<< Click to Display Table of Contents >> Navigation:  Wadiso 6 User Guide > Time/Water quality simulation > Time/WQ techniques and general concepts > Time simulation water quality model > Basic Transport

EPANet’s water quality simulator uses a Lagrangian time-based approach to track the fate of discrete parcels of water as they move along pipes and mix together at junctions, between fixed-length time steps. These water quality time steps are typically much shorter than the hydraulic time step (e.g., minutes rather than hours), to accommodate the short times of travel that can occur within pipes.

 

The method tracks the concentration and size of a series of non-overlapping segments of water that fills each link of the network. As time progresses, the size of the most upstream segment in a link increases as water enters the link, while an equal loss in size of the most downstream segment occurs as water leaves the link. The size of the segments in-between these remains unchanged.

 

For each water quality time step, the contents of each segment are subjected to reaction, a cumulative account is kept of the total mass and flow volume entering each node, and the positions of the segments are updated. New node concentrations are then calculated, which include the contributions from any external sources. Storage tank concentrations are updated, depending on the type of mixing model that is used (see below). Finally, a new segment will be created at the end of each link that receives inflow from a node, if the new node quality differs by a user-specified tolerance from that of the link’s last segment.

 

Initially, each pipe in the network consists of a single segment whose quality equals the initial quality assigned to the upstream node. Whenever there is a flow reversal in a pipe, the pipe’s parcels are re-ordered from front to back.