Oswaldo:
Assuming that you have sufficient inflow into the reservoir, it sounds as if the piping failure is not growing as fast as the overtopping failure is. As such, the water surface elevation for the piping failure reservoir model is rising further than the overtopping failure reservoir model.
Or, it could be something else that is wrong with the model.
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com