Predicting Rating Curves for Combined Sewer Overflows

Student thesis: Master Thesis and HD Thesis

  • Mathias Ulsted Jackerott
This project presents a method to predict the rating curves of combined sewer overflow (CSO)
structures based on the geometry of the structure and the connected sewer system. 3 important
parameters have been found to influence the rating curve; the weir coefficient, the capacity of the
discharge pipe and the resistance induced by potential screens. Methods to predict the weir coefficient
have been proposed for two types of CSO structures being side weir and stilling pond CSO structures.
Furthermore the location for installing a water level sensor giving the best prediction has been
determined. Based on CFD modelling of close to a thousand randomly generated CSO structures, it
was found that the weir coefficient can be predicted based on the Froude number and the geometry
of the weir crest. A method to relate the hydraulic resistance of a screen to the flow over a weir were
proposed, and the method has been validated and found accurate based on laboratory experiments
and CFD modelling. Based on the laboratory test it was further shown how the hydraulic resistance
of the screen increased as the screen clogged. A simple model of the CSO discharge pipe has been
proposed and validated against the results of CFD models.

Rating curves of 5 real life CSO structures have been predicted and compared with the results from
CFD modelling and real-life measurements. The predicted rating curves have been deemed accurate
for structures which falls within one of the two types of CSO structures.
Publication date9 Jun 2023
Number of pages100
ID: 533891761