What is it that happens to water underneath waterfalls that can cause swimmers to get into trouble? Keen swimmer and ex-paramedic Lynne Roper talks about sieves, syphons, flashy rivers and how bubbles rob swimmers of buoyancy.
Fatal accidents involving waterfalls are a tragedy during summer. What do you need to be aware of when you go swimming in waterfalls?
If there is white water, if the water is foaming or disturbed by rocks or falling a distance, then you lose buoyancy. ‘White water is water mixed with air, and air bubbles have less density than water. So you lose your buoyancy in it, are affected by gravity much more and may very well sink,’ says Lynne Roper. ‘If you’re not experienced that’s the kind of thing that can make you panic, and panicking can get you into trouble.’ It is instinctive to try to swim against the current, back up thorough the bubbles, but you have to exit that stream of air filled fluid through the water around you and then surface.
Water has a lot of power when it moves and one of the key hazards is a siphon. ‘Kayakers are the real experts in this,’ says Lynne. ‘But if there’s a gap between some rocks and water is forced through a smaller hold, then it’s the equivalent of a plug – a swimmer can get sucked in, plug the gap and then not move out again because they are pinned by the weight of the water.’ This is called a siphon.
‘A fallen tree or roots overhanging a bank can create the same effect, but this is called a sieve. In strong currents a swimmer can find they are pushed against roots or the bank, and the water holds them against it, and they can’t get away: the weight of the water holds them there.’
Flashy rivers are ones where conditions change quickly with rainfall. There are various dangers associated with a flashy river, like those seen on Dartmoor and Snowdonia. A stream that was friendly and benign can, after a short period of heavy rainfall, rise by as much as 2-3ft and then have completely different risks and hazards. For example: you may become much more likely to get trapped against rocks or a fallen tree and held there by the strength and weight of the water.
In a waterfall you are placing yourself under a weight of water which in a way is dangerous. The height of a waterfall affects its strength.
‘The power of the water varies with the flow rate, and when flow rates increase it gets heavier, and can trap a swimmer against a rock or obstruction,’ says Lynne Roper. ‘You have to be aware of that, and each time you get in you have to judge, particularly on that day and in that minute.’
It’s important to be aware of where the current is in a river. ‘In summer, with high spirits, teenagers are particularly likely to get into water that looks friendly, be moved along in it by the water and not be worried at all and then find they cannot get out because the bank is too high.’
Solutions to this include:
‘The biggest factor in people getting into trouble and drowning is the cold,’ says Lynne. ‘People think it’s hypothermia that’s the cause of that but you won’t get hypothermia for 30 minutes or probably longer in the warmer weather.’
Water in Britain is lower than 15 degrees much of the year. ‘At these temperatures you experience physiological responses when you get in including cold shock, the response where your body initiates a gasp reflex. If you are under the water or a waves comes over your head when this gasp reflex takes place you may inhale some water, and you don’t need to inhale much for it to cause serious problems with your breathing. You don’t have to completely engulf your lungs for there to be a problem.’
After the initial gasp of cold shock comes a period of hyperventilation that you can’t control. ‘Breathing is really rapid and you can’t control it, which can lead to a feeling of panic,’ says Lynne. ‘New swimmers also tend to go upright in the water, in this position we are less buoyant, and then panic and can really set in, increasing the chances of drowning.’
Getting in gradually and staying in depth until the cold water shock response has passed and breathing is under control is safer.
Cold water shock was identified as the cause of a fatal accident at Low Water in the UK in summer 2015. BBC News.
Where water is cool compared to our body temperature, it eventually starts to cool swimmers down. ‘After 30 minutes or so our body’s response to cold is to restrict peripheral blood supply,’ says Lynne. ‘The blood supply in arms and legs and hands and feet goes much more towards the core, so even the strongest swimmers and fittest person loses strength and dexterity and the ability to coordinate their body. Swimming becomes slower, ragged and short distances can take a long time to cover.
‘If the swimmer succumbs to early stages of hyperthermia they lose their fine motor skills and the ability to hold things in their hands.’ This can make it hard to get out, where it’s necessary to hold onto grass or tree roots with your hands in order to do so.
During the source to sea journey of a river, the water has a chance to warm up during warm and sunny periods. Waterfalls are often found at the rocky beginning of a river’s journey, and contain colder water than the more mature sections of the same river further downstream.
‘In mountainous areas you may still see patches of snow on north facing cliffs as late as May, and mountain stream water can be cold,’ says Lynne. ‘People are caught out by cold mountain streams.’
Fluctuating water levels mean that a popular jumping spot can become fatal if river levels have dropped, and currents can move underwater rocks: every episode of jumping needs it’s own risk assessment (going into the downstream section and exploring depth and underwater hazards), the fact that it’s a popular jumping spot or others have been seen jumping there before is no assurance of safety. The higher the jump, the deeper the water necessary beneath it.