Best design practices for holistic blue green infrastructure in the Nordics : a design-based case study of constructed wetlands and retention ponds in Lahti, Finland

Abstract

As urbanization accelerates globally, cities face unprecedented challenges that jeopardize their sustainability. Nature-based solutions (NbS) have emerged as multifunctional alternatives to conventional grey infrastructure, addressing societal challenges while providing ecological benefits. While research interest has bloomed, gaps have also emerged, leaving room for misuse and maladaptation. Two gaps in particular are the need for more site-level design analysis to supplement NbS implementation, and the need for more empirical evidence in the Nordic socio-ecological context. This thesis aims to bridge these gaps by conducting a detailed case study on the design and impacts of two blue-green infrastructure (BGI) projects in Lahti, Finland from a site-level design perspective. Based on the core components of NbS (addressing a societal challenge, providing biodiversity enhancements, and improving human wellbeing), three parameters were selected. Thus, this paper examines the effectiveness of these BGI projects in terms of stormwater management for improved water quality, hbiodiversity enhancement, and physical wellbeing.

The research employs a mixed-methods approach, combining quantitative data collection from two case studies with qualitative benchmarking of best practices. The methodology includes water quality sampling, habitat mapping, and robust physical well-being evaluations. The study integrates a Theory of Change model to link design decisions to observed outcomes, providing insights into the practical implications of BGI design and implementation. This is also supplemented by a benchmarking exercise to uncover best practices in BGI within similar contexts.

Key findings indicate that targeted design interventions, such as strategic plant placement and varied water depths, significantly improve water quality. The species-area principle is also observed as the larger site allows for more opportunities for enhanced biodiversity. While in general co-benefits are observed, the magnitude differs depending on whether targeted design interventions for that specific outcome were present. On that front, both case studies has the most room for improvement in terms of physical wellbeing. The study also highlights the importance of early integration of BGI in urban planning to maximize ecological and social benefits. Continuous monitoring and adaptive management are essential for optimizing NbS designs over time.

This research contributes to the empirical evidence base for NbS in the Nordic context, offering valuable insights for designers, planners, and policymakers. The findings underscore the need for detailed, context-specific design strategies and inclusive planning to ensure equitable access to green spaces. Future research should focus on long-term effective monitoring systems, precise targeting of design interventions for intended impacts, and exploring ways to integration BGI in space-constrained urban settings.