Flash floods: in valleys, underpasses and behind the wheel

Normally part of the rain soaks into the ground, part evaporates and only the rest runs off. In heavy rain, so much water falls in a short time that the ground cannot absorb it fast enough. On steep slopes, on rock, on dried-out, hard soil or on sealed surfaces such as asphalt and roofs, almost everything runs off immediately.

The water collects in channels, small streams and streets and flows to the lowest points: narrow valleys, gorges, dry stream beds, underpasses and basements. Because water from a large area gathers in a narrow place, the level can rise within minutes. The flood carries mud, branches and stones and develops great force.

The difference from a river flood is mainly speed. A large river often rises over days, leaving time to prepare. A flash flood leaves no such time – the decision has to be made in seconds.

According to the US agency NOAA, about 15 centimetres (6 in) of fast-moving water can knock over an adult and about 30 centimetres (12 in) can carry away most cars. A car is built like a boat: it contains a lot of air and gains buoyancy once the water is deep enough. Then the tyres lose grip and the water pushes the vehicle along.

Then there is what you cannot see. Murky water hides how deep it is, whether the asphalt underneath has been washed away and whether manhole covers have been pushed off. A dip that 'looks shallow' can be much deeper. That is why the well-known US rule is: 'Turn around, don't drown.'

This applies even if other cars drive through. Every situation is different, and barriers are never driven around. A detour costs time, not lives.

Warning signs are rapidly rising or suddenly muddy water, floating debris such as branches and leaves, a loud roar or rumble upstream, and heavy rain nearby or in the hills above. If you notice something like this, do not wait to see whether it gets worse.

The right direction is sideways and up, not downstream. If you flee with the water, it catches up; if you climb up the slope across the valley, you are quickly out of reach. Leave gear behind, because it can be replaced.

At home in heavy rain, do not enter the basement or an underground car park, not even to save belongings or the car – such spaces fill very quickly, and doors can hardly be opened against the water pressure. Go to upper floors. Switch off the power only if the fuse box is dry and safely reachable.

If a car is caught in rapidly moving water, Ready.gov advises: stay inside the vehicle. If water rises inside, climb onto the roof. Call the emergency number at once (112 in the EU). These are emergency measures – the aim is never to get into this situation in the first place.

When people are trapped in water, the temptation to go in is strong. But water rescue is a task for trained responders with equipment. You help most by calling the emergency number immediately, describing the location precisely and keeping eye contact from safe ground.

If a possible dam or levee failure is announced, follow evacuation instructions immediately and move to higher ground away from the watercourse. Do not wait to see whether it really happens, and do not drive to the dam to look.

When hiking and camping in valleys and gorges, check the weather beforehand – not only for where you are, but also for the area above where the water comes from, the so-called catchment. Never camp in a dry stream bed or right on the bank. A dry stream bed is dry until suddenly it is not.

In towns during heavy rain, masses of water shoot across sealed surfaces into streets, underpasses and basements. Keep away from drains and open manholes, because covers may have been pushed off and currents can pull. If you live in a dip or near a small stream, it is best to know the way up in advance.

This entry describes no water rescue techniques and cannot forecast flash floods. Following weather-service warnings about heavy rain is therefore the most important first step.