Fredrik :
If you are using a "gated structure", then you are defining the center elevation of the gate with the entry "Spillway Gate Center Elevation". This is because DAMBRK is using the orifice equation, which relies on the center of the orifice for its computations.
However, if your outlet structure is a free surface weir, then use the entry "Uncontrolled Spillway Crest Elevation".
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com