[(morteza)]

Dear all,

I am going to specified upstream boundary condition at inlet of bottom outlet of a high dam. Now, is it necessary to simulate all of reservoir upstream water elevation or not?

Thanks

[JBurnham]

No, it is not necessary to simulate all of the upstream reservoir, as long as you know the depth of the upstream reservoir acting on the outlet. Use a pressure-type boundary, and set the pressure at the boundary = Patm + RHO*g*h, with h = distance from the surface to the pipe.