I don't know of any waterproof trews that will stand up to the same knocks as cotton or ventile, maybe look at hunting or mountain clothing, they'll have reinforced knees and seat. The Haglofs Rugged Mountain and Norrona Dovre Heavy Duty offer water resistance but wet out eventually in persistent rain.
I tend to wear army goretex trews for knocking about in the bush, if they get ruined then it's not too expensive to replace them. I've got some Norrona Dovre goretex trews but they're only Pro Shell so will snag on a bramble probably, I tend to wear mine in the canoe and swap for my swedish splinter camo goretex if I'm in the woods.