This paper presents predictive discolouration modelling and subsequent field trial validation for a cast iron trunk main network. This enabled a UK water company to propose an ‘operational flow conditioning’ maintenance plan that reduces discolouration risk, improves network resilience and asset condition and yet does not require the trunk main to be decommissioned for invasive cleaning. This represents substantial time and cost benefits. Pre- and post-trial turbidity monitoring data are presented which identified a daily flux of material, a factor in the regeneration of material layers that have been shown to cause discolouration when mobilised. Additional data showing the occurrence of pressure transients are also presented, the latter being a possible cause of contaminant ingress and asset failure. After 6 months a second flow trial was conducted and modelled, confirming the regeneration of particulate discolouration material across the range of mobilising forces applied during the trial. It also indicated full layer development, or maximum discolouration risk, may occur in just over 2 years for this cast iron main. This highlights the need for appropriate maintenance strategies, such as periodic flow conditioning identified here, to mitigate discolouration risk and help safeguard water quality.