working with a digital elevation model
 

Hello everyone,
I'm very new to CFD and OpenFoam. I'm planning to simulate the flow of a River section. As data basis, I do have a very large DEM in GeoTiff format, which I converted into STL file.
I wanted to implement a structure/ bridge as an outlet in the STL. I drew the structure in a CAD program and exported it as STL file. I tried to merge the two STL files in Salome. I created water tight boundaries around the DEM surface. Now I'm having troubles to place the structure STL on the right position and replace a part of the boundary to make a water tight solid, which can be used in the snappyHexMesh utility.
Since it is very difficult to modify the STLs in Salome, I wanted to ask if there is a more convenient way, maybe in a CAD program, to work with terrain data to prepare it for the meshing?
Thank you so much in advance.

Regards, Theresa