Session D9 - SCM maintenance

Blue–green infrastructures (BGIs)—engineered, nature-based stormwater control measures such as green roofs and bioretention cells—are increasingly used to mitigate combined sewer overflows (CSOs) at catchment scale. However, their hydrological performance can deteriorate over time due to clogging, litter accumulation, and sediment deposition. This study investigates how such temporal degradation of bioretention systems can influence long-term CSO volumes. We developed a deterioration framework based on discrete condition-state transitions governed by a semi-Markov process, applied at a daily time step. Transitions were parameterised using Weibull-based deterioration curves derived from empirical reductions in hydraulic conductivity, porosity, seepage rate, and storage-layer void ratio. Using a calibrated SWMM model of a mid-sized Swiss urban drainage catchment, we simulated 53 sub-catchments from 1987 to 2012 under stationary, no-BGI, and dynamically deteriorating BGI scenarios. The results show that BGI deterioration leads to progressively higher CSO volumes, yielding approximately 17% greater cumulative CSO by the end of the study period. These preliminary results provide a basis and rationale for further analysis, such as whether all BGIs require continuous observation or whether a smaller subset disproportionately influences catchment-scale CSO dynamics.

Nature-based stormwater solutions (NBSS) have become a vital component of urban stormwater management, offering hydrological benefits and ecosystem services. However, incomplete data on their long-term performance, operation and maintenance poses challenges for their sustainable asset management. This study presents an agent-based model (ABM) to address these gaps by integrating Fault Tree Analysis (FTA), field inspection data, and stakeholders’ opinions. Agents represent utilities, maintenance practices, failure events, and NBSS components, simulating their interactions over time. FTA and field inspection results help in identifying NBSS failures and their prevalence as well as assessing their effects on NBSS components, while survey data offers insights into maintenance practices and their frequency. ABM’s capacity to model emergent behaviours and simulate management strategies offers a promising path toward optimizing NBSS performance. This study advances the understanding of the interplay between NBSS and their socio-ecological environment and supports the development of proactive, data-driven management frameworks to sustain the long-term benefits of these critical infrastructures.

Green stormwater infrastructure (GSI) is increasingly used to improve water quality, reduce flooding, and promote urban greening. However, many installations do not function as intended, and the underlying causes of these failures remain poorly understood. This study synthesizes insights from a global survey of 43 practitioners and 14 interviews across Canada, Australia, the UK, and the USA to assess perceived performance, common failure mechanisms, and factors contributing to underperformance in roadside GSI. Most practitioners (71%) rated GSI as moderately successful, highlighting recurring design, construction, and maintenance issues. Failures commonly occurred within 1-3 years of installation, reflecting design and construction deficiencies and inadequate early-stage maintenance. Three primary failure mechanisms were identified: flow bypassing, reduced infiltration due to surface clogging, and short-circuiting. Key design issues included undersized or inappropriate inlets and unmaintainable features, while construction challenges stemmed largely from poor quality control and limited contractor experience. Maintenance issues were dominated by sediment accumulation and human-related disturbances. Governance barriers centred on funding shortfalls, staffing limitations, and poor interdepartmental coordination. Findings highlight the need for dedicated maintenance funding, improved municipal collaboration, enhanced contractor training, and more rigorous initial site investigations to reduce failure risks and build resilient GSI.

The long-term performance and management requirements of bioswales remain inadequately understood. Asset managers often rely on personal judgment or adopt a run-to-failure approach, guided by inspection reports that are frequently unreliable, incomplete, subjective, and lacking in standardization. This practice contributes to the progressive degradation and, ultimately, the functional failure of bioswale systems. Accordingly, this research aims to develop a structured and standardized inspection protocol that enables a uniform procedure and objective condition assessment of bioswales. The proposed protocol is developed through the analysis of 507 inspection records, supplemented by a comprehensive literature review. It introduces a detailed catalogue of system components, associated defect typologies, and potential failure modes. Each defect is accompanied by numerical scores, formal descriptions and a visual quality ruler to ensure clarity and consistency in interpretation. Furthermore, the protocol specifies the parameters to be evaluated, and the sequence and tools required for effective inspection. The outcomes contribute to the advancement of evidence-based asset management, enabling effective planning and optimized allocation of resources.