Q: What causes quicksand? (continued)
A: Sarah Lewis (7 January 1995) has promulgated a possibly dangerous 鈥渕yth within a myth鈥� by saying that a human body will float in quicksand. This is true, but it will probably be a dead body. This is because the folklore tale of being sucked into the sand is closer to the truth. Because of the complex hydrodynamic interaction of sand grains and water, a body in quicksand is likely to exhibit a slow-motion version of the behaviour of someone who has jumped into water. The person will sink to beneath the surface and then rise again.
Unfortunately, the need to breathe is not slowed down correspondingly, and the person in the quicksand will run out of air before he or she surfaces again. It is almost traditional to point out here that both Albert Einstein and Osborne Reynolds wrote fascinating papers on the behaviour of sand/water mixtures. Reynolds even based a notoriously cranky theory of the Universe upon it.
Advertisement
A: On a beach in Wales there is a quicksand swirl at a meeting of the waves. Here, fine flakes of shell float and are gathered into a pool. The concentrated dispersion of very light, finely divided shell has an overall density that is less than water and so its buoyancy is too low to support a person half-in, half-out as stated by Sarah Lewis.
A: Sarah Lewis underestimates the danger. Quicksand can be expected where a rapid mountain river flows over sand. It will occur underwater and may be invisible from above the surface.
In Britain, walkers may never face this danger, but in wild country it may be impossible to find a safe place to ford the river. Quicksand is produced by an underwater spring and it is usually a tiny region in a vast area of sand, but its position can wander rapidly. For this reason, probing ahead with a stick does not necessarily show safe passage, and even if one person crosses safely, the same route may trap others even though they follow immediately.
One does not float in quicksand, one falls into it (I write from experience). The problem is that the act of falling causes the spring to divert slightly, perhaps less than a metre sideways, and the sand into which you have fallen sets instantly. With one鈥檚 legs trapped in wet sand and up to one鈥檚 armpits in cold water, life expectancy is short unless you have companions and they know the drill. A rope, tied with a bowline under the armpits, is essential and one must rope-up before attempting to cross. The quicksand victim has to take a deep breath before the head goes under and the companions must pull hard from a downstream angle. I am here to tell you this works.