Hi - I just replied to your other post, but didn't see that you had added more information here. I am not sure I understand your question fully (for example, why is the flow sinusoidal?), but here are some general comments...
Because you have an impulse turbine, the best way to model this in HAMMER is to approximate the gate closure by using a 'Discharge to Atmosphere' element:
The data shown above will close an initially open valve over 20 seconds. This should approximate your turbine gates closing over 20 seconds. As the valve shuts, the pressure will rise in the upstream pipeline.
If you want more control over the valve closure, you could use a TCV element instead. The TCV allows you to enter a curve of valve (gate) closure versus time. For example you could have the gates close quickly until they are 30% open, then close slowly for the final 30%.