The concern with wind-driven sand (or other material) is not so much with impact to the glass (using a hood will mitigate everything but a direct-to-the-face sand-blasting), but with the tiny particulate matter getting inside of the lens and camera (and to some extent into filter threads and hood attachment mechanisms). This is a danger even at very low wind speeds. And, even if you are using a weather-sealed lens/camera combo, there is a danger of getting sand or whatever inside your camera when you change lenses. I've had grit get under focus/zoom rings as well and it sounds horrible though I've not experienced any lingering problems as a result.
If you're planning to be out in the wind, there are a couple of things you can do:
- use a lens hood/shade (a good idea anyway)
- use a lens/camera rain cover or wrap
- carry a small soft brush and use that to somewhat vigorously brush off everything: the lens (except the glass!), camera, your hands — anything that's going to be near the opening — prior to changing lenses. And when you do change, obviously you're going to want to be out of the wind if at all possible.