The fast current of a flash flood is one of its biggest dangers. But if someone is trapped by a flash flood-- clinging to a tree branch or perched on the roof of a car--try using that speed to your advantage.
Tie a rescue rope to a solid object (a tree, for example) to anchor it against the weight of the victim and the flowing water's immense pressure.
Coil the rescue rope and throw it upstream of the person you're rescuing, allowing the current to carry the line to the victim. Instruct that person to tie the rope around his or her waist.
Once secured to the rope, the victim can leave his or her perch and work toward the shore.
This tip is from OL's Ultimate Survival Manual. When disaster strikes, time is of the essence. The Ultimate Survival Manual has 333 skills that will get you out alive whether you're facing a disaster in the wilderness or an urban catastrophe.