Six inches of rain in half an hour... Toowoomba, a town sitting in the caldera of an ancient volcano...
Personally, if I'm trying to get my Rover (or whatever) out of that situation, I'd forget the umbrella and messing with the tires, and just GET OUT! Releasing air from the tires may give better traction, but it also lowers your chassis deeper into the flood waters, so it's more likely to stall.
It Won't Happen Here.
It Won't Happen to Me.
Sure, I Can Get Across the Flood.
If I Hit It Fast, the Momentum Will Get Us Across.
Are the Kids Strapped In?
Twelve dead, so far.
Sue