Flash floods are a potential danger to travelers because they are the occurrence of very heavy rains within a short period.
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.
On other occasions, heavy rain can cause flooding due to flooding of the drainage system. In such cases, rainwater accumulates on roads and from there, can spread to sidewalks, driveways if applicable, and ultimately houses.
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 rural areas).
Areas that are at persistent risk of 'flooding' tend to have formalised flood plans, whose implementation starts even as the rain is falling. Listening to local media if rain seems likely may be worthwhile.
Like in the case of a tsunami, immediately try to get to high ground when there is a flash flood warning. Conversely low-lying and underground places like metro stations and underground shopping malls are extremely dangerous: 14 were drowned in Zhengzhou's metro during a massive flash flood in 2021. Try to keep a flotation device near you. When 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). There could also be hazards like missing manholes or bare electric wires underneath water, which are equally dangerous to water current.
In areas where flash floods can be expected, avoid camping near streams and rivers (including wadis, dry riverbeds), which obviously are where most of the water will go. You should also stay away from downstream directions of dams, especially if they were poorly maintained: dam failures are much more destructive than floods. Be especially careful with canyons, from where it is difficult to escape and where flash floods often really are worthy of their name.
In desert areas and canyons, keep a watch for rain off in the distance: flash flooding can occur in areas without rain in the immediate vicinity due to rain further away. In deserts, the sun-baked ground has little capacity to absorb even the slightest of rains. In canyons, the more narrow the canyon, the more force the water will carry.
In low-lying areas, a metre can be the difference between inundation and no damage. Shelter locations should be raised above their surroundings, a frequent practice in the construction of modern facilities and commercial buildings.
In a vehicle
When heavy rain strikes, drivers should totally avoid tunnels and underground parking lots. Underground flooding easily scraps your vehicle, if not take your life simultaneously.
Driving through flood water is a bad idea; two feet (60 cm) is enough to sweep your vehicle away; even less is enough to make steering impossible and highly likely 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 (as soon as possible) 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 the 48 hours following a severe flood, avoid roads unless absolutely necessary. Even if your particular area is not flooded, low segments of major roads could remain underwater until evaporation or drainage have their impact and it's common for vehicles to be damaged under such circumstances. If bad weather is forecast for an area, make sure you have adequate supply of fuel, non-perishable food and water prior to the storm so travel is unnecessary while roads remain flooded.