Towards the intentional multifunctionality of urban green infrastructure: a paradox of choice?

The research question addresses the gap between research on multifunctional Green Infrastructure (GI) and its holistic implementation in urban areas. The study's purpose is to provide specific guidance for coordinating the planning, design, and construction of GI by analyzing existing academic literature. The importance of the study lies in its potential to bridge the gap between theory and practice, leading to more effective and sustainable urban development. Green Infrastructure (GI), encompassing structural Nature-based Solutions (NbS), is intentionally placed in urban landscapes to provide various ecosystem services such as stormwater attenuation, climate regulation, and habitat conservation. A key aspect is its multifunctionality—the ability to simultaneously perform multiple services, promoting synergies and reducing trade-offs. The potential benefits of GI are numerous and well-documented, including heat mitigation, biodiversity protection, stormwater management, and improvements to human health. Despite these benefits, the potential of GI to address multiple ecological, social, and economic factors is often overlooked during the engineering design and construction phases at the municipal level. This disconnect stems from variations in terminology and the siloed approach of agencies governing different ecosystem services. Public and private entities often focus on single issues, leading to scattered GI elements without strategic connections. Multifunctionality is not yet an intentional consideration throughout the planning, design, and construction phases, resulting in missed opportunities to address pressing issues such as climate change and social justice. This study aims to provide specific guidance on aspects that could be jointly considered between GI planning, design, and construction entities, analyzing the academic literature and identifying areas needing further attention.

The literature review synthesizes existing research on green infrastructure (GI) elements and their associated objectives. It examines different terminologies used to describe GI across various regions and sectors, highlighting the inconsistencies in definitions (e.g., Green Stormwater Infrastructure (GSI), Sustainable Drainage Systems (SuDS), Water Sensitive Urban Design). The review explores the diverse perspectives on GI from different disciplines, including engineering, urban ecology, and urban greenspace planning. The review categorizes GI elements into 15 distinct categories (e.g., green roofs, urban streams, pervious surfaces, urban parks) and compiles a list of 15 broad objectives encompassing ecosystem functions, services, disservices, and benefits (e.g., flood risk mitigation, water management, heat mitigation, biodiversity, social justice, economic development). The review reveals a significant gap in the literature regarding the intentional consideration of multifunctionality throughout the GI lifecycle. The existing research frequently focuses on isolated objectives and elements, neglecting the potential synergies between different GI components and their contributions to multiple services. Many studies highlight the proven ecosystem services of GI and its potential to address multiple ecological, social, and economic factors, but these benefits are often considered after installation rather than integrated into initial planning and design. The review highlights the need for stronger coordination between entities involved in GI planning, design, and construction, advocating for a proactive, systems approach.

The study employed a comprehensive literature review using the Web of Science database. The researchers queried the database for a range of 15 GI elements and 15 objectives, resulting in 225 queries in total. The search terms for each element and objective are detailed in supplementary tables. The searches were conducted across all databases and collections, limiting document types to articles or review articles to ensure the analysis focuses on peer-reviewed literature. Urban keywords such as "urban," "built environment," "city," and "cities" were included in all queries to focus the analysis on urban areas. The results of the literature queries were summarized in tables, which show the number of publications discussing specific combinations of GI elements and objectives. The data was then normalized to calculate the percentage of publications for each objective represented in the GI element literature and the percentage of publications for each element represented in the objective literature. These percentages were used to create a matrix visualizing the relationship between GI elements and objectives, highlighting the extent to which different objectives are considered in the literature for different GI elements and vice versa. The matrix illustrates the dominance of certain objectives for particular GI elements and the underrepresentation of others, revealing existing silos in the literature and highlighting areas where further research and coordination are needed.

The study's key findings are presented in a matrix (E/O matrix) that maps 15 green infrastructure (GI) elements against 15 objectives. The matrix reveals significant silos in the existing literature. The literature on "engineered" GI (e.g., pervious surfaces, infiltration systems, detention basins) predominantly focuses on water-related objectives (stormwater management, water quality, water provision). Conversely, the literature on less engineered GI elements (e.g., urban parks, trees, bare earth) tends to concentrate on biodiversity and human well-being, largely omitting water-related aspects. The matrix shows that some objectives dominate the discussion for particular GI elements (e.g., heat mitigation for vertical greening systems, human well-being for urban parks, biodiversity for trees). Conversely, certain elements are largely absent from the literature for specific objectives (e.g., infiltration systems for non-water related objectives). Objectives like social justice and noise mitigation are significantly underrepresented across the board. The study identifies uneven discussion of GI elements across the objective literature; some GI elements dominate the discussion for a particular objective (e.g., trees for disaster mitigation, non-infiltrating water storage for water provision). The analysis reveals that only the literature on stormwater management and biodiversity considers all 15 GI elements. The overall finding points towards significant silos between objectives and elements, highlighting underrepresented elements and objectives and the need for greater integration of multiple objectives in the planning, design, and implementation of GI.

The identified silos in the E/O matrix hinder the coordination of multifunctional GI systems in practice. The study demonstrates a clear divide between the GI elements discussed in water-related literature and those addressed in non-water-related literature. This reveals opportunities for expansion of objectives within engineered GI, such as integrating carbon sequestration, air pollution reduction, and heat mitigation into the design and implementation of vegetated infiltration systems. The different entities controlling the elements and objectives within the matrix also control implementation and maintenance budgets; thus, understanding the gaps in information within the matrix can inform practitioners about opportunities for innovation and coordination. Entities focused on single objectives need to expand their scope to include other relevant objectives. The study suggests that increased funding for interdisciplinary projects can further facilitate coordination among academic disciplines. The findings indicate a critical need for a systems-thinking approach that integrates multiple objectives throughout the planning, design, and maintenance of GI. Challenges such as the lack of quantification for some objectives, difficulties in monetizing the benefits, and the need to account for potential disservices and trade-offs are highlighted. The study emphasizes the importance of considering the impacts of extreme weather events related to climate change and the need for climate-resilient GI designs.

This study demonstrates the need to address multifunctionality across a range of GI elements and objectives. The presented E/O matrix serves as a guide for researchers and practitioners to consider 15 elements and 15 objectives during GI planning, design, and implementation, promoting systems thinking and cross-sector coordination. Future research should focus on developing tools for multifunctionality assessments to determine optimal numbers of design objectives for specific sites, considering the surrounding GI system. Transitioning from unintentional to active and integrated planning decisions, coordinated across sectors and scales, is crucial. This assessment could be utilized by any entity managing a component of the E/O matrix and employed to support policy development. Trans- and interdisciplinary research collaborations are essential for successful implementation of multifunctional GI.

The study's analysis relies on a Web of Science query of peer-reviewed articles, which may not fully reflect municipal action and understanding. The distribution of research funding may have influenced the topics covered in the literature, potentially creating an uneven representation of objectives and elements. The study does not quantify the value of each objective, which limits the ability to conduct cost-benefit analyses and makes it challenging to fully understand trade-offs between objectives. The study primarily focuses on the academic literature and does not delve into the practical challenges and constraints faced by practitioners in implementing multifunctional GI systems.