Quick & dirty way...
- Mitre cut 30-degree six pieces of 6x1" timber, fit on perfectly flat
base, glue end grain until they form a 180-degree arch. Repeat from
the OTHER side so the joints are offset (stretcher bond in brickwork
speak), glue & screw the front & back together.
- Clamp to a flat surface. Create a bar of steel to link a plunge
router to the centre of the arch. Sweep the router repeatedly
increasing the depth each time until you have a perfect arch.
- Waterproof glue is required, kiln dried timber joinery grade 4
(IIRC) a good idea.

Very slow way...
- Buy veneer of whatever you want. Create former for the arch, screwed
& glued layers of MDF. Fold veneer around arch, glue, repeat until a
sufficient large number of bent layers built up. Leave to set as long
as necessary.

The quick & dirty way works fine, I have seen it done in both UK &
USA. I suspect, however, in the USA that curved arches are available
readily if not quite off the shelf (California for one). If you do not
have a plunge router you could jigsaw (decent pendulum ideally), then
simply plane & sand into the desired shape. I have seen that done on a
UK porch arch, vastly cheaper than the several hundred some fool place
wanted for a steam bent frame and built up as a laminate with
waterproof glue it was immensely strong).