Dear Miroslawa Hirna :

The model might be experiencing some tailwater submergence effects that are preventing you from lowering the piping elevation at the bottom of the dam. Or, there might be some other effect that is causing this problem.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com