The Summer of Sand
This article is a modified version of one that appears in the October issue of The Illawarra Flame Magazine
If you’ve gone to the beach lately in NSW, you’ve probably noticed there’s a LOT of sand. Beaches are wider than we’ve seen for years and judging from the massive sand banks, there’s more coming – it’s going to be the ‘Summer of Sand’.
There’s a few reasons why the sand is returning. We’ve just come out of a triple La Nina weather pattern that brought lots of rain, storms and big waves that severely eroded our beaches. In some places, it seemed like the beaches would never recover. But they have. The simple rule is that big waves strip sand from beaches and dump it offshore and small waves bring it back, but there’s more to it than that.
While beaches are fairly resilient to large clean swell waves, they’re not so good at dealing with large messy waves generated by storms such as East Coast Cyclones. That’s what erodes beaches and it’s the finer (smaller) sand grains that are transported the furthest distances offshore. Exactly where the sand goes, nobody really knows - offshore somewhere! It’s a big unknown. But we do know that clean swell with average, or below average, wave heights brings that sand back onshore. The reason for that comes down to wave physics.
Waves travelling offshore are fairly symmetric in shape and the onshore movement of water under the wave crests is balanced by the offshore movement under the troughs. As waves enter shallow water and slow down, they become increasingly asymmetric in shape - with narrow, peaked crests and long, flat troughs. When you get smaller swell, only the water motion under the wave crests is strong enough to transport sand lying on the bottom. So you get onshore sand movement under the crests and not much under the troughs and the net transport direction is towards the beach. These smaller waves are also best suited to moving fine sand grains, which is why we’ve also seen a lot of fine sand being brought back lately. Wave direction is also important and slight differences between prevailing wave direction between El Nino and La Nina conditions may also be the key to bringing that sand back.
Will the Summer of Sand be good for swimming and surfing conditions? Yes and no. It will make conditions a lot shallower and you’ll be able to wade a long way offshore, but those massive shallow sand bars have created a lot of close outs for surfers and peaks are hard to find. We’ll also likely see some channelised rip currents get locked into place through the sand bars and while that might be bad for swimmers, they’ll at least create some decent surf breaks.
Of course, we might get a massive messy storm wave event that will erode the beaches again. It’s unlikely for this time of year - but you never know.
If you want to know more about this topic, a colleague of mine Dr Mitch Harley from the UNSW Water Research Laboratory recently talked about the build up of sand on our beaches in the Sydney Morning Herald.
A very, very wide Coledale Beach in the Northern Illawarra in September 2023.