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[netcdf-java] Remove z levels

Hi all,
I'm new to netCDF and I've a simple (I think) problem.
I've a netCDF file containing currents forecast (u and v variables) for 3 days and 33 deep ocean levels. Now I would to remove all deep levels but the first one (0.0 - surface level).
How I can do it?
I tryed this but does't works:

        String fileIn = "C:/Users/Test/Desktop/latest.nc";
        NetcdfFileWriteable  aa = NetcdfFileWriteable.openExisting(fileIn);
        List<Variable> vars2 = aa.getVariables();
        for (Variable va : vars2) {
            System.out.println("Variable name :" + va.getFullName());
            List<Dimension> dims = va.getDimensions();
            for (Dimension di : dims) {
System.out.println("dim name: " + di.getName() + " len:" + di.getLength());

                if (di.getName().equals("Depth")) di.setLength(1);
            }
        }
        aa.finish();
        aa.finish();
        aa.close();

This is my output:
Variable name :u
dim name: MT   len:3
dim name: Depth   len:33
dim name: Y   len:30
dim name: X   len:38
Variable name :MT
dim name: MT   len:3
Variable name :Depth
dim name: Depth   len:1
Variable name :Latitude
dim name: Y   len:30
dim name: X   len:38
Variable name :Longitude
dim name: Y   len:30
dim name: X   len:38
Variable name :v
dim name: MT   len:3
dim name: Depth   len:1
dim name: Y   len:30
dim name: X   len:38


What could be wrong?

Best regards
Giovanni