Water Distribution System Asset Deterioration and Impact on Water Quality — A Case Study

This paper reports on the results of a repeated uni-directional flushing programme over a 4 year period as part of an asset maintenance study to prevent water quality failures through discolouration. By monitoring the resultant turbidity as accumulated material layers are eroded due to changes in system hydraulics it is shown that material regeneration/asset deterioration is a continuous, repeatable and therefore potentially viable modelling process. The monitored site had originally been selected following rehabilitation in 2001 which for the most part involved the scraping and lining of larger diameter cast iron pipes. The first monitored field study was 1 year after rehabilitation, the next a year later, then 2 years on and the final exercise a further year again. Flushing was conducted using a planned stepped increase in flow to assess the shear strength characteristics of the developing pipe wall particulate cohesive layers, primarily of iron and manganese composition in the UK, responsible for discolouration if mobilised. Once cleaned from the pipe walls it is hypothesised that the layers regenerate and are conditioned by the hydraulics found within the system. The results from the monitored pipe sections all show a similar and repeatable response to increasing system pipe wall shear stress, with each step mobilising additional material, corroborating the particulate cohesive layer approach to describing discolouration. Results also suggest that flushing was as effective as rehabilitation for controlling deterioration in this network and that the magnitude of a potential discolouration event is determined by the conditioning system hydraulics. These results can be used to inform operation and maintenance strategies to reduce the likelihood of a discolouration incident.