Define interface of Air-water with fine mesh as free surface level
 

Hi everyone,

I am currently simulating open channel flow with wave boundary conditions. Problem definition includes rigid plate, which has been considered as a rigid body, and it is floating over the water and should move along waves. I am getting many errors while applying such boundary conditions, the main issue with defining setup is about interface at the free surface.
Can anyone suggest regarding mesh at the interface?
Any kind of help or suggestions in this kind of simulation will be appreciated.
Thanks!