Watch out for weirs.

The tragic death of two international tourists on a rafting trip down the Highveld Crocodile River at the weekend marks the first fatal incident in commercial rafting in South Africa for several years. The incident highlights some river safety rules that everyone should know and observe.

A weir is a manmade wall built across a river to hold it back, forming a pool above with water pouring evenly over the wall. See illustration. Below the wall the water drops to the bottom of the channel but on the surface there is a back-flow that sets up a continuous cyclical motion. This is called a hydraulic. It's bad news for paddlers. Weirs may not be easy to see or hear from upstream, as all the action is below the wall and it may be hard even to notice the horizon line as the river pours over.

A natural weir is one formed by a rock ledge, having the same effect as a manmade wall. The safest weirs are those that you know where they are and you walk around. Ignorance is the greatest killer. The illustration shown here is just one of the many instructional slides used by Adventure Standards Africa when training river guides. Other river hazards like "holes" (rock pourovers) and "strainers" (trees in the current) also feature in these safety sessions. A weir may trap you and your boat in an endless churning maelstrom. If you fall off, as you are likely to, you will be spun as in a washing machine, down, around, up and down again. In theory you should try to swim out at the bottom where the water is escaping downstream. Having been recycled in quite a few hydraulics myself over the years, I can tell you that's easier said than done.

Often you are being spun so violently that you can't tell which way is up or down. Also, the foam pile near the surface is very aerated and doesn't support a victim, who stays beneath the surface. So catching a breath or looking to see where you are is difficult if not impossible. This is a drowning situation. Bodies and boats can stay trapped at the foot of a weir for hours - and rescuers who attempt to reach them may be sucked in too.

Americans call a bad hydraulic a "reversal" because on the downstream side the current actually flows back upriver towards the wall. Anyone approaching from below can be drawn into the hydraulic and suffer the same fate as those they were trying to save. This has happened many times at weirs all over the world. Nowadays firemen and river rescue units are trained to use rope taglines on boats or other techniques to reach people in the "slot" between the foam pile and the smooth water flowing over the wall.

Apart from trying to swim out along the bottom there may be other ways to get out of the thing:

• Someone may throw you a rope and haul you out

• You may be able to stand on the river bottom and walk out (this has saved me twice)

• There may be an "eye" in the slot where the current flushes through without reversal

• It may be possible to swim along the slot to safety, or stand with your back against a column supporting the wall until rescue is effected

• Sophisticated techniques are used by trained teams to reach trapped swimmers.

Here are some no-nos:

• Don't drive across a flooded bridge where your vehicle may float and get washed over: there is likely to be deadly weir action below the bridge.

• If hiking, don't regard a wall under water in a current as a convenient crossing point

• If paddling go armed with knowledge, a map with hazards marked if possible, rescue gear such as throwbags and whistles, and (if new to rivers) go with a qualified and cautious guide or guides.

AsAfrica runs River Proficiency courses at regular intervals throughout the year. Anyone may attend, from club safety officers to scouts, casual paddlers and hikers.

WhatsApp +27 84 245 2490.

The contents of this article do not reflect in any way on the event in the news report.

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