Without looking at the model, there are few items one could examine:
1. Are you running the model as a mixed flow regime? This may impact the computed WSEL profile.
2. Is it possible that the model is going through a hydraulic jump just downstream of the dam? This could be caused by having a higher Manning's roughness value or a constriction in the channel.
3. Do you have an instability in the model at the cross section in question? Perhaps changing from Option 1 to Option 13 would assist in solving that.
4. Have you reviewed the Reservoir Depletion Table that DAMBRK generates to make certain that everything looks OK? Perhaps this can shed some light into where your model is failing.
Sincerely,
Chris Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com