Blue-green infrastructure (BGI) plays a significant role in the resilience of Dutch cities to the rising societal challenges and climate change effects. However, their condition is often neglected by asset managers, possibly resulting in operational failures and rapid deterioration. This research aims to identify the failure mechanisms of three commonly used BGI systems (permeable pavements, bioswales and retention ponds) during their whole life cycle. A comprehensive review of literature was conducted first, along with experts' assessment. Fault tree analysis was used to explore failure modes, degradation processes and root causes. Root causes were evaluated through minimal cut sets analysis and ranked by their criticality, propagation and impact. The findings were validated by a team of experts and structured in three fault trees accounting for 400 possible failure mechanisms: 84 for retention ponds, 115 for permeable pavements and 201 for bioswales. Clogging and hydraulic overloading emerged as key failure mechanisms in permeable pavements and bioswales, triggered by diverse root causes. Failures in retention ponds are largely linked to permanent pool conditions. The results provide a foundation for developing standardized inspection and monitoring protocols that address current gaps in the asset management of BGI.

Co-located infrastructure networks such as road, water, and sewer in theory offer the possibility for integrated multi-infrastructure interventions. However, how closely these networks are aligned in space and time determines the practical extent to which such coordinated interventions can be realized. This study quantifies the spatial alignment of the aforementioned infrastructure networks and demonstrates its application for integrated interventions and potential cost savings. It proposes two metrics, namely 1) shared surface area and, 2) shared trench volume, to quantify the spatial relationship (i.e., degree of co-location) of infrastructures. Furthermore, the study demonstrates how the degree of co-location can be used as a proxy for cost-saving potential of integrated interventions compared to silo-based, single-infrastructure, interventions. Through six case studies conducted in Norwegian municipalities, the research reveals that implementing integrated interventions across road, water, and sewer networks can result in potential average cost savings of 24% in urban areas and 11% in rural areas. Utility-specific savings under different cost-sharing scenarios were also analysed. To identify the yearly potential of integrated multi-infrastructure interventions, future work should add the temporal alignment of rehabilitation of infrastructures (i.e., time of intervention need for the infrastructures).

Urban drainage systems have developed way beyond the traditional piped combined or separate sewer systems. Many ‘new’ systems are being introduced, ranging from stormwater infiltration facilities to green roofs. However, the widely advocated blue-green infrastructures are typically overlooked by asset managers, which will very likely have detrimental effects on their performance, service life, and wider adoption. In this paper, the working group on Urban Drainage Asset Management (UDAM – https://udam.home.blog/) of the IWA and IAHR Joint Committee on Urban Drainage discusses whether the state-of-the-art knowledge based on conventional sewer asset management is sufficient to develop asset management for blue-green infrastructures (BGIs). The discussion is structured around the five preconditions for effective control and asset management. Results show that asset management for BGIs is still underdeveloped due to a lack of monitoring techniques covering the broad range of BGI benefits and performance indicators, inspection techniques covering relevant failure mechanisms and models describing these mechanisms, maintenance and rehabilitation options, and sufficient support tools to aid inhabitants in the operation and maintenance of their individual BGIs such as green roofs or vegetated swales.