I have done this before. It consisted of 3 arches. Center one ,1 ft taller than the 2 sides. Came out very strong. It replaced a wall that to my surprise had contained a double pocket door I found during demo. This was done on the bottom floor of a triple decker. Not a bit of movement in the five years I remained living there.
Figure the radius you want for the inside and don't forget the sheetrock covering it. Using 2X4 or 6 (whatever). Score a cut at a depth approx 1/2 the thickness of the stud at the desired radius from the end of the workpeice. Clamp to a true flat surface (lathe bed). Raise peice up until the sawcut closes up and you will get the spacing for the sawcuts you just made 1 of, to acheive the radius you desire.
When I did this, I left 1 1/2 ft of straight stick below and put a half lap to marry the rest of the upright to.
The Little Old Man, Working in His Shop