Determining Dispersion Coefficients in Sewer Networks

• Main Author: Wagstaff, Joshua G.
• Format: Others
• Published: BYU ScholarsArchive 2014
• Subjects: sewer; dispersion; coefficient; Advection-Dispersion; pipe flow; Civil and Environmental Engineering
• Online Access: https://scholarsarchive.byu.edu/etd/3979; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=4978&context=etd

This work determines a suitable value for a dispersion coefficient to be used in the One-Dimensional Advection-Dispersion equation to model dispersion within sewer collection systems. Dispersion coefficients for sewer systems have only recently begun to be studied, and there is not yet an established value that is commonly accepted. The work described in this paper aimed, through observational study, to find a suitable value to be used. Salt tracers were placed in two separate reaches of sewer line. The first line studied was a straight, linear reach of sewer that included three manholes. The tracer was placed in the first manhole and the conductivity was measured at the two consecutive manholes downstream. These measurements were compared to a model developed using the 1D Advection-Dispersion Equation. The flow information and sewer network geometry was used in the model and the dispersion coefficient was adjusted to find a best fit. It was found that a value of 0.18 m2/s for the dispersion coefficient provided the best statistical match. The next reach of sewer that was studied was a reach with a 90 degree angle. This section was chosen to observe the effect that mixing has on dispersion, because of the change in direction of flow. The same procedure was applied, and an optimal dispersion coefficient of 0.22 m2/s was found. These values represent optimal dispersion coefficients under a specific set of conditions. It should not be assumed that they will provide accurate results in all circumstances, but are rather a base point for average flows under dry, stable conditions. Using these values inferences can begin to be made about dispersion characteristics throughout the entire sewer network. This can lead to specific engineering applications, and well as applications in other fields of study.