re: Good question....
I know what you mean. Have you ever driven through the narrow canyon between Loveland and Estes Park, Colorado? There is a sign posted saying something about taking higher ground in the event of flash flooding. The canyon walls are straight up, so the sign is almost humorous. Unless you are a very good mountain climber, I don’t know how you could scale up the cliff if the canyon started flooding- but I would sure give it my best shot.