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At least 13 people have been swept away and drowned as they try and cross flooded rivers in the past few days, despite continued warnings from everyone that this is highly dangerous.

We had a pirate taxi carrying 10 people swept away in the Midlands, with three people drowning although the other seven managed to reach the river bank.

We had four people swept away as they tried to cross Runde River after it rose in flood after an upstream weir started spilling, although without that weir it would have already been in flood.

Four others in the same group managed to keep their footing, hanging on to trees or bushes, and although marooned in the midst of the rushing waters were rescued by the Air Force of Zimbabwe who scrambled a helicopter to the area at dawn.

The same helicopter was then able to pick up three men along the same river who were caught by the rising waters, but again managed to hang on until rescued.

And then we had the weekend tragedy, when a double cab pick-up carrying six people and towing a trailer took a chance in crossing a flooded low-level bridge on the Gweru River and was swept away.

The trailer was found by Sunday night, several kilometres downstream, but the truck was only recovered yesterday, with two of the six passengers accounted for.

The real problem, and the real danger when crossing rivers in flood, is not so much the depth as the speed of the flow of the water.

We all learnt in the early stages of high school that energy of moving bodies, and water flowing down a river is definitely a moving body, is proportional to the mass and to the square of the velocity. But few of us apply this knowledge. We just learn it for an exam, not for life.

But what it means is that if a river doubles in depth the energy and the resultant force is only doubled, since the depth measures the mass of the water. And many of us can wade across quite a deep still pool without much trouble, even if the water is waist high.

But to show the incredible forces that can result as the river starts flowing faster think about what most of us have done when driving across a ford on a remote farm road or rural road when a river is hardly flowing.

So long as our exhaust pipe is above the level of the water, crossing a ford when a river is hardly moving, flowing at say 1km/hr, is no big deal. We take it slow and splash through without much thought and little danger.

Some SUVs and pick-ups even have a vertical exhaust pipe to make fording a still river safer, but that will not help if the river is flowing.

Take that same river, at the same depth, and increase the flow to 5km/hr, the sort of speed almost anyone can walk at for long distances, and you have 25 times the force.

Speed up the river to 10km/hr, what an unfit middle-aged man can easily maintain while jogging, and you have 100 times the force. Go up to 20km/h, and most adults can run quite a lot faster than that for a short distance, and you have 400 times the force.

Many rivers in flood will at times be flowing at this speed and no matter how shallow they are they will exert powerful forces that can knock you over and sweep you downstream.

Yet if you double the depth of water on that 1km/hr gentle flow, you only double the force. So the danger comes when someone looks at a flooded ford, or a flooded bridge, and reckons the water is not that deep and so drives across or wades across without taking into account how fast the river is flowing. And they are swept away by the incredible forces that moving water can exert, especially fast moving water.

That bridge on the Gweru River was not that far under water, and the driver of the large double cab probably reckoned that the vehicle had a high enough ground clearance and a big-enough engine to get across.

If the water had been still, or flowing very gently, the driver might well have been correct. But the flow was strong enough to sweep a trailer carrying a heavy mine compressor downstream for several kilometres.

Most drivers should know the rapidly rising forces of speed from ordinary driving.

Hit a tree on some slippery farm road at 10km/hr and you have a small irritating dent. Do it at 60km/hr, the speed limit on suburban roads and you 36 times the force; you probably walk away, but need a tow truck to move your car.

Do it at 120km/hr on the open road, at the limit, the force is 144 times as great and the rescue workers are probably going to extract your broken body.

And do it at 150km/hr, the sort of speeding some men boast about in bars, then forces are 225 times as great and if there are two people in the car then rescue workers have the problem of trying to figure out which body parts they scrape off the wreckage go into which box.

Low level bridges have done so much to open up the more remote parts of Zimbabwe. They are safe and allow an easy crossing 350 or 360 days of the year. But they will be flooded after heavy rains and so you have to wait until the water levels drop.

This rarely takes long, and you either turn back or camp on the river bank for a few hours, or even a day or two. Experienced travellers keep a loaf of bread in their car, on their bicycle carrier or in their backpack when on minor roads in the rains.

Even 60 years ago there were still low-level bridges on some major highways and travellers would organise a little party as a group of cars waited.

There was one infamous river crossing a little south of Musina in South Africa where several scores of cars would have to park after a bad storm or two, but the point is that people knew the danger and waited.

Impatience kills. Rivers in flood are powerful, cutting gorges and moving hundreds of tonnes of earth, so a one tonne car is just a bit of lightweight flotsam and a human body is far lighter than some of the logs that bounce along in the current.

We need to respect the forces of nature, realise the dangers, accept advice and think clearly. We saw this at New Year when tropical storm Chalane hit Chimanimani and dumped 100mm in one day. The highly experienced communities were told exactly what to do and did it.

So no one was injured, let alone killed. We can learn from other people’s mistakes; we can follow expert advice; we can take precautions; we can live, even with a minor disruption for a day or two.