How do I release a screw whose head is flush with the surface and the notch is so worn down that the screwdriver can't make contact?

My driveway is slanted downhill, and at the bottom is a drain. The drain apparently is clogged with debris, and I want to open it up to get rid of the nasty muck. However, the drain cover is held down on one side with a screw that cannot be unscrewed. I tried prying along the edges of the circular drain lid, which loosened it a bit, but there does not appear to be an easy way to open it.

Is there a way short of breaking the whole drain cover to open this screw/cover?