you can use a non-linear deformer called bend to do it. That should work. One other way is to create a cylinder then duplicate it and scale it down a bit but make sure it extends beyond the edges of the original one (length ways) then use a boolean to get the hole for the arch. Then delete the bottom half of the faces and voila one arch As long as the cylinders have the same properties you should get very clean geometry.

If you need more help or a better explanation just post your questions

Alan