How can I model turbidity in WaterGEMS or WaterCAD?
First, it is important to understand the nature of the problem you are trying to solve.
Unlike other constituents, there is no Law of Conservation of Turbidity. Essentially it is suspended solids that are conserved. For a research paper on this subject, see "Can there be a Law of Conservation of Turbidity" by Walski, Minnich, Sherman, Strause, Whitman.
As long as settling and suspension isn’t involved and there is a linear relationship between turbidity and suspended solids, then you can consider modeling turbidity as a constituent.
If you are trying to model settling and resuspension, that is entirely different. There is a model called PODDS from the Univ. of Sheffield in the UK which can predict settling and resuspension. But, it is focused on rusty water and isn’t a true network model.
Once you understand that the problem is one of resuspension, the Flushing tool could be considered, as it helps with planning a flushing program. Meaning, instead of modeling the solids, just clean them up by providing scour velocity.