Myth of the collapsing sand bar

Every time I give one of my Science of the Surf community talks I always get asked the same question óìDo sand bars collapse?ó and I always give the same answer óìNo, sand bars do not collapse unless you blow them up with explosivesó.

Sand bars are big piles of sand that are pretty solid and heavy. Sure, they shift around from time to time, but this takes days, weeks and even months. They never, ever, ever implode on themselves. It just doesn’t happen.

But you still hear stories of collapsing sand bars getting people into all sorts of strife. So where did this myth come from? Well, if it’s wacky and related to the beach, it must come from… Bondi.

Sand Banks Clear Water Eye Level Bondi
In this @aquabumps photo you can see the deeper areas of the water (green/blue) and the shallow areas, or sandbanks (brown areas: see top right hand corner).

Sunday 6 February 1938 at Bondi Beach was a beautiful summer day and many of the 30 000 people at the beach were splashing around in the water standing safely on the shallow sand bars. According to eyewitness reports, everything was fine until three large waves approached the shore and broke.

Soon after that, a bunch of swimmers suddenly found themselves in deep water being dragged into a deep channel and out to sea. Approximately 60 surf lifesavers conducted a mass rescue during the ensuing hysteria and panic. In the next 30 minutes, 250 bathers required assistance, of which 35 were rescued unconscious and revived, while tragically five drowned.

Today this mass rescue is still often reported as being caused by a collapsing sand bar, which is unfortunate because that’s not what happened at all. The report of three large waves and a deep channel tells an interesting story though. The deep channel was probably a rip channel and the larger waves were probably a wave set coming in.

Waves in the ocean tend to travel in groups of 3 to 10 bigger waves with lulls in between. Surfers call them sets. When the waves in the set at Bondi that day broke, the water level would have risen and swimmers standing on the sandbars near the rip would have lost their footing and floated or been carried into the rip by the water draining sideways off the sandbars.

Then the rip would have pulsed. All rips have a nasty habit of suddenly doubling their speed for about 30 seconds before going back to normal. It usually happens after a wave set has broken. All that extra water coming in has to get back out and it basically óìpumpsó the rip. So all these people who were washed into the rip channel would then have been taken way offshore when the rip pulsed.

The stories of collapsing sand bars are not uncommon, but it’s almost guaranteed that in each case it was a rip pulse. However, although not proven, there is some evidence to suggest that the waves that came in at Bondi that day were not part of a wave set, but those of a small tsunami! But that’s a whole different story.

First published in November 2012


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