If you don't mind how it looks, buy some wide fabric tape e.g binding from a haberdashers, and lay it over the carefully lined up tear. I'd sew it carefully by hand, but you could try good quality fabric adhesive. You could try carpet binding tape, but I don't think this is waterproof. Depending how long you are prepared to spend on the repair job, you could get a fairly neat, not very expensive repair. Oviously if you can match colours for the tape and thread it will look better, and you can give it a spray over with waterproofer from the camping shop if required.