EPANET Model

Regarding Water Quality and Reverse Flow:

Since version 1.1c, EPANET has kept proper account of reverse flow in its water quality module. The program divides each pipe into sub-segments to correctly model advective transport of water quality constituents during a hydraulic time period. The number of segments equals the pipe length divided by the product of flow velocity and water quality time step. When a new hydraulic period begins, a new set of flows exists and a new pipe segmentation is made. Mass in the oldsegmentation is transferred (overlaid) into the new segmentation. If the flow in a pipe reverses, the order of the segments is also reversed to keep mass moving in the proper direction.

Although a default quality time step is computed by EPANET based on the shortest time of travel through any pipe, it may be safer to use a user-specified value for this parameter. I tend to use a value between 3 and 5 minutes.

More details on water quality modeling algorithms can be found in the paper I wrote with P. Boulos, "Numerical methods for modeling water quality in distribution systems: a comparison" which appeared in Jour. of Water Resources Planning & Managment, 122(2):137-146,1996. In this article, the method used by the current EPANET version is called the Discrete Volume method while the more efficient method to be used in Version 2 of EPANET is called the Time Driven Method.

Lew Rossman
EPANET Author