The Myth of the Collapsing Sand Bar
From the December 2021 issue of 2508/2515 District News Magazine
After I give one of my Science of the Surf community talks I often get asked a question along the lines of ‘Why do sand bars collapse?’ and I always give the same answer ‘They don’t. The only way to collapse a sand bar is to use explosives!’ And it’s true. Sand bars don’t collapse. They are big piles of sand that are pretty solid and heavy and while they shift around in response to changing wave, current and tidal conditions, this takes days, weeks and even months. They never, ever, ever implode on themselves. Where are they going to ‘collapse’ to? It simply just doesn’t happen. It’s a big myth that can be traced back to the infamous ‘Black Sunday’ event at Bondi Beach in 1938.
February 6, 1938 was a beautiful day and according to eyewitness reports, everything was fine until three large waves approached the shore and broke. Soon after, a large number of swimmers suddenly found themselves out of their depth, being dragged into a deep channel and out to sea. In the next 30 minutes, 250 bathers required assistance from lifesavers, of which 35 were rescued unconscious and revived, while tragically five drowned. This event is still often reported as being caused by a collapsing sand bar, but that’s not what happened.
The deep channel was likely a rip current channel and the larger waves were probably a wave set coming in. Waves in the ocean tend to travel in groups (sets) of 3 to 10 bigger waves with lulls in between. When the waves in the set at Bondi broke, the water level would have risen and swimmers standing on the sandbars near the rip would have lost their footing. It would have seemed to them as if the sandbar had suddenly dropped away, hence ‘the collapsing sand bar’. In reality, they would have been carried into the rip by the water draining sideways off the sand bars and taken quickly offshore by a rip ‘pulse’. All rips have a nasty habit of suddenly increasing in speed for a short burst, usually after a wave set has broken because all that extra water coming in has to get back out and it basically pumps the rip and creates a ‘pulse’.
Locally, something similar happened in January 2011, when it was incorrectly reported that a ‘dramatic sand bar collapse’ resulted in a fatal drowning and a mass rescue of 25 swimmers at Stanwell Park. Again, the situation was most likely due to sudden large waves and a rip current. When a wave set, or even several large waves, breaks on a shallow sand bar, the water level rises and can create a short-lived flash rip heading offshore taking swimmers with it. The sand bar? It didn’t shift or collapse at all.
Nice sandbars on either side of the rip. Where are they going to collapse to?