwater but also supports the cooling of urban microclimates, reduces the urban heat island effect, and lowers building energy costs [9]. Almaaitah et al. emphasize that blue–green infrastructure has significant potential to reduce surface and air temperatures, to contribute to the cooling effect [14]. Studies show [15,16] that green roofs can be up to 4 °C cooler than traditional roofs, confirming the effectiveness of such solutions in mitigating urban heat islands [10]. Moreover, research by Przestrzelska et al. [16] highlights that BGI can reduce temperatures in cities by up to 2 °C, which in turn lowers the demand for energy used to cool buildings. Analyses revealed that implementing blue and green infrastructure can result in energy savings of 8.12% and 4.78%, respectively, while their combination (BGI) leads to savings of around 6.73% [16].

These studies confirm that implementing blue–green infrastructure in urban environments not only mitigates the urban heat island effect but also leads to significant energy savings in buildings. Additionally, the development of BGI supports urban biodiversity conservation, enhances ecosystem services, and creates resident-friendly public spaces [1,2,17]. Elements of blue–green infrastructure include the following, among others: retention ponds, bioretention basins and swales, infiltration trenches, containerized rain gardens, green bus stops, green roofs, green façades and walls, permeable surfaces, and structural soils [1].

Table 1. Blue–Green Infrastructure Solutions: Types, benefits, limitations.

Solution Name	Short Description	Benefits	Limitations
Retention Ponds	Artificial reservoirs collecting excess rainwater	Flood risk reduction, microclimate improvement, biodiversity support, and pollutant filtration	High construction and maintenance costs, requires large spaces
Bioretention Basins	Depressions allowing infiltration and water retention	Pollutant filtration, increased soil moisture, flood risk reduction, microclimate improvement, biodiversity support	Possible silting, maintenance required
Bioretention Swales	Trenches collecting and filtering stormwater	Effectively reduce stormwater runoff volume and improve water quality by filtering pollutants through vegetation and soil	Requires sufficient space, which can be a limitation in dense urban areas. Effectiveness depends on proper design, soil type, and maintenance
Infiltration Trenches	Depressions filled with permeable material for infiltration	Reduces surface runoff, supports groundwater recharge, biodiversity support	Limited efficiency in clay soils, needs maintenance
Rain Gardens in Containers	Containers with plants capturing and filtering rainwater	Easy to implement in cities, aesthetic improvement, biodiversity support	Requires maintenance, limited capacity
Green Bus Stops	Bus shelters covered with vegetation	Reduces urban heat island effect, aesthetic enhancement	Maintenance costs, technical requirements
Green Roofs	Roofs covered with vegetation layers	Thermal insulation, water storage, reduces urban heat island effect, aesthetic enhancement	High installation costs, structural load concerns
Green Facades and Walls	Building walls covered with vegetation	Air purification, improved insulation, reduces urban heat island effect, aesthetic enhancement	Requires irrigation, maintenance costs
Permeable Pavements	Materials allowing water infiltration into the ground	Reduce surface runoff, improves soil absorption, groundwater recharge, aesthetic enhancement	Higher cost than traditional surfaces
Structural Soils	Special substrates allowing tree growth in cities	Better conditions for urban greenery, enhancement of urban vegetation	Requires appropriate material selection, maintenance

presents the main advantages and disadvantages of the most popular blue–green infrastructure solutions for cities [2,3,4,7,18,19,20].

Recent studies have highlighted both the benefits and limitations of implementing blue–green infrastructure (BGI) in urban environments. While BGI offers many ecosystem services and contributes to improving urban resilience, it also poses some practical challenges.

Table 2. Benefits and limitations of blue–green infrastructure in urban areas [27,28,29,30].

Benefits	Limitations
Flood risk reduction: BGI elements like retention ponds and rain gardens effectively manage stormwater, reducing urban flood risks.	High initial investment costs: Implementing BGI can require significant upfront financial resources.
Improved air quality: Green roofs and facades absorb air pollutants, leading to cleaner air in urban environments.	Ongoing maintenance requirements: BGI elements need regular care and monitoring, increasing operational costs.
Urban heat island mitigation: Vegetation cools urban environments, mitigating the urban heat island effect.	Limited space availability: Densely built-up urban areas may lack the space for larger BGI installations.
Increased biodiversity: BGI creates habitats for various plant and animal species, supporting urban biodiversity.	Seasonal performance variations: The effectiveness of certain BGI elements depends on seasonal and climatic conditions.
Enhanced urban aesthetics: Features such as green walls and rain gardens improve the visual appeal of urban areas, benefiting residents’ well-being.	Need for public awareness and education: Successful BGI implementation requires informing and engaging local communities and decision-makers.

summarizes the key benefits and limitations of selected BGI solutions based on the recent literature. Key benefits include stormwater retention and flood mitigation, improved air quality, a reduction in urban heat islands, and the promotion of biodiversity. In addition, BGI can significantly improve urban aesthetics, positively impacting psychological well-being and social cohesion. However, these systems often require significant upfront investment, as well as ongoing maintenance and monitoring. Space constraints in densely urbanized areas, seasonal variations in productivity, and the need for public education and community engagement remain critical constraints to widespread adoption. Addressing these barriers is essential for the successful integration of BGI into sustainable urban planning strategies [21,22,23,24]. Additionally, a key factor in selecting the appropriate practice lies in understanding the site-specific conditions [25,26].

Recent studies underscore the need for context-sensitive and scalable BGI solutions, particularly in the face of accelerating urban growth and intensifying climate extremes [24,31]. Moreover, comparative assessments across different urban scales remain limited in the literature, highlighting a research gap that this article seeks to address [22,32].

The aim of this article is to present the latest trends and applications of blue–green infrastructure (BGI) in urban environments, using case studies as valuable sources of knowledge for understanding practical implementation, identifying context-specific challenges, and deriving transferable lessons for future planning and policy development. Special attention is given to the comparative analysis of BGI implementation in two urban catchments of different scales: a large, central urban catchment and a smaller, residential sub-catchment. This comparison provides a basis for evaluating the effectiveness of BGI in varying urban contexts and identifying optimal strategies tailored to both types of areas.

By applying multi-criteria analysis (MCA) alongside SWOT assessments, this research evaluates alternative BGI strategies to enhance urban resilience. The aim is to contribute to the advancement of climate-resilient urban development by providing practical insights for decision-makers and planners.

2. Materials and Methods

This article presents a comparative analysis of stormwater management in the context of sustainable development for two selected urban catchments. The analysis of BGI includes case studies describing the application of such technologies in different urban settings, allowing for an assessment of their effectiveness and adaptability to local environmental conditions. Two different urban catchment areas were selected to review and compare different BGI strategies. The first case study focused on a residential estate in Wrocław, covering a catchment area of 24.488 m². The second catchment, with a surface area of 239.82 m², comprised the roof area of four terraced single-family houses located near Niemodlin, Poland. A multi-criteria analysis (MCA) was carried out for each site, complemented by the SWOT method to identify the strengths and weaknesses associated with the implementation of each solution.

2.1. Case Study I

The analyzed residential complex consists of two mixed-use buildings containing a total of 48 residential units and 8 commercial premises, as well as 17 multi-family residential buildings, each comprising 6 individual apartments. To manage stormwater runoff from the development, the following solutions were implemented:

• Rainwater from the rooftops is directed via downpipes into stormwater drainage manholes. Water collected from the buildings and inlets is conveyed through PVC pipelines and non-access polypropylene inspection chambers (DN425 PP). The main junctions are equipped with PP pipelines and concrete manholes (DN1000 and DN1200), while retention channels feature large concrete chambers (DN2000).
• Runoff from hardened surfaces, such as roads, sidewalks, and parking areas, is collected and conveyed via PVC lateral drains and street inlets with 0.5-meter sediment traps. A stormwater pumping station and an oil-pollutant lamella separator are used before discharging into the so-called “dirty” stormwater drainage system.
• Rain gardens were implemented as a measure to delay surface runoff [33].

Due to the existing soil and hydrogeological conditions, it was not possible to fully manage all stormwater on site. Therefore, additional retention support measures were implemented along the road adjacent to the investor’s property. After analyzing various factors, the following supplementary solutions were introduced:

• Installation of retention crates and a flow regulator to reduce the volume of water entering the main receiving channel.
• Road profile shaping to direct runoff toward a green area on the opposite side of the street.
• Construction of a new stormwater drainage channel.
• Installation of rain gardens, infiltration swales, French drains, and retention crates in locations where the stormwater network is too shallow. The goal of this system is to collect runoff via rain gardens and convey it through drainage pipes and retention crates toward the outflow point.
• Installation of a stormwater pumping station with a maximum flow capacity of 2.0 L/s and a discharge limit of 10 L/s.
• Installation of DN1500 concrete manholes and stormwater inlets.

The following formula is used to calculate the volume of stormwater runoff:

$$Q=\Psi ·A·q,$$

(1)

where

A—drainage area (m²);

q—design rainfall intensity (L/s·ha);

Ψ—runoff coefficient, which varies depending on the type of surface.

Surface Runoff Coefficient Ψ

The coefficient Ψ characterizes the catchment area in terms of its surface characteristics and shape, and expresses the ratio of the amount of water running off the surface of the catchment area (Qₛₚ) to the total precipitation volume (Qₒₚ) [9]:

$$\Psi ={\displaystyle \frac{Q_{sp}}{Q_{op}}},$$

(2)

where

Qₛₚ—amount of water running off the surface of the catchment area, dm³/s;

Qₒₚ—total precipitation volume, dm³/s.

The surface runoff coefficient is most commonly selected based on tables that specify surface sealing (imperviousness) and land slope. The selected surface runoff coefficient is Ψ = 0.95. The rainfall intensity, expressed in units of L/(s·ha), represents the relationship between the frequency of rainfall occurrence and its duration, as defined by the Błaszczyk formula [34]:

$$q={\displaystyle \frac{6631·\sqrt[3]{H^2·c}}{{Td}^{2/3}}},$$

(3)

where

H—average annual rainfall height (taken from meteorological data for a given region), mm;

c—frequency of rainfall occurrence, once every c years;

T_d—rainfall duration, minutes.

The average annual rainfall height H for Wrocław is ≥ 720 mm. The rainfall occurrence frequency c is a dimensionless value expressing the ratio of the probability of rainfall with a minimum number of occurrences (once every c years) as a percentage, to a 100-year period expressed as 100% (according to sheet ATV-A138—dimensioning of rainwater infiltration systems) [35]. This means, for example, that for a rainfall probability of p = 20%, the value c = 5—which indicates a likelihood of occurrence once every 5 years—resulting from the following relationship:

$$c={\displaystyle \frac{p}{100\%}},$$

(4)

where

p—probability of rainfall occurrence, %.

The value c = 5 was adopted.

Table 3. Summary of the catchment area and maximum 15-minute retention capacity—own elaboration based on [36].

Surface Type	Area [m2]	Runoff Coefficient	Reduced Area [ha]	Calculated Flow Q [L/s]	Flow Qp. [L/s]
Total Built-up Area	5318	0.96	0.51	71.88	25.16
Geogrid Parking	250	0.25	0.01	0.88	0.31
Perforated Paving Parking	638	0.3	0.02	2.69	0.94
Solid Paving Parking	331	0.72	0.02	3.36	1.17
Solid Paving Sidewalks	1289	0.72	0.09	13.07	4.57
Internal Roads (Solid Paving)	2242	0.72	0.16	22.73	7.95
Green Roofs (Parking)	1282	0.25	0.03	4.51	1.58
TOTAL	11,350			119.11	41.69
Solid Paving Sidewalks	300	0.72	0.02	3.04	1.06
Solid Paving Road	488	0.72	0.04	4.95	1.73
TOTAL	788	-	-	7.99	2.8
GRAND TOTAL	12,138	-	-	127.1	44.49

presents the input data and the results of these calculations. For the calculations in Table 3, a rainfall intensity of q = 140 L/s·ha was used (The value of q was calculated using the Błaszczyk formula for the given precipitation height (H). Rainfall duration of heavy rainfall: T_d = 15 min; duration of post-precipitation runoff: t = 15 min.

Since each type of surface occupies a different area and is characterized by a different runoff coefficient, separate calculations had to be performed for each one.

Taking into account Formulas (1) and (3), as well as the presented coefficients for different surfaces, the total stormwater flow was calculated as Q = 77 L/s.

All the above calculations, along with the tank selection calculator based on the DWA-A-177 guidelines, enable the proper selection of a retention tank [37,38]. After entering the required data, the software generated two tank systems that meet the established conditions for two different rainfall frequency values, as presented in

Table 4. Tank selection based on rainfall frequency—own elaboration based on [39].

Rainfall Frequency (C)	Required Tank Volume [m3]	Discharge Time [h, min]
5	91.8	2 h 33 min
10	116	3 h 13 min

.

A DN700 stormwater pipe with a length of 134 m has a capacity of V₁ = 51.5 m³, while a series of retention crates with a length of 115.2 m has a capacity of V₂ = 39.6 m³, which together provide a total retention volume of V = 91.9 m³. Taking into account the additional retention capacity of the French drain, the designed system is sufficient to accommodate the assumed rainfall intensities [7].

The pipelines responsible for stormwater retention were made of PP SN8 pipes with diameters DN200 and DN700, connected using lip seals and socket joints, and laid on a 15 cm gravel-sand bedding. Three concrete manholes with a diameter of DN1500 were installed along the pipeline, placed on a 20 cm sand bedding.

An important element of the system is the stormwater inlets, responsible for collecting runoff from the roadway. These inlets are connected to the main pipeline via lateral pipes also made of PP SN8 DN160 pipes. Each inlet is equipped with sediment buckets, which serve to trap solid contaminants and prevent them from entering the main collector.

2.2. Case Study II

The catchment area was 239.82 m² (which corresponds to the roof area of a row of four single-family houses) and located at an elevation of approximately 167.10 m above sea level. The groundwater table, with a free water surface, was located at a depth of about 2.5–2.6 m below the ground surface, occurring in medium-grained, moderately compacted sands with a filtration coefficient of k_f = 10⁻⁴ [40]. The average annual rainfall for Niemodlin is approximately 600 mm (in 2021, it was 596.2 mm) [41]. Calculation of the rainfall intensity was also based on Equation (3).

The catchment area was divided into four parts, each corresponding to a separate catchment area (based on the division of the four houses). The same type of device was selected for each part.

The individual catchment areas had the following surface areas:

• Segment 1: F₁ = 60.90 m² = 0.006090 ha;
• Segment 2: F₂ = 59.01 m² = 0.005901 ha;
• Segment 3: F₃ = 59.01 m² = 0.005901 ha;
• Segment 4: F₄ = 60.90 m² = 0.006090 ha.

Since the areas of F1 = F4 and F2 = F3, meaning they are the same, F1 and F2 are used for the calculations.

2.2.1. Selection of Devices

The selected devices are soakaway wells (infiltration wells) in the shape of a cylinder with the following internal dimensions:

• H_s—well height: 1.40 m;
• H_d—depth of underground inflow: 0.6 m;
• h_c—height from the bottom to the inflow level: 0.8 m;
• d_c—bottom diameter: 0.6 m.

Each segment was assigned one soakaway well (SC) with underground inflow.

The soakaway wells were constructed identically:

SC1 = SC2 = SC3 = SC4 with V = 0.45 m³

The calculated volume V is smaller than the volume of rainwater runoff from the catchment area V_d because V_d should be reduced by the volume of water that infiltrates the ground during rainfall event T_d. Infiltration—understood as the outflow rate of water from the soakaway well—is calculated using Maag’s method, which assumes infiltration into the ground occurs only through the bottom of the well. This is expressed by the following formula [42]:

$$Q_f=2·\pi ·d_c·h_c·k_f,$$

(5)

where

Q_f—water outflow rate from the soakaway well through infiltration, m³/s;

d_c—diameter of the soakaway well, m;

h_c—water height (filling level) in the soakaway well, m;

k_f—soil filtration coefficient, m/s.

The water outflow rate from the soakaway well as infiltration was calculated using Maag’s method, as outlined below:

$$Q_f=2·3.14·0.6·0.8·{10}^{-7}=0.00030 {\displaystyle \frac{{\mathrm{m}}^3}{\mathrm{s}}},$$

(6)

For a rainfall duration of 15 min, the water outflow rate Q_f = 0.0003 m³/s. It represents approximately 40% of the inflow rate Q in Segment 1 and just over 40% in Segment 2.

2.2.2. Required Volume of the Soakaway Well (V_w) According to Rainfall Duration

The required volume of the soakaway well is directly dependent on the duration of the rainfall, expressed by the following Equation (3) according to [40]:

$$V_w=\left(Q-Q_f\right)·T_d·60,$$

(7)

where

V_w—required volume for the given rainfall duration T_d, in m³;

Q—inflow rate, in m³/s;

Q_f—outflow rate, in m³/s;

T_d—rainfall duration, in minutes.

The required volume for a rainfall duration of T_d = 15 min for Segment 1:

$$V_w=\left(0.00076-0.00030\right)·15·60=0.410 {\mathrm{m}}^3,$$

(8)

The required volume for a rainfall duration of T_d = 15 min for Segment 2:

$$V_w=\left(0.00074-0.00030\right)·15·60=0.398{ \mathrm{m}}^3,$$

(9)

The verification of the required volume of the soakaway wells was conducted for various rainfall durations—T_d = 5, 10, 20, 25, 30, 35, 40, 45, 50, and 55 min—resulting in V_wmax for T_d = 55 min (Figure 1) [34,41].

By analyzing the above results and the obtained curves, the highest required soakaway well volume was determined as V_w = 0.422 m³ for the catchment area with surface F₁, and V_w = 0.404 m³ for the catchment area with surface F₂.

As can be observed in both cases, this value occurs at a rainfall duration of T_d = 10 min.

Therefore, T_d = 10 min should be adopted as the design rainfall duration. To verify the condition below, the highest required volume obtained for the catchment area with surface F₁, where V_w = 0.422 m³, was used. The condition is fulfilled as 0.450 m³ exceeds 0.422 m³.

3. Results

Many studies focus on the hydrological aspects of BGI solutions, which are crucial for determining the size of facilities and their performance. However, a comprehensive assessment of the effectiveness of these systems should also consider cost, environmental, and social factors. This allows us to evaluate the impact of the solutions on quality of life, climate change adaptation, and long-term infrastructure maintenance costs. Such a multi-criteria approach makes it possible to assess the effectiveness and sustainability of implemented BGI strategies in the context of sustainable urban development.

3.1. Catchment Area A—Residential Estate

As part of the project, comprehensive technical solutions were applied, covering both the main components and the necessary auxiliary equipment to ensure effective stormwater management in the urban area.

The primary stormwater drainage system was constructed using polypropylene (PP) pipelines and manholes with diameters ranging from DN1000 to DN2000, enabling the efficient transport of runoff from paved surfaces.

To treat the collected stormwater, street inlets equipped with sediment traps and hydrocarbon separators were installed, allowing for a reduction in pollutants before further discharge.

In situations of increased rainfall intensity, the system’s operation is supported by a stormwater pumping station, which allows for controlled redirection of excess water to selected retention elements.

An integral part of the system consists of retention crates with a flow regulator, serving for the temporary storage of stormwater and its controlled release.

The system is complemented by blue–green infrastructure elements, such as rain gardens, infiltration swales, and French drains, which enhance water infiltration into the ground, improve the local water balance, and support natural purification processes.

3.2. Catchment Area B—Single-Family Housing

As part of the designed drainage system, each of the four segments is equipped with soakaway wells with a diameter of 0.6 m and a height of 1.4 m. The inflow point is set at a depth of 0.6 m, while the operational water level in the wells is maintained at 0.8 m.

Each soakaway well provides a storage capacity of 0.45 m³, offering adequate buffering for localized runoff. For the hydraulic calculations, a rainfall intensity of 132.62 dm³/(s·ha) was adopted. Based on this value, the required retention volume for a 10-minute rainfall event was estimated to be 0.410 m³ for Segment 1 and 0.398 m³ for Segment 2.

The capacity of the designed soakaway wells exceeds these requirements, confirming the effectiveness of the adopted solutions. This ensures the efficient operation of the system during periods of heavy rainfall, reducing the risk of local flooding and supporting natural infiltration processes.

Focusing on the need for evaluating the solutions, a multi-criteria analysis (MCA) was applied. In the first stage, various technical solutions suitable for residential catchment areas were analyzed (

Table 5. Blue–Green Infrastructure Solutions: Types evaluation [16,24,31,43,44].

Criterion	Retention Tanks	Green Roofs	Rain Gardens	Permeable Pavements
Investment cost (EUR/m2)	50–150 EUR/m2	100–250 EUR/m2	30–100 EUR/m2	40–120 EUR/m2
Life expectancy	30–50 years	30–40 years	10–20 years	20–30 years
Retention efficiency (%)	60–90%	40–80%	50–85%	30–70%
Space requirements	Large (depends on capacity)	Integrated with building structure	Small/medium	Integrated with road infrastructure
Additional benefits	Flood protection, water reuse	Building insulation, improved microclimate	Aesthetic value, supports biodiversity	Runoff reduction, enhanced infiltration

).

Subsequently, based on the criteria provided, the BGI solutions were evaluated for their suitability to the catchment type (

Table 6. Evaluation criteria with scores and assigned weights.

Criterion	Description	Weight [%]	Green Roofs	Retention Tanks	Rain Gardens	Soakaway Pits
R—Retention Efficiency	System’s capacity to retain stormwater	30	2	5	3	4
K—Implementation and Maintenance Costs	Total investment and operational cost	15	4	2	5	3
E—Environmental Impact and Durability	Ecological benefits, e.g., microclimate, biodiversity, durability	20	5	4	3	2
S—Scalability and Space Demand	Adaptability of the system to other areas considering spatial demand	20	5	3	4	2
Z—Sustainability Compliance	Consideration of social, environmental, and economic aspects	15	4	2	5	3

). To evaluate each solution, a rating scale from 1 to 5 was adopted. The criterion values are linearly transformed, as described in [45]. Determination of weights for each criterion was established based on the literature, expert opinions, and local priorities.

Normalization was applied to ensure that all evaluation scores are presented on a uniform scale from 0 to 1. This allows for direct comparison between variants, regardless of the original units or value ranges. Rain gardens received the highest overall score, offering a good balance between effectiveness and cost. The final results are presented in Figure 2.

Rain gardens represent a cost-effective and adaptable solution applicable across diverse residential typologies, not exclusively within dense urban environments. In conjunction with green roofs—known for their capacity to enhance localized stormwater retention and contribute to microclimatic regulation—these nature-based interventions offer multifunctional benefits that extend beyond hydrological performance. As emphasized in [46], nature-based solutions (NBS) are increasingly recognized as integral to urban climate adaptation strategies, simultaneously mitigating flood risk and enhancing urban livability. Furthermore, the concept of urban rewilding, as explored by Russo et al. (2025), introduces the restoration of ecological processes within urban fabrics as a means to increase resilience, support biodiversity, and improve natural infiltration and retention capacities [47]. Empirical insights from Swedish municipalities also illustrate that the successful implementation of green infrastructure necessitates institutional collaboration between public authorities and private landowners, with clearly delineated responsibilities and sustainable financing models [48]. These findings are consistent with earlier research [48,49], reinforcing the necessity of mainstreaming decentralized, nature-based stormwater management practices across various residential development contexts as a response to increasing climate-related hydrometeorological challenges.

Using the evaluation of individual solutions, a comprehensive analysis of Catchment A (

Table 7. Catchment A—residential area.

Criterion	Rating (1–5)	Justification
R	5	High retention (approx. 70% of annual rainfall due to retention tanks and rain gardens)
K	3	Moderate investment and operational costs; cleaning and maintenance required
E	4	Green infrastructure supports microclimate and biodiversity, reduces urban heat island effect
S	4	Solutions applicable in other multi-family housing areas
Z	5	Comprehensive approach combining ecological, social, and economic aspects

) and Catchment B (

Table 8. Catchment B—row housing development.

Criterion	Rating (1–5)	Justification
R	2	Low retention, lack of infrastructure—surface runoff almost without retention
K	5	Practically no investment and operational costs
E	2	No environmental effects—minimal biologically active area
S	3	No system in place—but easy installation of tanks, e.g., above ground
Z	2	Minimal involvement in sustainable development ideas

) was performed.

Table 9. MCA (multi-criteria analysis) evaluation for the applied solutions with normalization.

Criterion	Weight	Catchment A	Catchment B
R	30%	0.5	0.6
K	15%	0.45	0.75
E	20%	0.8	0.4
S	15%	0.6	0.45
Z	20%	0.9	0.4
Total	100%	0.64	0.52

presents the MCA evaluation for the applied solutions with normalization.

Catchment A shows a clear advantage in terms of retention efficiency, ecological impact, and compliance with the principles of sustainable development. The use of solutions such as rain gardens and underground retention tanks allows for a significant reduction in surface runoff and local water retention. Catchment B, despite low maintenance costs, is characterized by minimal retention capacity. The lateral overflow results in direct runoff, which can cause overloading of the storm sewer system or local flooding [50].

The MCA analysis showed that comprehensive blue–green solutions used in urban conditions (Catchment A) are more effective in the long term, despite higher initial costs. There is potential for the adaptation of some elements of Catchment A on smaller scales, e.g., installation of above-ground rainwater tanks, green roofs, or seepage boxes in single-family housing (Catchment B).

Catchment A scored 0.64 in the MCA analysis—this confirms high efficiency and compliance with the principles of sustainable development. Blue–green solutions are more expensive but bring measurable ecological and social benefits.

Catchment B scored 0.52—despite low costs, the lack of full retention infrastructure limits its efficiency. Introducing even simple solutions (rainwater barrels and seepage boxes) could significantly improve the result.

The use of multi-criteria analysis allows not only for quantitative assessment (retention) but also qualitative—taking into account the impact on the environment and adaptation possibilities.

To complement the multi-criteria assessment (MCA), separate SWOT analyses were conducted for both case study catchments—Case Study I (residential estate) and Case Study II (single-family housing). These evaluations provide a structured reflection on the internal strengths and weaknesses of each BGI implementation, as well as external opportunities and threats affecting long-term success. Case Study I demonstrates high performance in terms of retention efficiency and sustainability alignment but requires greater investment and maintenance (

Table 10. SWOT analysis—Case Study I (residential estate).

Strengths	Weaknesses
✓ High retention efficiency—approx. 70% of annual rainfall stored. ✓ Comprehensive BGI system: retention tanks, rain gardens, infiltration swales. ✓ Improved stormwater quality and pollutant removal. ✓ Enhanced microclimate and reduced urban heat island effect. ✓ Alignment with sustainable development goals.	✓ High initial investment costs (underground infrastructure, separators, pump stations). ✓ Ongoing maintenance requirements (cleaning, inspections). ✓ Complex technical coordination between system components.
Opportunities	Threats
✓ Availability of EU and national funding. ✓ Replication potential in other urban districts. ✓ Integration with climate adaptation and smart city policies. ✓ Potential for environmental education and public awareness.	✓ Legal and administrative barriers to project permitting. ✓ Delays or lack of funds for maintenance. ✓ Risk of overload during extreme rainfall events. ✓ Low public acceptance if implementation fails.

). In contrast, Case Study II offers low-cost, decentralized solutions that are easy to adopt but lack broader environmental and infrastructural impact (

Table 11. SWOT analysis—Case Study II (single-family housing).

Strengths	Weaknesses
✓ Low implementation costs (simple systems: soakaways, rain barrels). ✓ Easy adoption by individual property owners. ✓ Quick deployment and decentralized management. ✓ Potential for resident engagement in stormwater practices.	✓ Limited retention capacity (no collective infrastructure). ✓ No measurable impact on microclimate or biodiversity. ✓ Poor integration with broader urban infrastructure. ✓ Minimal contribution to sustainability objectives.
Opportunities	Threats
✓ Possibility to install additional features (green roofs, infiltration boxes). ✓ Educational campaigns can boost community interest. ✓ Rapid effectiveness improvements with small investments.	✓ Lack of motivation among residents for individual investments. ✓ Inefficiency during heavy rainfall events. ✓ Risk of improper installation or neglected maintenance. ✓ No coordination may lead to drainage overload.

). These findings reinforce the need for scale-appropriate, context-specific strategies that balance ecological, economic, and social objectives in urban stormwater management [44]

The SWOT analysis reveals that blue–green infrastructure can bring a number of benefits in terms of climate change adaptation and improving the quality of life of residents. However, its effectiveness depends on proper implementation, maintenance, and public acceptance [44,51]. The implementation of BGI also provides an opportunity to raise external funds and develop environmental education, although it may be limited by institutional and financial barriers [51,52].

4. Discussion

As numerous studies have demonstrated, indicators employed to optimize sustainable green–grey infrastructure and assess its stormwater management performance are often based on predefined criteria [53,54,55,56,57]. For instance, Tameh et al. [56] introduced a comprehensive environmental sustainability indicator integrating reliability, resilience, vulnerability, and hydrological sustainability to optimize green–grey infrastructure layouts. Similarly, Hosseinzadeh et al. [57] applied a multi-criteria decision analysis (MCDA) framework to identify optimal detention pond configurations in urban drainage systems. In this study, rather than proposing a new indicator, the authors applied MCA to evaluate BGI solutions, facilitating a holistic assessment of various performance criteria [52].

The results of this study reinforce the growing consensus that integrated, multi-scale BGI systems offer superior long-term resilience, especially in dense urban complexes. Studies by Pietrzyk et al. (2017) and Oorschot et al. (2017) show that combining gray and green infrastructure—particularly rain gardens, permeable pavements, and retention tanks—yields synergistic effects on flood mitigation, urban cooling, and pollutant reduction [58,59]. Our findings from Case Study I support this view, confirming that while such systems involve higher costs, they deliver a more robust and multifunctional performance.

Case Study II demonstrates a typical decentralized approach to stormwater management, often promoted for suburban or low-density areas. Although systems like soakaway pits are effective on a small scale, they lack the cumulative benefits observed in collective infrastructure. This is consistent with Janiszek et al. (2023) [1], who found that decentralized systems often underperform when implemented in isolation without systemic urban hydrology coordination.

Importantly, the MCA results highlight how retention efficiency alone is insufficient to determine overall system value. Environmental co-benefits—such as biodiversity promotion, microclimate regulation, and social amenity creation—are crucial to the success of BGI, as stressed by Hodson and Sander (2017), who call for broader sustainability metrics in infrastructure planning [60]. Our results also mirror the findings of Franzeskaki, who emphasizes that institutional coordination and stakeholder engagement are vital for maximizing the performance of BGI [61]. Case Study I, based on systemic design, aligns with such integrated governance approaches, while Case Study II, following an individual responsibility model, risks uneven maintenance and inconsistent functionality.

Another critical insight concerns spatial flexibility. While green roofs and permeable surfaces show strong adaptability across various urban typologies, large-scale retention structures often face implementation barriers in densely built environments. This reflects the challenges described by Wong and Brown (2009), who emphasized that retrofitting water-sensitive urban design (WSUD) elements in established areas is frequently limited by legal, financial, and spatial constraints [62]. Finally, the effectiveness of BGI is highly climate- and site-dependent. As Pappalardo et al. (2017) argue, hydrological performance can vary significantly with soil type, rainfall intensity, and urban morphology [63]. This contextuality supports the need for hybrid planning models combining centralized and decentralized elements tailored to local geophysical conditions.

5. Conclusions

This study presents an analysis of the potential for implementing blue–green infrastructure (BGI) in various urban contexts, based on two case studies. These case studies facilitated a comparative assessment of BGI applications in different spatial settings, specifically large residential complexes and small catchments. The analysis revealed that large urban residential areas offer a broader array of opportunities for stormwater management, as reflected in the multi-criteria analysis (MCA) results. In contrast, smaller catchments with a limited spatial extent necessitate solutions with specific characteristics and parameters, as confirmed by the analysis.

Employing the MCA method, which is based on a set of criteria derived from a literature review on BGI, proved beneficial for the comparative evaluation of different solutions. While the method has limitations, such as subjective criteria weighting and restricted scenarios, its ease of application and transparency render it valuable in urban planning decision-making processes.

It should be noted that the findings of this study may not be fully generalizable to other urban contexts, particularly those with differing climatic, social, or infrastructural conditions. The analysis was confined to case studies in Poland, which may influence the broader applicability of the results. Nevertheless, case studies remain a valuable and widely accepted approach in urban planning and environmental research, as they allow for an in-depth examination of real-world implementations and context-specific conditions. Furthermore, case studies provide practical knowledge for spatial planning by offering insights into the applicability of BGI at various spatial and functional scales. When local specificities are appropriately considered, the presented framework can be scaled up and adapted for use in other diverse urban environments.