Modeling Low Velocity/High Dispersion Flow in Water Distribution Systems

Under most circumstances, dispersive processes can be neglected in water distribution systems due to high velocities and low dispersion coefficients that ensure that advective transport dominates constituent spreading. However, during periods of low flow, dispersive effects may become important if the velocities are significantly decreased (e.g., during the night). Thus, the question that arises during a 24-h simulation is whether dispersive processes may dominate the transport of chlorine in a water distribution system. Given that advective transport models (e.g., EPANET, PICCOLO, NET, and DWQM) cannot account for dispersive transport, it is likely that these models would underpredict the required concentration of chlorine at locations behind the advective front and overpredict the required concentration at locations in advance of the advective front. Therefore, when is a purely advective transport model an appropriate solution for contaminant transport in a water distribution system? Two constraining equations are presented to aid the user in assessing the applicability of an advective transport model.