Large domain free surface simulation
 

I have a control volume of size (1000,500,50) in my simulation. The problem is to model a large two phase release in an open area . However, I find it difficult to mesh this domain. I am getting a error that "software has encountered unhandled exception" . I have cleared the generated mesh many times . Still I get this error".
I want to refine the mesh near the inlet and the free surface. The control volume has an atmospheric boundary layer and an inflation is required in the region near the ground and this is where the free surface also lies.
I would like to know how to mesh this geometry properly for multiphase simulation. I have attached the a picture of geometry with this file. The box was done using enclosure option. A small dot in the left hand side of the geometry is the release point.