Breaking out of the silo: collaborative approaches to implementing blue-green infrastructure in urban areas

The effects of climate change are being felt in urban areas as increased heavy rainfall, heatwaves, droughts, and more frequent storms. These developments mean that managing water in urban areas has become a significant challenge. To guarantee the efficiency of urban water management systems, it is essential to adapt them to the prevailing conditions.

To adapt urban water management to these external pressures, blue-green infrastructure (BGI) – a subcategory of nature-based solutions – has been discussed and evaluated as a promising solution (Eggermont et al. 2015; Kabisch et al. 2016; Bush & Doyon 2019; Dorst et al. 2019; Frantzeskaki et al. 2019). BGI is intentionally designed to provide environmental, economic, and social benefits (Dorst et al. 2019). Depending on the specific approach taken, implementing BGI can influence a range of services, including wastewater drainage and treatment, water supply, the provision of urban green spaces, and the management of urban health (Ghofrani et al. 2017; Kabisch et al. 2017; Raymond et al. 2017). It fulfills several functions, including restoring a natural water cycle in urban areas to cope with heavy rainfall or storing water (e.g., in cisterns) that can provide rainwater for irrigation. Furthermore, BGI has the potential to mitigate the effects of urban heat islands by establishing natural shading and evapotranspiration, creating habitats for wildlife and therefore increasing biodiversity, and contributing to noise protection and filtering air pollutants. The planning, implementation, and management of BGI involves stakeholders from different sectors (Frantzeskaki et al. 2014; Willems et al. 2020), such as road planning, civil engineering, green space management, private companies (e.g., housing associations), and civil actors with conflicting values and objectives (Frantzeskaki 2019), and requires the integration of different policy domains (Wamsler et al. 2020). In the majority of Organisation for Economic Co-operation and Development (OECD) countries, however, urban infrastructure management is characterized by strictly defined sectoral responsibilities and regulations that often hinder collaboration (Willems et al. 2023). The management of urban infrastructure services is typically the responsibility of dedicated municipal departments, which are required to adhere to clearly defined tasks and procedures (Wong 2006; Graaf & van der Brugge 2010). The literature has identified such constructions as ‘siloed’ structures that constitute significant organizational barriers to the successful integration of BGI (Uittenbroek et al. 2014; Kabisch et al. 2016; Sarabi et al. 2020; Wamsler et al. 2020; Almaaitah et al. 2021; Dorst et al. 2022).

The objective of our research was to ascertain how to facilitate coordination among actors and stakeholders to accelerate the planning, implementation, and management of BGI. As a foundation for our research, we draw on the recently introduced ‘infrastructure transition canvas’ (ITC) (Hohmann & Truffer 2022), a tool based on a business model approach from the field of management literature. This framework was applied to an empirical study in the German city of Bochum, where interconnected tree trenches have been successfully integrated into existing urban water infrastructure, thereby enhancing the overall system's capacity to adapt to climate change. Bochum is among the first German cities to employ BGI as a standard urban drainage solution. Often, similar projects stall at early planning stages, despite being widely considered promising solutions to tackle emerging infrastructure challenges, e.g., in the context of climate change. The coordination among different municipal government departments proved to be a crucial factor in Bochum's case, as this helped to overcome major barriers resulting from established sectoral planning routines and procedures. Our research question is: How can the ITC facilitate collaboration between actors and stakeholders when implementing BGI?

The paper is structured as follows: In Section 2, we define BGI and explain why its integration into urban infrastructure requires the coordination of actors from different sectors. We introduce the ITC to identify related coordination tasks among the various actors. Section 3 presents our methodological approach. In Section 4, we discuss the key lessons from the Bochum case and identify the main collaboration interfaces between the different actors. Section 5 concludes by discussing the criteria that must be met for the successful integration of BGI in urban infrastructure management in Germany and the lessons learnt from Bochum that can be transferred to an international context.

BGI has developed into a promising option for climate change adaptation in urban environments and for improving infrastructure quality. However, its implementation still faces many challenges. We elaborate on the range of BGI approaches and argue that one of the key conditions for their success or failure is the integration of the stakeholders who need to be involved in the planning, implementation, and management of urban infrastructures. This elaboration will provide guidance on how to structure the multiple coordination tasks involved when implementing BGI.

The concept of BGI integrates nature-based components into the design of urban infrastructure with the objective to enhance the resilience of urban environments (Eggermont et al. 2015; Wang & Foley 2023). BGI brings together elements of green (land) and blue (water) urban spaces, as well as sustainable urban drainage systems (Voskamp & van de Ven 2015). It combines forms of rainwater harvesting, infiltration, and management of rainwater runoff, as well as rainwater collection through a combination of natural and technologically enhanced elements. These include ponds, wetlands, permeable pavements, trenches for water retention and conveyance, ditches, open channels, and infiltration trench systems (Suleiman 2021). These techniques are combined with urban green spaces, including parks, public green areas, street trees, urban forests, green roofs, and vertical greening. Combining urban blue and urban green elements in natural and designed landscapes elements makes it possible to fulfill a wide variety of functions, e.g., storing water for irrigation, providing flood protection and relief for sewers and sewage treatment plants, and creating wetlands as a habitat for wildlife or for purifying water (Ghofrani et al. 2017).

The characteristics of BGI diverge fundamentally from those of the conventional (gray) infrastructure that currently prevails in OECD countries and represent a significant departure from established urban planning standards and procedures of urban planning. Implementing BGI often requires changes in established municipal structures (Kabisch et al. 2016; Frantzeskaki 2019). The main reasons are that BGI projects go beyond the policy domain of any single infrastructure sector, as these measures demand a wide range of expertise and resources while contributing to broader environmental, social, and economic values (Raymond et al. 2017). The implementation of BGI projects frequently involves both public and private actors, necessitating the application of collaborative governance approaches. The selection of an appropriate governance approach is contingent upon the objectives of the project and the stakeholders involved. Consequently, the effective planning, implementation, and management of BGI requires methods that identify and integrate the relevant public and private stakeholders (Frantzeskaki 2019; Frantzeskaki et al. 2020). These stakeholders often have diverse interests, objectives, resources, priorities, and legal mandates, which may support or hamper the provision of key resources, e.g., knowledge or funding. Identifying and integrating the multiple resources and value concerns involved and facilitating collaboration between relevant stakeholders represent significant challenges when implementing BGI (Almaaitah et al. 2021; Winker et al. 2022).

As an example, we consider the case of urban water management. BGI is a new approach to urban drainage (Fletcher et al. 2014). This infrastructure stores, uses, and infiltrates water during heavy rainfall events, relieving existing storm water infrastructure. The department responsible for urban water management is, therefore, the most obvious actor to take the lead in implementing such BGI projects (e.g., Willems et al. 2020). At the same time, BGI requires expertise and resources beyond the remit of the urban water management department that are found in different municipal departments and among non-state actors. The department responsible for urban water management must, therefore, collaborate with them to mobilize the necessary resources.

However, traditional municipal routines and procedures are often ill-suited to such tasks (Willems et al. 2020). To implement BGI more widely, two key factors must be considered. First, the relevant actors and their resources must be identified, and second, the added value of BGI must be evaluated to engage key actors and stakeholders and ensure successful implementation.

The ITC can help to identify relevant actors and stakeholders of specific implementation projects and analyze their roles and responsibilities based on their respective value considerations and resources. This provides a foundation for suitable coordination strategies, incentives, and compensation measures, which can then be evaluated in terms of their efficiency. The tool has two analytical layers (see Figure 1). The basic layer (orange elements) represents a direct extension of the conventional business model canvas (Osterwalder & Pigneur 2010). This covers all elements related to individual key actors and stakeholders: their activities and resources (key actors); the segments, relationships, and communication channels (stakeholders) in the upper part; and the generated values, costs, and risks for these actors and stakeholders (lower part). Systematically mapping the key activities, resources, and capacities, values, costs, and risks can identify imbalances or obstacles in terms of neglected value propositions, and unequal distribution of costs and risks among the various actors and stakeholders. The second layer (green elements) depicts the specific coordination structures that come into play when different actors with diverging rationalities have to be coordinated to achieve an overall balanced outcome. This encompasses prospective intermediaries and the incentive and compensation mechanisms designed to address the aforementioned imbalances and obstacles.

The ITC, therefore, represents a tool that can be used to identify the relevant actors and stakeholders involved in the planning, implementation, and management of BGI and outline their potential roles at an early stage. The next section will show how this framework can be used to identify key coordination interfaces in BGI implementation.

Understanding the coordination challenge posed by BGI draws on analytical steps from both layers of the ITC. The activity-oriented (orange) layer leads to the main actor- and value-related elements that must be considered in BGI implementation. The coordination-oriented (green) level refers to the conditions that facilitate effective stakeholder collaboration, often organized by intermediaries, and incentive and compensation mechanisms. The latter refers to the allocation of roles, tasks, and the distribution of cost-benefit streams when implementing BGI.

In a first step, we distinguish between actors with a direct impact on the shape and form of a BGI outcome and stakeholders with a more passive role. The former group is the key actors in the ITC framework and provides the resources for BGI implementation. Stakeholders are those affected by BGI measures but without any direct influence on planning, implementation, or management. Actors and stakeholders are not predetermined but are based on the requirements of a respective BGI measure. The first coordination task among the key actors is to determine a leadership structure for the project. While there is always an actor who initiates a project, further analysis of who controls which type of key resource suggests that the responsibility for certain tasks can be assigned to others, or is taken over by a consortium of partners, often coordinated or supported by an existing or newly founded intermediary (Kivimaa et al. 2019). This may be the case, for example, when a district is undergoing a process of regeneration and BGI is being considered as part of this process. In such cases, the urban planning department may initiate BGI. However, the designated areas may fall under the responsibility of other actors, including housing associations and municipal departments. The responsibility for spearheading the implementation of a particular BGI measure falls upon the respective actors involved. The leading actor(s) are responsible for implementing the BGI project in compliance with budget and capacity constraints. Actors with the potential to assume leadership will assess the infrastructure project based on their specific interests, values, and expertise and will ultimately shape the actual value propositions associated with it.

Besides the leading actor(s) and intermediaries, other organizations can also be of crucial importance, as they are necessary for generating value, whether in the planning, implementation, or management of BGI. The form and intensity of the coordination between the leading actor(s) and these value-creating actors can vary throughout the development phases of a project. In addition to the value-creating actors, other key actors play an important role, particularly through their control of resources, e.g., specific knowledge or land. As their input is essential, but not at the core value creation of the BGI measure itself, we refer to them as enabling actors. The leading actor(s) will have to involve these enabling actors in the implementation process to ensure that their resources are available to the project.

Stakeholders are differentiated into those with and those without formal decision power. The former assumes a political, regulating, and/or decisional role in BGI projects. Typically, these are political decision-makers, approval authorities or the management board of municipal governments. Stakeholders with formal decision power can create favorable or unfavorable conditions for BGI integration. Their political, managerial, or regulatory interventions are usually mostly important in the planning stage, when legal and infrastructural embedding are at stake. The coordination task vis-à-vis these stakeholders consists of convincing them of the manifold benefits of BGI measures. Barriers must also be considered, e.g., the challenges surrounding a non-state-of-the-art technology (especially in pioneering projects), which may place new responsibilities on stakeholders, e.g., approval authorities. The coordination task is to create a favorable environment including the necessary legal conformity for BGI projects.

Stakeholders lacking decision power, like local residents or the environment, are either positively or negatively impacted by BGI measures. Depending on their perception of the specific costs and benefits, they can advocate or oppose the implementation of BGI measures. This means leading actor(s) are interested in considering their positions and strategies. One example is that of private landowners affected by construction or maintenance work as part of BGI projects. Such impacts may be experienced in the form of noise, pollution, or restricted land access. Interacting with such stakeholders presents a significant challenge, as it necessitates the consideration of a multitude of parties with disparate perspectives. Any deficiency in transparency or communication could give rise to concerns and spark opposition. It is the responsibility of the leading actor(s) to facilitate the exchange with these stakeholders through effective communication and participation processes. Table 1 summarizes the interfaces and core coordination tasks of the key actors and stakeholders.

The complexity of the coordination tasks can vary from case to case. Primarily, the number and heterogeneity of the different parties as well as the heterogeneity of resources and interests determine the effort required by the leading actor(s). In order to gain insights into how these different tasks could play out in reality, we use an empirical case to illustrate the feasibility of the approach.

The ITC has been evaluated in several BGI cases in Germany. This case study illustrates the ITC approach to interconnected tree trenches in Bochum, a case of medium technical complexity. It demonstrates how the ITC can facilitate collaboration in BGI planning, implementation, and management.

In Germany, urban infrastructure is often managed vertically within municipal governments and utilities. Roles and responsibilities are clearly defined, as are costs, risks, values, and processes. At the same time, many actors and stakeholders have acknowledged the need to adapt infrastructure to mitigate climate change. In these discussions, BGI plays an important role, as gray infrastructure alone is often unable to cope with the impacts of climate change, and BGI offers several complementary benefits (Wingfield et al. 2019; Willems et al. 2020). However, the procedures and criteria for implementing BGI are not yet institutionalized and often only partially fit the prevailing municipal structures (Sieker 2019; Frantzeskaki et al. 2020). The current socio-technical system is characterized by high path dependencies, which constrain adaptation to changing conditions.

The rationale behind the detailed examination of this case is that, over time, the municipal authorities in Bochum have been able to establish interconnected tree trenches as a standard solution for urban drainage. Our analysis has two objectives: to verify the advantages of the ITC as a framework for more transparent and efficient planning and to showcase a BGI success story, which offers insights into the conditions required for collaboration and shared value creation. The findings are used to develop a comprehensive overview of coordination requirements in BGI planning, implementation, and management (Section 4.2).

The data were collected by the first author from civil engineering representatives to reconstruct the challenges faced by project leaders between 2020 and 2023. In 2020, an online workshop was piloted using the ITC template with the civil engineering department as the leading actor and the environment and green spaces department as a second value-adding actor. In 2021 and 2023, semi-structured expert interviews were conducted with experts from various municipal departments. Table 2 provides an overview of the organizational background of the experts recruited for the interviews and workshop. The interviews were transcribed verbatim and coded using MAXQDA. The data were analyzed using a coding scheme derived from the ITC.

Overall, the interviews and workshop captured the perspectives of the actors most relevant for implementing interconnected tree trenches at different times of the process. One weakness in the data collection was the lack of perspectives from road planning experts in 2020 and 2021, as they did not participate in the workshop or the first round of expert interviews. This may have been due to their initial reservations about BGI projects. However, their perspective was included in the 2023 series of interviews.

The findings are based on the interviews, the online workshop, and further discussions with key actors from the civil engineering department responsible for urban drainage. The tree trenches are now a standard option for urban and infrastructure planning in Bochum. The learning process, in particular with regard to integrating internal organizational collaboration with other municipal departments, occurred over several stages. In the initial stages, the project team of the urban drainage division within the civil engineering department approached the process following their established routines and procedures. Over time, they acquired the necessary expertise to identify the relevant actors and stakeholders and suitable planning approaches for integrating interconnected tree trenches into the urban and infrastructural planning process. They now act as facilitators for implementing BGI in other cities. The following chapter outlines the learning process and coordination activities developed over time.

Over time, the urban drainage division within the civil engineering department has had to address a number of challenges. Firstly, the standard structures and processes of urban infrastructure planning in general, and urban water management in particular, were found to be unsuitable for planning, implementing, and managing interconnected tree trenches. This was due to the specific and, in some cases, unknown requirements that they entailed in terms of technical standards, space requirements, or organizational arrangements. Another pivotal element was the identification of so-called windows of opportunity. Such opportunities may arise from the necessity to renew urban infrastructure, including entire districts, roads, or sewers. It was, therefore, essential to engage the civil engineering department from the outset of these renewal processes to exploit these opportunities. However, the civil engineering department typically assumes responsibility for urban drainage only at a much later stage. It was, therefore, necessary to convince the relevant municipal staff about the advantages of interconnected tree trenches so that they would alter their standard procedures and involve the civil engineering department at an earlier stage. Secondly, the lack of resources was a significant impediment to the successful implementation of tree trenches from the outset. The relevant departments, such as the civil engineering department and the environment and green spaces department, initially had to obtain the necessary expertise about tree trenches and the trees within them. Moreover, the division of the civil engineering department responsible for road construction had to learn how to adapt road profiles to align with the locations of the tree trenches, thereby ensuring that rainwater would be effectively channeled into the trenches. Thirdly, the civil engineering department faced challenges during construction due to a lack of information on existing gas, water, and telecommunications grids and unclear access rights to both above ground and underground areas. Fourthly, the approval process was challenging due to the requirements for tree trenches. Three authorities were engaged in the approval process: the lower water authority, the soil protection authority, and the nature conservation authority. Each authority has their specific responsibilities and protected assets. Implementing interconnected tree trenches was a novel undertaking for all of them. The civil engineering department had to ensure the interconnected tree trenches would not harm groundwater, trees, or soil to obtain approval. This added to the workload of all parties involved and the approval process caused considerable delays to the initial implementation of interconnected tree trenches.

The civil engineering department had to deal with these diverse requirements that cut across sectoral domains, each with their own specific setting, and they learned how to create new interfaces for collaboration and to engage with relevant key actors and stakeholders. However, they acquired their knowledge in a rather fragmented and unsystematic manner, and their coordination efforts encountered manifold resistance. The department received important support from two associations that focus on water management, the Emschergenossenschaft and the Lippeverband. Both have launched a so-called future initiative for climate change adaptation (‘Zukunftsinitiative Klima.Werk’) through BGI, which networks municipalities in the Ruhr region. It organizes expert forums and enables networking of the associated municipalities to exchange expertise and experiences, and provide funding for corresponding projects. The network has been important for knowledge building in the civil engineering department, the environment and green spaces department, and the approval authorities.

This section provides an overview of the main roles and interfaces of the key actors and stakeholders involved in the tree trench project in Bochum. The ITC was applied to this case study as an analytical approach. The objective was to provide a structured overview of the civil engineering department's learning process, focusing on actor coordination and collaboration.

The urban drainage division within the civil engineering department was the leading actor for BGI implementation and the actor–stakeholder network from the outset. It formulated the value streams (key actors) and value propositions (stakeholders) and provided the most important resources for implementation. The civil engineering department clearly acted as the project leader and was never challenged in this regard. To integrate complementary value-creating actors, the department had to specify core components of the interconnected tree trench concept and reach out to other departments that could contribute to its successful planning, implementation, and management. Core components are the trenches, the interconnected drainage channels, and the trees. For the trenches, it was important to agree on whether they should be used primarily as water management facilities or as tree sites. As no nationwide regulation exists, the civil engineering department decided to define the trenches as water management facilities. This meant that they came under its remits and could be financed via rainwater fees. If the trenches had been defined as ‘tree sites’, the responsibility for the project would have moved to the environment and green spaces department, where funding possibilities were less obvious.

The environment and green spaces department is responsible for planning, planting, and maintaining urban trees and greenery, financed from the municipal budget. In this role, it participated in value creation for interconnected tree trenches. It took some effort on the part of the civil engineering department to win over the environment and green spaces department to manage the trees, as there were uncertainties about the new technology, its benefits and the maintenance efforts required. Providing information about the impacts on the tree site, such as the larger root space and access to rainwater as irrigation water, helped to engage the environment and green spaces department as a partner. This enabled the mobilization of personnel and time resources from the department. Prior to the tree trench project, there was minimal to no exchange between the two departments. Establishing this interface required significant effort. It was necessary to provide instructions to employees of the environment and green spaces department on the conditions applicable to trees in interconnected trenches. In addition, the distribution of tasks and resources had to be negotiated according to their internal goals, objectives, and possibilities, and new communication channels had to be established. The two departments have now established a collaborative relationship, which we refer to as a value-creating partnership. They conduct regular planning meetings and ad hoc meetings on day-to-day business. They also inform each other regularly of long-term planning activities. This value-creating partnership contributes the most important resources, such as material, expertise, and finances, as well as planning, implementation, and operation activities.

Establishing interconnected tree trenches as a regular urban infrastructure element required the engagement of other key actors and stakeholders. For the project to be successful, it was crucial to gain the support of the urban planning department and the road construction division within the civil engineering department, particularly with regard to identifying suitable ‘windows of opportunities’ for implementation. The urban planning department initiates projects in cases where existing districts are renewed as part of a holistic urban development concept funded by the state. Previously, it involved the urban drainage division only at a late stage in the planning processes. It was thus imperative to change this in order to guarantee the requisite conditions, for example, with regard to the required public areas. The road construction division is responsible for road works and road design. It can suggest interconnected tree trenches as part of road renewal and construction projects. For this purpose, road surfaces must be redesigned to channel rainwater into the trenches. Close cooperation between the urban drainage and road construction divisions had to be established – a connection that did not exist before even though both divisions are part of the same civil engineering department. Communication between the divisions' experts is now based on a combination of planning meetings, day-to-day management, and joint long-term planning.

A third type of enabling actors were landowners and infrastructure providers from other sectors due to the new demands on the distribution of above ground and underground areas, which typically need to be reallocated in tree trench projects. These actors had to be involved early on in the planning processes of the civil engineering department. Examples include the department for light rail construction, public utilities responsible for the supply of drinking water, gas and electricity, and telecommunications providers responsible for providing communication pipelines.

Stakeholders with formal decision-making power set the rules for value-creating actors. These include political actors and approval authorities. To ensure the successful implementation of interconnected tree trenches, the civil engineering department had to engage with local politicians to educate them about the benefits of these trenches in terms of climate change adaptation. Experts in approval authorities for water, soil, and nature conservation have the mandate to approve interconnected tree trenches. In Bochum, for example, the involvement of the lower water authority at an early stage helped to streamline the approval process. Today, the lower water authority advises the civil engineering department in the early planning stages of interconnected tree trenches.

The project leader also had to consider different types of stakeholders without formal decision-making power that are still affected by the project, such as citizens and the local environment. Citizens generally demand a healthy, safe, and livable urban environment. In Bochum, they are involved through public participation processes and public information events, depending on the scope and scale of the respective project. In addition, citizens' initiatives were established in response to the construction of interconnected tree trenches. These initiatives either advocated interconnected tree trenches, for example, as a means of adapting to climate change, or opposed associated construction measures, such as the necessary felling of existing trees. Considering the environment as a stakeholder, interconnected tree trenches can provide several ecosystem services, e.g., increase the amount of water in a nearby river, decrease the amount of fresh water used for tree irrigation in summer, and improve the local microclimate and evapotranspiration. On the other hand, however, interconnected tree trenches can also lead to environmental risks, for example the discharge of pollutants from urban runoff through tree trenches into water bodies. This is a much-discussed topic in Germany, as urban runoff is currently generally channeled into sewers. The environment and green spaces department and the approval authorities played an important role taking care of environmental concerns.

Finally, intermediary actors were important in enabling the transition to more sustainable infrastructures via BGI in Bochum. The most important intermediary in the Bochum project was the so-called Zukunftsinitiative Klima.Werk. Its aim is to implement BGI measures on a broad scale in the Ruhr region. The initiative mediates between individual local projects (e.g., Bochum) and mainstreaming BGI measures on a broader scale by aggregating the experiences from these individual projects. The civil engineering department, but also other important actors such as the lower water authority, plays an active role within the Klima.Werk as knowledge brokers and thus act as a kind of mediator themselves.

Table 3 presents an extract from the results of applying the ITC to the implementation of interconnected tree trenches in Bochum. It provides an overview of the resources, key activities, value creation, costs and risks for the two most important key actors, i.e., the civil engineering department (urban drainage) and the environment and green spaces department.

Although BGI has been identified as a potentially effective strategy for adapting urban infrastructure to climate change, there are several barriers to its implementation. In our paper, we focused on the prevailing organizational structures and siloed decision-making processes within municipal governments, which are regarded as contributing to the implementation deficit of BGI. In this context, our research focused on the need to develop new organizational arrangements to enable collaboration. We examined how the relevant actors and stakeholders can define their new roles within the setting of BGI by thoroughly analyzing their resources, key activities, costs, and value streams, and based on these identify forms of collaboration and interfaces for implementing BGI.

In order to conduct our analysis, we selected the city of Bochum in Germany, where interconnected tree trenches have already been established as a BGI measure in infrastructure planning. This has been achieved despite the fact that Germany is known for its rigid administrative structures in the field of urban infrastructure management. In this example, the requirements for the planning, implementation, and management of BGI mark a considerable departure from the established rules and procedures of urban water management, as shown by whether to define tree trenches as tree sites or wastewater treatment facilities with the resulting consequences for responsibilities and management (Section 4.2). This necessitated proactive engagement across departmental silos, despite the relatively straightforward technical process and the involvement of primarily municipal government actors. The organizational effort was considerable and often not immediately apparent. The strong motivation and leadership on the part of the urban drainage division within the civil engineering department was identified as a key factor contributing to the successful planning, implementation, and management of this BGI measure, as policies and regulations for BGI integration do not yet exist in Germany.

The Bochum case provides an example for the organizational complexity of a relatively straightforward technical BGI measure. It focuses on the challenges associated with implementing BGI measures on public land, excluding private property. The primary challenges concern the redistribution of roles and responsibilities and the availability of public space. Only a limited number of public actors and a relatively small number of stakeholders were involved here.

In general, the configuration of actors and stakeholders in BGI projects can be either homogeneous and relatively easy to manage or heterogeneous and therefore require more complex coordination structures. The size of the municipality may be a contributing factor, with smaller municipalities exhibiting a more homogeneous configuration of actors and stakeholders, and larger cities displaying a more heterogeneous configuration. In the case of Bochum, there was no need to implement additional incentive and compensation mechanisms. Furthermore, most of the financing needed could be covered by municipal budgets and rainwater fees, and additional costs were covered by research funds. This is not always the case. Implementing BGI frequently reveals significant coordination challenges that go beyond the organizational challenges described in the Bochum case (see, e.g., Almaaitah et al. 2021). For instance, in projects involving both public and private stakeholders, comprehensive incentive and compensation mechanisms must be developed and negotiated to facilitate the implementation of BGI. These may be required, for example, if property owners invest in BGI measures that mainly benefit either their tenants (e.g., improving the microclimate or the local environment) or municipalities (e.g., by alleviating the sewer system).

Applying the ITC enabled us to reveal the learning processes and coordination activities retrospectively and identify the new actor and stakeholder roles and interfaces relevant to the Bochum case. However, we also believe that the tool could be used during early planning stages to provide a basis for identifying the relevant actors and stakeholders and allocate roles. In light of our findings, we are confident that a more systematic approach to the various coordination tasks could significantly improve the likelihood of success and expedite the BGI implementation process.

Although the approach has only been validated based on a single case, we believe that it could be applied to a wider range of similar BGI-focused projects through its conceptual generalization of management tasks for cross-actor coordination: (i) Forming value-creating partnerships that include the key actors responsible for value creation and other key actors who facilitate value creation in participatory planning. (ii) Early involvement of stakeholders with formal decision-making power who set the legal and operational boundary conditions through directives. (iii) Early and proactive exchange with stakeholders without formal decision-making power, who may act as proponents or opponents of BGI. (iv) Leveraging intermediaries who support implementation through higher-level coordination activities.

Further empirical analysis is required to determine whether our results can fully be applied to more (or less) complex cases or whether additional coordination is needed between actors and stakeholders, particularly in terms of leadership, intermediation, incentive, or compensation mechanisms. In this context, it is particularly important to identify suitable collaborative governance approaches aimed at facilitating the involvement of the relevant actors and stakeholders in planning, implementation, and management processes. A key question here is the role of leadership in BGI's success and how BGI can succeed without prominent leadership roles in municipalities. In order to obtain a comprehensive overview of the range of coordination tasks for the successful integration of different BGI measures with their varying complexity of actor and stakeholder constellations in the urban structures, further research should conduct comparative analyses of different BGI measures with varying complexity.

In any case, the analysis of the Bochum case provided valuable insights into and lessons on how collaborative processes and projects can evolve over time and how this leads to greater efficiency. These should be taken into account when designing collaborative governance for urban climate adaptation that involves BGI in other cities or contexts. Furthermore, we believe that the ITC has the potential to make a substantial contribution to climate change adaptation beyond BGI measures by facilitating collaboration between the relevant actors and stakeholders at an early stage.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.