Hi Mahmut,
Based on the current description it is difficult to gauge the exact cause of the issue.
However, I guesstimate the issue could either be due to improper selection of model, physical domain, boundary condition, or inappropriate initialization.
The example best suited to model your flow physics would be VOF Dam Breaking simulation. You would locate various video example to model the same for your reference.
In your case kindly check if you are modelling with the following
1.    VOF Method is used with species transport in case if the mixing of wastewater with ambient water is required.
2.    If the case is initialised appropriately i.e. if it has patching is done correctly, or the hydrostatic pressure is applied correctly.
3.    Check if there are any wall that are created in the domain which is not allowing the flow to pass.
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