boundary conditions in open rectangular channel

surya

please give me reply(urgent)

I am doing simulation of open rectangular channel using vof model. i have given the velocity at the inlet is 0.75m/s. i applied the boundary conditions as inlet end----------velocity inlet top free surface----mass flow inlet output end----------pressure outlet. I didn't applied patch for that. Even if i apply, i am not getting perfect curve. But we are getting the velocity profile as straight line.not a curve. I tried different combinations also, but still i am not getting the exact curve of openchannel flow.

can any one suggest what is the problem for me? whether i have to change any other boundary conditions? please give me reply?