Intense rain with insufficient areas to absorb the water can result in flash floods. In the US the National Flood Insurance Program defines it as “rapid flooding of low-lying areas in less than six hours, which is caused by intense rainfall”.
Flash floods often occur where the soil is too dry to quickly start absorbing water, where the topography forces the water into narrow valleys, and where wetlands have been drained and streams walled. In some areas quite moderate rain much upstream can cause a flash flood with little warning.
Another source of flash floods is volcanic activity under a glacier, which will melt extreme amounts of ice. If the water is contained by an ice barrier, even billions of tons of water may suddenly get loose. Similar outburst can occur without volcanic activity, when barriers of ice lakes break for other reasons. Such jökulhlaups are recurring events in Iceland, destroying roads and bridges on their way to the sea (mostly in uninhabited areas).
Like in the case of a tsunami, immediately try to get to high ground when there is a flash flood warning. Try to keep a flotation device near you. When the flash flood strikes, don’t try to walk or swim — the current is stronger than you and as little as 6 inches (15 cm) of rapidly moving water can knock you down (not to mention items from trash cans to branches that may be carried by the water).
Driving is also a bad idea; two feet (60 cm) is enough to sweep your vehicle away; even less is enough to make steering impossible and probably also to stall the engine. If you see a large strip of standing water on the road in front of you, do not pass through it, as you might not know if it's deep enough to stall your car and strong enough to sweep you away. You’re actually more of a sitting duck if you’re in a car and the water is rising. Open the car door if you can. If the water level is high enough to block the door, you'll have to roll down a window or break it to escape.
In areas where flash flood can be expected, avoid camping near streams and rivers (including wadis, dry riverbeds), which obviously are where most of the water will go, causing flooding. Be especially careful with canyons, from where it is difficult to escape and where flash floods often really are worthy of their name.