I want to add a hydrodynamic pressure using the Westergaard added mass interface for a flat surface that is submerged completely underwater (does not extend to the water free surface). I've consulted the documentation on the method here: Westergaard's added mass for hydrodynamic pressures: a simple case - GeoStudio | PLAXIS Wiki - GeoStudio | PLAXIS - Bentley Communities, but the example does not give insight on how the element actually works and the example is for a surface that extends to the water free surface.

For my case, can I simply have the interface extend to the top of my surface I want to apply the hydrodynamic force to (below the water free surface) and the program will calculate the pressures appropriately? Or will I have to apply a hack which extends the interface all the way to the water surface and have an opposite reacting force above my surface?