Back-up flushing cannot be applied in cases of meshed networks or low line gradients. Instead of the required difference in the water level that produces increased flow velocities, the water levels in front of and behind the back-up device oscillate until they reach equilibrium conditions. In that case, the flow velocity between the device and the sewer wall does not suffice for the removal and transport of deposits. Furthermore, there is the risk that the artificially backed up sewage enters the surface waters at storm water overflows.