Distribution and Properties of Defects in Sewer Systems: An Analysis of Auckland’s Sewer Network

Abstract Sewer systems are essential for public health and environmental protection; however, they deteriorate over time, potentially leading to significant impacts. Although numerous models have been developed to assess pipe condition, most rely on aggregated closed-circuit television (CCTV) scores and do not incorporate detailed defect-level analysis. Although defect-based approaches have shown strong potential, they often don’t account for defect properties, leaving a gap in the comprehensive analysis of individual defect types and their characteristics. This study addresses this gap by conducting a descriptive analysis of sewer defect types and their properties across different pipe materials. The analysis is applied to CCTV data from Auckland’s sewer network, covering five pipe materials: three rigid—asbestos cement (AC), concrete (CONC), and vitrified clay (VC)—and two flexible—polyvinyl chloride (PVC) and polyethylene (PE). The analysis includes an assessment of data set representativeness and a graphical evaluation of defect patterns. The results reveal clear material-dependent patterns. Rigid pipes exhibit higher defect densities and are mainly affected by Joint Faulty and Root Intrusion defects, likely related to their short, segmented construction. In flexible pipes, PVC is also strongly affected by Joint Faulty, whereas PE is predominantly affected by Dipped Pipe. Defect property analysis further highlights distinct patterns. Cracks tend to be larger in PVC pipes, whereas Surface Damage is more pronounced in rigid pipes. Spatial analysis shows that Joint Faulty defects mainly occur at pipe ends, whereas Debris Grease and Surface Damage are typically observed near the pipe inlet and often extend along the pipe length. Circumferential analysis showed that mechanically induced defects (e.g., Joint Faulty and cracks) tend to occur near the pipe top, whereas Obstructions and Surface Damage concentrate at the pipe bottom. These findings provide new insights into sewer deterioration processes, support more informed decision-making, and offer a transferable methodology for application to other drainage systems.