Below is a simple solution, not yet taking into account that you might want to have different inner sections for the ‘arches’. The loft component is set to rebuild the offset curves which results in a nice polysurface.
PS: please not that in your unnamed Grasshopper definition above, you are attempting to orient the closed polysurface from Rhino more than 20000 times on your curve. That is a bad idea
Although I think you’d have better luck just scaling a curve at those points then sweeping all the curves afterwards so you’re not affecting the section thickness of those arcs with the scaling.
Again, I appreciate your time and effort! I am trying to adjust Ftzuk’s work. Hmm, it would be possible to adjust the size more flexibly like the image that I attached?
What Ftzuk did would work but what if straight-line - curved up - straight line - curved down - straight line again, something like that more than the wave. Just like more controllable. Sorry if I am asking too much lol.
You can achieve this by creating a linear representation of your tunnel and drawing a curve which defines the height. The distance between the linear axis and the height curve can either be remapped to whatever domain or it can be divided by the height of your master section.