Laboratory studies investigating the processes leading to discolouration in water distribution networks

Results are reported from laboratory experiments conducted to investigate the processes of discolouration within a water distribution system and specifically the concepts underpinning an empirical model proposed by Boxall et al. [Boxall, J.B., Saul, A.J., Skipworth, P.J., 2001. A novel approach to modelling sediment movement in distribution mains based on particle characteristics. Water Software Systems 1, 263-273.] and field validated by Boxall and Saul [Boxall, J.B., Saul, A.J., 2005. Modelling discolouration in potable water distribution systems. Journal of Environmental Engineering ASCE 131(5).]. The model is based on the hypothesis that discolouration is caused by the erosion and transport of fine particles, typically dominated by iron and manganese in the UK, that are attached to the pipe walls of the system by forces in addition to self-weight. These particles display cohesive-like properties and build up in layers on the pipe wall, conditioned by the usual daily flow patterns within the system. Discolouration events are caused by erosion of these layers due to changes in the system hydraulics and specifically changes in shear stress at the pipe wall, for example due to change in demand, a burst or the opening of a fire hydrant. Once cleaned from the pipe walls the layers re-accumulate under the usual conditions within the system. Experiments to determine cohesive layer behaviour and strength characteristics involved development periods followed by the measurement of the resultant discolouration when accumulated material was eroded by an increase in pipe-wall shear stress. The results support the empirical model concepts and hence its application. The results also suggest that the generation of material layers is influenced by the range of daily flow patterns, with greater variability reducing material accumulation, but not by the magnitude of steady state hydraulic conditions.