How Fast Is Flash Flood?

Flash floods rise and surge at about 5 m/s — fast enough to carry a car at just 30 cm depth. Six inches of moving water can knock an adult over, which is why emergency services drill the message: turn around, don't drown.

Flash floods can carry boulders the size of cars — the deadly 1972 Rapid City flood moved 100-tonne rocks several kilometres downstream.

• Same pace as a galloping horse
• Just 30 cm of water can carry a car