I have an issue with modelling a dam breach flood (unsteady flow analysis) using HEC RAS 5.0.6. I have done this in the past, by using early versions of the software (5.0.3, 5.0.4) and it worked. Now, I need to simulate another dam breach flood but after I run the model without any error, I get an inappropriate result downstream of the dam. The software does not model the flood accordingly, all I get is a pool of water downstream, instead of a dynamic water flow. The reservoir is empty and all the water is concentrated in the downstream of the Flow Area, near the External Boundary Condition. I have tried with different water volumes and I still get the pretty much same result, which is absurd. I have done the simulation using the same methodology applied in the past, when all worked well but now it seems that it doesn't. Did someone experience similar issues? If yes, how did you manage to solve it?