SKS thermoplastic fenders have worked very well for me commuting. Use the plastic quick releases to avoid a bad situation if you hit debris.
As for debris, whenever I've gone over gravel on the commute, I can hear the tire kick up the gravel bits and they get sent all the way round the tire rolling along the fender gap. I have a feeling this can be a very bad thing if you factor in even light mud collection, along with grit and gravel bits. The gap is good for water, but not stones, possibly giving a sudden nasty result.
I'd imagine for wet gravel a better option is a high tail type fender, like a motocross bike. It'd prevent major muddying of your back and face, but leave room for debris clearing. Strategically placed guards could keep drivetrains a little cleaner but sounds fiddly