I have a client supplied model with river reaches that surcharge and flood onto the 2D mesh with further exchange of flow as one reach spills into the next one downstream via the mesh. However, when surcharging of the reaches occurs, there is significant instability with flows that are nominally ~2m3/s oscillating wildly between -10m3/s and +60m3/s even when running with a 1 second timestep. The river reaches need to flood as there is a dam and throttle at the end of the valley and both the reaches and mesh provide storage. The question I have is: does the software allow for flooding in this manner to occur, or is it a quirk of the model that I am using that generates this instability.