Restoring Historical Watercourses to Cities: The Cases of Poznań, Milan, and Beijing

Abstract

The increasing frequency of extreme weather events, combined with the historic degradation of urban water systems, has prompted cities worldwide to reconsider the role of water in urban planning. This study examines the restoration and integration of historical watercourses into contemporary urban environments through blue and green infrastructure (BGI). Focusing on three case study cities—Poznań (Poland), Milan (Italy), and Beijing (China)—this research explores both spatial and regulatory conditions for reintroducing surface water into cityscapes. Utilizing historical maps, contemporary land use data, and spatial planning documents, this study applies a GIS-based multi-criteria decision analysis (GIS-MCDA) to assess restoration potential. The selected case studies, including the redesign of Park Rataje in Poznań, canal daylighting projects in Milan, and the multifunctional design of Beijing’s Olympic Forest Park, illustrate diverse approaches to ecological revitalization. The findings emphasize that restoring or recreating urban water systems can enhance urban resilience, ecological connectivity, and the quality of public space.

1. Introduction

In the face of growing environmental problems and the increasing frequency of extreme weather events related to climate change [1], cities around the world are re-evaluating their historical relationship with water. Urban development throughout the 19th and 20th centuries often prioritized the regulation of natural watercourses, leading to the canalization of rivers, streams, and canals within the cityscape. This has limited the ecological value of these areas and contributed to problems related to stormwater management, urban overheating, and biodiversity loss. The reintroduction of surface water along historical watercourses presents an interesting possibility to address the highlighted problems while also referencing the heritage of the cities. However, such projects can be difficult to implement due to various constraints resulting from the current land development, as well as legal, economic, or social circumstances.

This study explores the potential for restoring historical watercourses or creating new water features as part of contemporary urban blue–green infrastructure (BGI) strategies. It focuses on three urban contexts—Poznań (Poland), Milan (Italy), and Beijing (China). These cities, despite their differences in size, climate, and urban composition, share a common characteristic: the transformation or loss of surface water networks and the recent efforts to recover their presence in the urban fabric. Poznan, Milan, and Beijing have a rich, hidden heritage of historic watercourses. In the past, all were crossed by networks of rivers, streams, and canals. Nowadays, significant parts of them have been sewered or removed. Their removal or change of course has had a significant impact on water conditions, including rainwater retention. In times of climate change, we are observing an increase in heavy rainfall, which the existing sewage network is insufficient to handle.

The objective of this study is to investigate spatial, planning, and ecological conditions that allow for the restoration of water-related infrastructure in urban areas. Through the use of GIS-based multi-criteria decision analysis (GIS-MCDA) and a comparative analysis of case studies, this paper presents opportunities for integrating historic water structures with modern needs such as flood mitigation, biodiversity, public space design, and cultural identity.

The final outcome of this study is to answer the following research question: Which historical waterways can be restored, and how do the structural conditions of individual cities influence the choice of possible forms, strategies, and scope of their reintroduction? To specify the detailed research objective, the following auxiliary questions can be formulated:

• What specific factors (e.g., spatial layout, land cover and land use, infrastructure, and planning and legal conditions) affect the possibility of restoring historical watercourses in different urban environments?
• To what extent can a uniform analytical method (e.g., GIS-MCDA) be applied in cities with different structural and contextual circumstances to identify possible recovery scenarios?

Examples ranging from the re-naturalization of streams in Poznań and the renewal of canals in Milan to the creation of ecological infrastructure in Beijing’s Olympic Forest Park illustrate a wide range of approaches and outcomes in reintroducing water into the city [2]. These examples highlight practical and regulatory challenges that must be addressed to make such transformations realistic and sustainable.

1.1. State of the Art

In the research conducted so far on urban water body restoration or the application of blue–green infrastructure (BGI), two main trends can be distinguished:

• Research on the benefits of introducing BGI to cities;
• Research on the conditions, possibilities, and limitations of introducing research on the benefits of introducing BGI solutions.

Studies regarding the benefits of water restoration projects focus on various aspects of the beneficial impact of blue–green infrastructure on urban quality. There are several types of benefits taken into consideration. The first of them are environmental benefits, including water quality improvement [3,4,5], biodiversity enhancement [6], and flood risk reduction [7]. Another category of benefits that appeared as a result of research is social benefits: recreational functions [8] connected with an impact on mental and physical health. There is research available combining more than one aspect, such as ecological, social, and economic benefits in stormwater management, heat island mitigation, and increased recreational and esthetic value of urban spaces [9,10]. The restoration of urban rivers and lakes has been studied as a means to introduce natural hydrological functions and reconnect urban populations with water landscapes [11]. Several studies have emphasized the co-benefits of BGI, integrating green spaces with blue elements to support urban ecosystem services [12,13]. Furthermore, interdisciplinary approaches have explored the role of public perception, stakeholder engagement, and governance in shaping successful BGI projects and water body revitalization [14,15].

The second type of studies focuses on the conditions, possibilities, and limitations allowing for the reintroduction of BGI in the place of historic waterways. These studies include economic feasibility studies, the influence of site-specific factors, and spatial policy approaches. In this context, an interesting comparison of community-based and top-down approaches has been conducted using examples from New York and Nice [16]. Limitations, including land availability, funding constraints, maintenance challenges, regulatory fragmentation, and unequal access to benefits, have been discussed for the case study of Kunming City [17]. Moreover, integrating BGI into existing urban systems often requires overcoming institutional inertia and aligning the interests of various stakeholders. Ongoing research explores how participatory planning, adaptive governance, and evidence-based design can help navigate these limitations and maximize the multifunctional benefits of BGI.

1.2. Historical Background

1.2.1. In Poznan

In the histories of Poznań and Milan, the networks of watercourses played an important role in shaping the urban and social development of both cities. A significant portion of these watercourses in both cities have not survived to the present day, having been removed, buried, or redirected into underground channels or storm sewer networks.

Poznań once had an extensive network of natural and artificial watercourses. The most well-known of them was the old riverbed of the Warta River, characterized by a tight bend between the Old Town and Chwaliszewo. Several tenement houses in Chwaliszewo, located directly on the riverbank, created a unique waterfront of the so-called “Poznań Venice,” known from old postcards, which is no longer present in the cityscape. However, it has been preserved in the city’s toponymy, including commemoration in the name of Wenecjańska (pol. “Venetian”) Street. The old riverbed was buried during the major regulation of the Warta River in 1966–1970 (designed 1964), which included, among others, redirection of the central section of the main navigable riverbed to the previous relief channel between Chwaliszewo and Ostrów Tumski [18]. The regulation was intended to improve the city’s flood protection and navigability conditions. The first of these goals was achieved; the city has not been exposed to floods since then, although the disruption of the continuity of underground permeable layers disrupted the natural groundwater flow, with some consequences for the underground parts of the buildings [19]. However, the navigability potential was never fully exploited, and the former river port ceased to function. The regulation moved the river far from the city center, and the area of the former riverbed was left unused for several decades.

Apart from the major change in the main waterbed of the Warta River, there were many more watercourses that existed in the past but were removed, redirected, or canalized at various stages in history. The following watercourses are worth mentioning:

• Mała Warta (Zgniła Warta, Leniwa Warta)—Flowed along the current Mostowa street, separating the island of Grobla from the city; buried in 1896–1898.
• Struga Karmelicka (Kamionka)—Located in the southern part of the city center along Łąkowa street, together with a branch of the Noteć stream on today’s Rybaki Street (toponym of a former service village with a name derived from the Polish word rybak, meaning “fisher,” indicating the ancient presence of water), disconnected from Warta (as a result of the construction of the Droga Dębińska causeway) in 1820 [20], ultimately buried in 1888–1896.
• Former moats around the medieval city walls.
• Bogdanka—A stream flowing along the western wedge of greenery. The section that once ran through the old town and fed the medieval city’s moats was completely removed, and its waters were joined with the Wierzbak stream and canalized in the downtown area. There is also another hidden section between the artificial lake Rusałka and the ponds of Sołacz.
• Wierzbak—A stream in the northwestern part of the city, formerly flowing parallel to Bogdanka—Currently connected to its waters in the area of Sołacz and then, via an underground channel, to the Warta River.
• Seganka—A stream flowing along the spatial layout of the former oval village of Jeżyce (currently a district of Poznań), nowadays entirely hidden underground.
• Obrzyca—A stream in the valley between the lower and upper terraces of the Rataje district, most of it non-existent on the surface.

1.2.2. In Milan

In Milan, the system of artificial canals (Navigli) and moats served both a defensive and economic function [21]. The network, whose construction dates back to the 12th-15th centuries, enabling the transport of goods and people, consists of five main canals: Naviglio Grande, Naviglio Pavese, Naviglio Martesana, Naviglio di Paderno, and Naviglio di Bereguardo. They connected with each other in Milan via several circular channels with interconnections; the internal ring was Cerchia Interna (also known as Naviglio Interno or Fossa Interna). The outer one, Cavo Redefossi, was created in the 18th century. The majority of them were buried between 1894 and 1930. To date, only a couple of canals remain in the cityscape, such as the Naviglio Grande and Naviglio Pavese, which connect the city to the Ticino River and other parts of Lombardy. They meet at the Darsena—the former city port—which, today, has regained its importance as a recreational space.

2. Materials and Methods

2.1. Research Materials

The choice of Poznań, Milan, and Beijing as research materials allowed for the assessment of the applicability of the GIS-MCDA (Geographic Information System-based multi-criteria decision analysis) method in diverse urban settings. The selected cities differ significantly in terms of scale, climate, and geographical, cultural, and planning conditions. Such diversity allowed for testing the universality of the analytical approach and assessing its flexibility in identifying the potential for restoring historical watercourses in different urban contexts. Due to the availability of relevant comparable spatial data, it was decided to limit the scope of the research to the cases of Milan and Poznań, leaving the example of Beijing as a reference.

The research materials consisted of historical watercourse areas in Poznan and Milan (Figure 1), comprising the subject of analysis. The sources of data used to study these areas included historical maps of former riverbeds, contemporary land use maps, and spatial planning documents. These sources provided essential information for identifying the past and present conditions of the land, enabling an assessment of spatial transformations and the potential for restoring or integrating historical water systems into the current urban fabric.

The data sources included both historical and contemporary sources, allowing for a comprehensive analysis of spatial transformations related to former watercourses. Historical maps of former riverbeds [22,23] were used to identify the original layout and flow of non-existent or hidden streams. Contemporary land use maps provided insight into current development patterns and how they correspond with historical water networks. Additionally, spatial planning documents were analyzed to understand urban planning decisions, regulatory frameworks, and potential strategies for restoring or integrating these former water systems into the modern urban landscape.

2.2. Methods—GIS-MCDA Implementation

A multi-criteria assessment of the possibilities for restoring or recreating waterbeds in a manner adapted to contemporary needs and circumstances was conducted. The analysis was carried out using the GIS-MCDA (GIS-based multiple-criteria decision analysis) method, which combines multi-criteria analysis with geographic information to evaluate and select the most appropriate solutions to spatial decision-making problems [24,25,26]. It integrates spatial data with various selection criteria.

The GIS-MCDA method provides three types of evaluation at subsequent stages: screening criteria, site evaluation criteria, and site evaluation constraints. The adopted decision-making process scheme (Figure 2) is analogous to solutions previously used in similar decision-making problems (e.g., Rikalović et al. [27]), adapted to the needs of assessing the potential of watercourse restoration.

In terms of screening criteria, this research superimposed historical maps of watercourses (former riverbeds, canals, ponds, etc.) onto contemporary land cover and space use data and spatial planning documents (i.e., local spatial development plans). The aim of this screening was to identify the following:

• Gaps and discontinuity of former watercourses and related greenery wedges or corridors;
• Potential places of their reconnection and reconstruction as a basis for further elaboration of the possibilities and limitations of their restoration.

At the stage of screening, there were several issues resulting from the inaccuracy of the historical maps obtained. First, the available maps came from different periods and differed in accuracy, scale, and scope of showing individual watercourses. In addition, the challenge was to transform raster maps without georeferences into a digital vector form suitable for GIS processing. The main problem to be solved here was the issue of projection. The old maps did not correspond to any of the currently applicable coordinate systems, so it was necessary to georeference the raster image. The Helmert transformation algorithm with cubic resampling (core 4 × 4) was used for this purpose. The raw results of this transformation were unsatisfactory, showing, among other things, distortions of the geographic grid on old maps in comparison to the contemporary coordinates. Therefore, additional manual corrections were necessary in several areas after processing, based on traces preserved in the topography and ownership structure of the land. These inaccuracies, although causing some difficulties at the data collection stage, did not have a major impact on the results, because they mainly concerned defining the course of water routes and not the parameters of their assessment.

The following were adopted as site evaluation criteria for assessing the possibilities, needs, and potential benefits of restoring water:

• Contemporary land use in areas formerly occupied by watercourses—as a crucial parameter defining if it is physically possible to undertake restoration;
• Provisions of applicable planning documents concerning the studied areas—allowing for the assessment of, on the one hand, legal restrictions resulting from local law acts, and, on the other hand, the compliance of the proposed solutions with the strategic directions of the spatial development of the city;
• Flood risk, particularly vulnerability to flash floods [28] (analysis using Scalgo Live software, https://scalgo.com/en-US/scalgo-live-documentation/about/whats-new/national-polish-land-cover-map-in-high-resolution, accessed on 30 May 2025)—allowing for the identification of areas at high flood risk to prioritize locations where restoration could provide significant hydrological and protective benefits;
• Zones lacking access to public green spaces or surface water—to determine the usefulness of the intervention for providing residents with better access to public recreation areas, which can improve social equity;
• Important public spaces and key pedestrian routes—to assess the importance and contribution of the land to urban livability, attractiveness, and usability for residents;
• Elements of cultural identity and heritage—to underline and strengthen the local identity [29] and the historical heritage of the city.

The analysis included the classification of the contemporary land use of the area. Functional areas were assigned to the following categories:

• Built-up areas;
• Temporary development;
• Public greenery areas and sports and recreation areas;
• Communication infrastructure areas (roads, streets, and railway lines);
• Parking lots;
• Unused areas (wasteland).

At the same time, an analysis of the planned functions of these areas was carried out in accordance with the applicable planning documents. The following designations were taken into account:

• Areas for development (including a specific intensity: residential, service, and industrial);
• Public spaces;
• Public green areas and sports and recreation facilities;
• Surface water forms;
• Communication infrastructure;
• References to the protection of water resources and provisions in local plans.

Current development limitations, which restrict the possibilities of water restoration, were identified as site evaluation constraints:

• Built-up areas (with a high density and no gaps);
• Communication routes—especially those with no alternative ways;
• Valuable public spaces;
• Valuable natural greenery;
• Complex underground infrastructure;
• Restricted public accessibility—physical barriers (fences, landform, etc.);
• Restricted public accessibility—legal barriers (ownership structure, etc.).

Based on the multi-criteria assessment, it was possible to classify sections of former watercourses into one of the following recommendation categories:

• Full restoration of the watercourse in its original form (or possibly similar to the original one) in its original course;
• Restoration of the continuity of the watercourse in a modified form in its original course;
• Restoration of the continuity of the watercourse in an alternative course;
• Creation of a new, other form of surface water in the place of the former watercourse (no continuity required);
• Restoration of the continuity of the ecological corridor in the trace of the former watercourse in an alternative form (i.e., greenery);
• Point forms of resemblance (continuity not possible);
• No possibility of restoration.

For each section of the former watercourse, an assessment was prepared of the compliance of the current and planned states with potential forms of restoring the watercourse. The results were processed according to the matrix of circumstances and recommendations (

Table 1. Matrix of the site evaluation constraints in relation to the recommendations.

Built-Up Areas	Valuable Public Spaces	Transportation Routes	Valuable Greenery/Natural Value	Valuable Historical Heritage	Complex Underground Infrastructure	Restricted Public accessibility—Physical Barriers	Restricted Public Accessibility—Legal Barriers	Recommendation: Maximum Possible Level of Restoration
0	0	0	0	0	0	0	0	A. Full restoration of the watercourse in its original form (or possibly similar to the original one) in its original course
0	0	0	1	0	0	0	1	B. Restoration of the continuity of the watercourse in a modified form in its original course
0	0	0	1	0	0	0	1	C. Restoration of the continuity of the watercourse in an alternative course (at least one alternative continuous course meets the requirements for category B)
0	1	0	1	1	0	1	1	D. Creation of a new, other form of surface water in the place of the former watercourse (no continuity required)
0	1	1	1	1	1	1	1	E. Restoration of the continuity of the ecological corridor in the trace of the former watercourse in an alternative form (i.e., greenery)
0.5	1	1	1	1	1	1	1	F. Point forms of resemblance (continuity not possible)
1	1	1	1	1	1	1	1	G. No restoration possibility

). The numbers in the table represent the following: 0 points means the criterion is not present, 1 point means the given circumstances are present, and 0.5 points means the criterion is partially met.

3. Results

As a result of the GIS-based multi-criteria analysis process, several key outcomes were identified. These include detailed statistics and balances for the study area, as well as an assessment of spatial possibilities and final recommendations grouped into seven categories (

Table 2. Final recommendation of the form and scope of possible restoration of historical watercourses in Poznań as a result of the GIS-MCDA analysis.

Category	Length (km)	Share (%)
A. Full restoration	2.96	2.8%
B. Restoration in a modified form in its original course	19.23	18.3%
C. Restoration in an alternative course	3.37	3.2%
D. Creation of a new form of surface water	15.46	14.7%
E. Restoration of the continuity of the ecological corridor without surface water	9.78	9.3%
F. Point forms of resemblance	20.83	19.8%
G. No possibility of restoration	27.46	26.1%

,

Table 3. Final recommendation of the form and scope of possible restoration of historical watercourses in Milan as a result of the GIS-MCDA analysis.

Category	Length (km)	Share (%)
A. Full restoration	0.46	0.9%
B. Restoration in a modified form in its original course	0.39	0.8%
C. Restoration with an alternative course	0.00	0.0%
D. Creation of a new form of surface water	9.74	19.6%
E. Restoration of the continuity of the ecological corridor without surface water	0.00	0.0%
F. Point forms of resemblance	26.08	52.3%
G. No possibility of restoration	3.31	6.6%

and Figure 3), with the length and percentage share of each category provided. The maps (Figure 4) present an overview of their spatial distribution.

3.1. Detailed Recommendations

3.1.1. Poznań

Descriptive recommendations for individual watercourses are presented below as the conclusions from the GIS-MCDA analysis.

In Poznań, the scope of possibilities for reconstructing former watercourses is very diverse. In some areas, the current land use along their former paths makes it easy to identify their former courses. In other cases, the continuity of these corridors has not been preserved due to development or functional and ownership divisions. However, a relatively large portion of the traces of former watercourses are located within green wedges. The detailed findings of the analysis for each case are presented below.

Regarding Zgniła Warta, the former western riverbed of the Warta River resembles the shape of Mostowa Street. Given the limited street width and the dense, compact structure of the surrounding tenement buildings, the only feasible intervention is a point-based commemoration of the former river.

The northern part of the Struga Karmelicka stream, located in the city center, is fragmented—either already built-up or a part of a street. The term “much more potential” refers to the southern part, located in the green wedge, that includes a few undeveloped fragments, although they do not form a coherent whole, interrupted by point development or sports facilities. Two of them provide special opportunities for the introduction of blue–green infrastructure: the green area along ul. Dolna Wilda and the area of the former Warta Stadium, currently a wasteland overgrown with spontaneously spreading ruderal vegetation. The latter was the focus of the architectural competition in 2022/2023 for students and professional architects.

The restoration potential of the Bogdanka and Wierzbak rivers varies across their historical courses. Uncovering and restoring the section of the Bogdanka from the artificial Lake Rusałka through Golęcin to Sołacz is fully feasible. In contrast, the original central part of the course of Bogdanka, running through the present city center, no longer exists and cannot be easily reconstructed due to dense urban development. Below Sołacz, the waters from Bogdanka and Wierzbak share the same waterbed and further underground canal from Wodziczki Park to the Warta River. This part can largely be uncovered, either along the original course or along an alternative route. The upper course of the Wierzbak River, from Podolany through Winiary to Nad Wierzbakiem Street, is also restorable.

3.1.2. Milan

In Milan, much of the circular canal network overlaps with the street layout, offering limited possibilities for intervention without interrupting the traffic system. For example, the internal circular channel Cerchia dei Navigli overlays the M4 metro line under construction. The design for the construction of the metro line included the former waterway as an underground canal, Naviglio Interno Internato, at a depth of 6 m, which, despite being invisible on the surface, is important for improving water conditions by serving as a retention reservoir. Above- and underground transportation infrastructure make full-range restoration impossible; however, partial forms of reconstruction can be implemented, such as rainwater reservoirs (i.e., in the form of oblong ponds), green corridors, or visual markers indicating the former course within the pavement. The waterway most likely to be restored due to the current land cover is the Conca di Viarenna, which is partially preserved. There are plans to restore a two-hundred-meter-long section of Naviglio Vallone, connecting the Conca di Viarenna with Darsena [30]. Among other restoration projects in Milan, it is worth noting the design for the area of Parco Giovanni Paolo II [31]. The above-mentioned examples from Milan show that despite limited spatial possibilities, it is possible to find alternative ways to carry out projects in the field of blue–green infrastructure, making use of the potential of past watercourses [32,33].

3.2. Design Examples

Selected cases of newly implemented or designed blue–green infrastructure, serving as either restoration or a functional replacement of former watercourses in Poznań, Milan, and Beijing, are presented below. These examples represent different approaches and varying degrees of blue–green infrastructure integration in relation to former watercourses, and they can be classified into the different categories defined in Section 2.2.

3.2.1. Rataje Park in Poznan

In 2020, the Poznań University of Technology, commissioned by the City Green Board, developed a landscape architecture redesign regarding urban water management that employed nature-based solutions for Rataje Park in Poznań (Figure 5; authors: dr hab. inż. arch. Anna Januchta-Szostak, prof. PP; dr hab. inż. arch. Jerzy Suchanek, prof. PP; dr inż. arch. Wojciech Skórzewski; mgr inż. Jerzy Kosmatka, 2020) [34]. The project applies urban water management principles based on nature-based solutions [35] to address persistent problems related to high groundwater levels and stagnant rainwater following heavy rainfall. Excess water regularly accumulated in the park’s lowest areas, rendering recreational spaces unusable for extended periods. Rainfall analyses carried out for the project using two different methodologies related to current weather data showed that current standards are not keeping pace with climate change [36,37].

The project adapts the shape of the terrain, vegetation, and small architectural elements to allow functionality during both dry and wet conditions. A central feature is a multifunctional stone circle, serving as a meeting place when dry and as a walkable route over water under wet conditions. The interior is filled with gravel and fine pebbles to support retention. Concrete benches and connector stones enable access and serve dual purposes: seating and elevated walkways.

To manage excess rainwater, the design introduces a series of shallow grassy infiltration basins, allowing water retention and flow through culverts beneath paths, which also serve as amphibian passages. Linear drainage channels and a gravel-filled depression (after tree transplantation) enhance infiltration. Existing hammocks and three young trees are relocated to higher ground to accommodate the new layout while preserving recreational functionality and ecological balance.

The design may be classified as a point form of resemblance (category F) in relation to the non-existent Obrzyca stream in a slightly different location; however, the continuity of the green corridor is preserved.

3.2.2. “Uwolnić Bogdankę”—Student Competition for the Design to Uncover the Bogdanka Stream in Golęcin, Poznan

The idea of uncovering the Bogdanka stream was the focus of a student architectural competition held in 2023. The topic referred to a fragment of the watercourse that is currently covered underground, located between the artificial lake of Rusałka and the green areas of Park Sołacki with multiple ponds. A collage of the selected students’ works from the competition is presented in the picture below (Figure 6). The idea of recreating the Bogdanka stream in the form selected in the competition, but more or less in its original course, should be classified into category B (restoration in a modified form in its original course).

3.2.3. Case Study Corresponding to the BGI Solution: Case Study and Background of Beijing Forest Olympic Park

Many reports have focused on Beijing Olympic Forest Park (BOFP), a large-scale urban park developed as part of the green infrastructure to support the 2008 Olympic Games in Beijing, China. The BOFP represents an example of using strategically placed and well-designed public open spaces to relieve stress from high-density living in compact urban areas [38].

At 1680 acres, Beijing Olympic Forest Park is the largest public green space ever built in Beijing. Built as part of the Olympic Green for the 2008 Summer Games, the park is surrounded by an ultra-urban environment of high-density development and high-volume traffic. The park provides a variety of spaces for urban residents and visitors, including Mount Yangshan, a 50-acre man-made lake, woodlands, wetlands, grasslands, educational facilities, paths, playgrounds, and sports fields. The design incorporates traditional Chinese landscape arts and principles that emphasize the harmony between humans and nature, while modern ecological concepts and techniques were widely employed in the design of the park to address the goals of zero waste and zero stormwater discharge. Since its completion, the park has become an important public green space that provides recreation, educational opportunities, and environmental benefits to Beijing residents and visitors alike (Figure 7 and Figure 8).

The BGI approach applied in the Beijing Olympic Forest Park:

Concerning the biodiversity of the park, the environment, including grasslands, hills, and forests, gradually became richer, and the park turned out to be an ideal habitat for birds, along with the growth of plants and the expansion of the park.

In addition to protecting existing habitats, new habitats were created by installing over 2 million new plants of ~300 species, including ~0.53 million trees. The planting was designed to mimic communities that naturally occur around Beijing [39]. Native species were planted to the maximum degree allowed by local nursery resources. Additionally, a 218 m-long, 60 m-wide eco-bridge was installed to maintain wildlife and pedestrian connections across the 80 m-wide Fifth Ring Road. Other habitat features included a 24 m tall bird tower designed to accommodate over 1500 swifts, which are culturally important to Beijing.

In addition to the above sustainable features, the set of design strategies for creating a philosophically meaningful place and a lively social space is particularly worth highlighting.

The former ensures the preservation of the distinct Chinese and Scape culture, while the latter honors the most significant function of large parks in dense cities—to serve their people [38].

Self-sustaining water system in Beijing Olympic Forest Park:

Through research, we found that a self-sustaining and self-regulating water system has been formed to address Beijing’s high water supply evaporation rate. To address this issue, a four-hectare integrated vertical-flow constructed wetland was created in the park. The wetland treats 2600 m³ of reclaimed water from the Qing River Wastewater Treatment Plant per day, which then recharges the main lake. This was the first domestic application to use reclaimed water and stormwater to supply 100% of an urban park’s waterscape.

Moreover, a demonstration greenhouse and underwater observational corridor accompany the wetland to provide public environmental education.

To deal with stormwater, especially during the torrential summer storms, a comprehensive stormwater treatment system, including porous paving, bioswales, ponds, infiltration wells, and cisterns, was installed to store, reuse, and, most importantly, infiltrate stormwater. An intelligent landscape irrigation system was also installed to further reduce water use [38].

4. Discussion

A case study of the dragon-shaped river in the Beijing Olympic Forest Park was developed to examine the challenges associated with reclaimed water usage. The dragon-shaped river is a good example of reclaimed water implementation in China, and its experience can be compared with similar projects elsewhere in China and around the world. However, our study shows that there are still persistent challenges, particularly relating to sustained nutrient removal across the broader spectrum (nitrates, nitrites, etc.) and the sustainable management of resulting plant biomass [40].

However, this study has certain limitations resulting from location-specific conditions. In particular, this applies to the following criteria, which are most vulnerable to local variables:

• Provisions of applicable planning documents concerning the studied areas—due to different legal systems for spatial planning in individual countries, allowing for more or less flexible ways of shaping space;
• Flood risk—due to different climatic conditions concerning the total and maximum peak values of rainfall;
• Elements of cultural identity and heritage—due to different forms of protection of monuments.

BGI is considered a niche, competing with the gray conventional infrastructure of the current regime. The benefits of applying socio-technical transition theory to facilitating transitions from conventional infrastructure to BGI have been discussed. Adopting this perspective highlights the fact that this shift occurs within a more dynamic and complex system, drawing more attention to societal and institutional realms. Without configurations in these aspects, the transition toward BGI cannot take place [41].

The results of the GIS-MCDA analysis not only indicate spatially diverse possibilities of implementing BGI but also emphasize the need to balance the potential benefits and limitations related to such projects [42]. Each decision to introduce or recreate blue–green infrastructure requires taking into account technical aspects (e.g., hydrological possibilities and land availability), social aspects (e.g., local needs and social functions of space), and legislative framework (e.g., planning documents and heritage protection). The applied GIS-MCDA methodology allows for assigning weights to individual criteria in a way adapted to the priorities of a municipal spatial policy, which makes it a flexible tool supporting decision-making in a complex urban context. Thus, the obtained results can serve as support for both planning practices and theories of water management in cities.

Spatial errors due to historical map deviations are another uncertainty factor, although they were taken into account during the data-gathering stage. Uncertainties were reduced using raster georeferencing methods and comparative analysis with the current spatial structure, as described in the Methods section.

As written in [43], the primary motivations for using BGI include optimizing stormwater management in urban areas and diminishing flood risks. BGI aims to leverage natural processes, such as infiltration and retention, to manage stormwater in a way that mimics the natural hydrological cycle. BGI can significantly reduce the flood risk by decentralizing infrastructural elements, and by using solutions such as rain gardens, bioswales, and green roofs, cities can better manage intense rainfall, enhancing resilience to floods and inundations. In cities where traditional drainage systems are overwhelmed, BGI can provide alternative, more sustainable, and practical solutions.

Although the BGI solutions are applied in national parks and other city landscape areas, much work remains to be completed, especially in stormwater management. Most case studies have addressed issues such as stormwater management, urban heat islands, and habitat loss by integrating elements like green roofs, rain gardens, and water features. To improve the ecological, social, and economic well-being of communities, further development and practical application of BGI are still needed.

5. Conclusions

Introducing blue–green infrastructure to cities offers benefits in water retention, microclimate regulation, and biodiversity enhancement. In cities with a rich history of historical watercourses, these features can be an incentive for activities involving the discovery, restoration, and exposure of cultural heritage. The discussed examples from Poznan, Milan, and Beijing show that the restoration of historical watercourses in cities is possible under certain circumstances. The potential and desirable scope of restoration investments depends on constraints related to the current development, maintaining continuity, and planning conditions. GIS-based multi-criteria decision analysis (GIS-MCDA) can be a helpful method for assessing the potential to restore former watercourses.