Flash Floods
Flash floods happen suddenly (hence the name), forming in less than six hours. This sets them apart from “regular” floods, such as river floods, which can often be predicted days in advance. They are caused when a heavy rain falls over low-lying areas, especially when the soil is already saturated. They happen when rainfall intensity and duration match up, in a very bad way.
More than half the fatalities happen when people try to cross flooded intersections in their cars. This is because people are generally not very good at judging the depth and danger of flooded streets. (They can’t actually see the roadway surface, which might be a big deep hole.) While two feet of water doesn’t seem like much at all, especially if you are safely inside a big, heavy car, it is enough to carry you and the car away. (Another fact we tend to forget: cars are buoyant.)
According to the NOAA , “Water weighs 62.4 lbs. per cubic foot [1,010 kg per cubic meter] and typically flows downstream at 6 to 12 miles an hour [10 to 20 kph]. When a vehicle stalls in the water, the water’s momentum is transferred to the car. For each foot the water rises, 500 lbs. [226 kg] of lateral force are applied to the car. But the biggest factor is buoyancy. For each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water. [For each meter the water rise up the side of the car, the car displaces 2,040 kgs of water.] In effect, the car weighs 1,500 lbs. [2,040 kg] less for each foot [meter] the water rises.”
Do not be tempted to drive across a flooded roadway.(”Don’t drown; turn around.”) If you find your vehicle stalled in a flooded road way, leave it and seek higher ground. (”Better wet than dead.”)
This video on YouTube of a flash flood in Merriwa, NSW, shows just how quickly a flash flood can occur.