Extracting values 1m above a surface


I have a solved OpenFoam case with a 3D seabed geometry embedded in a mesh using snappyHexMesh. I would like to extract flow velocities 1 m above the surface (1m higher than the surface in z direction at each cell), which are stored in internal mesh. I would like to simply build a slice which is 1 m higher than a surface at every point (cell), and then export the values to create an ASCII raster for GIS purposes. Is there an easy way to do it?