Sand In The Engine
It's the basis for beachbreak bliss, but can you have too much of a good thing?
Sand In The Engine
Sand: It’s the final frontier of surf forecasting.
It’s a grand statement, and one that only applies to coasts where beachbreaks dominate, but stop and have a think about it. It’s 2023, powerful computers forecast swell to the nth degree, breaking it down into size, period, and direction. We know what’s going to happen up to a week before it occurs. Similarly, the wind rarely takes us by surprise; whether it be using computers or our native knowledge, we can predict that days out with startling accuracy.
Yet swell and wind are only two-thirds of the equation on a beachbreak coast. Great conditions can still fall in a heap if the sandbanks are terrible. And short of very obvious examples - say a coastal lake breaking open - there’s no real way of predicting exactly how they move about. Great banks follow a puzzling timetable, coming and going often without explanation.
Hence sand remains the great unknown, and it’s likely to stay that way for a while yet.
If you live on the East Coast, chances are you’ve seen your local beach grow an extraordinary amount over the last six months.
“Last year we could barely run Nippers during high tides,” says Steen Barnes about the state of his local beach. “Now we could set up the 100 metre sprint across the beach, and we’d still have room to spare.”
If you’ve heard of Steen before, it’s likely because of his photography around Wollongong. Steen, who lives at City Beach, rarely walks across the road without a camera in hand and has documented the local beaches through all their cycles and changes.
Right now, there’s more sand than he can ever recall. So much so, says Steen, that ”at one stage a secondary dune formed that created its own shadows.” More importantly, however, there are almost zero banks. “There are no gutters,” says Steen. “Pretty much no channels to speak of either.”
I live ten kilometres to the north of Steen and the same situation is repeated on every beach in between. In fact, it extends all the way up and down the East Coast. Gavin Scott lives on the Mid North Coast, among the East Coast’s most prized beachbreaks, and he can’t recall the banks being as bad. Again, there’s no shortage of sand. “Places that are usually stripped of sand at this time of year [early autumn] are deserts without a rock to be seen,” says Gav.
Further north, the excess of sand is playing out in different ways. As might be expected on a coastline where sand-bottomed points dominate, sand is usually seen as a blessing. “Generally sand surplus is better for wave quality on this stretch of coast than sand deficit,” explains Steve Shearer who lives at Lennox Head, yet it’s becoming borderline. “The sand excess is almost too much for Lennox Point.”
Quality aside, Steve illustrates the point by noting that through June this year and again through August, surfers could come in on sand after surfing Lennox Head - no need to negotiate the treacherous rocks. Most prominent at low tide, a rare bank had attached itself from the impact zone to the point itself without a nearshore gutter.
The East Coast currently has more sand than it’s had in a long while. It arrived after a period of heavy erosion and during a time of year when the sand volumes are traditionally at their lowest. The result has East Coast surfers wondering what the hell is going on. Why is there so much sand? Where has it come from? And why is it in such bad shape?
To these questions, we went searching for answers.
Before we get into it, a quick refresher. Though they give shape to lovely waves for playing in, sand has a more important function. Beaches serve as the defensive barrier against storms and large waves. Left untouched, beach dunes grow during times of small swell, and then absorb energy during storms. Big waves will erode the beaches, transporting the sand into the surf zone where it mitigates wave energy, making waves break further offshore.
On the East Coast, autumn and winter are when we get most of our storms, so come September beaches are traditionally at their thinnest, having survived the six month onslaught. Sand then gathers during the spring and summer swell cycles and the beaches restore their width.
As per Steen and Gav’s observation, and that of many other East Coast surfers, beaches are not just bucking that annual trend, but some are as wide as locals can remember. “They are incredibly wide at the moment,” said Dr Mitchell Harley of the Water Research Laboratory at the University of NSW in an article that appeared in the Sydney Morning Herald last month.
The same article claimed, “Bondi Beach is about 25 metres broader than it was this time last year, Manly is 26 metres wider, Coogee is 22 metres bigger and Maroubra has grown by a generous 42 metres in the past 12 months. North Narrabeen is the undisputed king of beach growth, adding a whopping 59 metres of sand since last year.”
However, as with most things, more is not always better. Many surfers that I spoke to said that, yes, their local beaches were wide, but there was also a corresponding loss of shape. A common observation was the loss of an inshore gutter, meaning at low tide the dry sand extended in one gentle slope towards the outer bank. In coastal science parlance, the outer bank was ‘welded’ to the shoreline. A bank becomes welded when wave energy is so low that it no longer carves out channels and gutters for water to escape back out to sea.
Take a look. If your beach has welded banks there’s a very good chance it’s also suffering from bad shape. Sure, good waves can still happen. Tuesday the 19th and Wednesday the 20th of September showed what can happen when swell alone - particularly a combo swell, as was the case on Tuesday - meets straight banks. Most of the time, however, the nearshore bathymetry has to do the heavy lifting and right now the architecture just isn’t in place.
If you live on the East Coast then you don’t need me to tell you that this past winter has been a lean one. You saw it with your own eyes. It makes sense why banks have attached themselves to the beach as there’s simply not been enough swell to start the pattern of water circulation.
Anecdotes are one thing, raw data is another. Actual scientific proof. And that’s what Manly Hydraulic Laboratory have supplied, charting all readings from Sydney’s wave buoy over the past 36 years. The surf wasn’t just down a little this past winter, it was down a lot.
Yet is one quiet season enough to account for the Saharan sandscape overtaking most beaches?
To answer that question I spoke to Associate Professor Ian Goodwin: surfer, sand expert, and soon-to-be-published author on a book of coastal processes. With his scientific training, Ian doesn’t see lone events, he sees patterns and cycles. Some of them are given handy names like El Niño and La Niña, while others stretch out beyond human lifespans making them hard to grasp. When everyone’s talking ‘right here, right now’, Ian’s the guy who provides historical perspective. He thinks in terms of decades, centuries, millenia.
“This current state of beach volume is typical for transition to El Niño years,” says Ian. “Very similar to 1982-83 and 1993-1994.” This is similar to something Steen said: “I’ve seen this type of buildup before, it’s not unheard of, though I’d have to go back to the 1980s - yet even then it wasn’t as extreme as now.”
One reason for the current situation, explains Ian, is that for the last three years - i.e during the La Niña three-peat - there was very little sand transport. Beaches were being eaten away by relentless easterly energy; the sand taken offshore. The current sand volume, says Ian,” is delayed sand transport during la Nina.”
Usually the transition out of La Niña is more subtle than has occurred this time, hence the seemingly sudden appearance of sand, and lots of it. However, what’s also boosted the situation is the wave climate over the past six months. You see, the graph above about wave height only tells part of the story about the East Coast’s past winter. Sure, it might’ve been small, but what swell did come ashore was often long period, generated by the same Southern Ocean storms that flicked the switch on Victoria’s season of plenty.
Long period swells - particularly sustained periods of them - are the key to sand returning to shore. “They’re essential for supplying sand to our coast,” says Ian. The reason for this comes down to a bit of ocean science, namely long period energy traveling deeper in the water column. It’s the reason long period swells refract earlier and more acutely - they feel the ocean bed further out to sea.
Similarly, long period waves disturb the ocean floor further out to sea, cycling sand and silt towards the coast. “Waves interact with the seabed at a depth of half the wavelength,” explains Ian, “so let's say we've got a 150-metre wavelength, those waves are going to be mobilising sand at 75 metres water depth.”
Therefore, the sand that had been taken offshore during La Niña has had six months of near-uninterrupted migration towards the coast. There have been no big swells to move it back offshore so we've had a relentless constructive phase with sand steadily piling up on the beaches. Plus, and this is where things get very interesting, there’s also been an uninterrupted flow of sand mobilising slowly across continental plain towards the mainland.
This last statement represents the pointy end of ocean science here on the East Coast. It requires a bit of a digression but as we’re talking about sand let’s follow those grains.
For many years, no-one quite knew where the sand that stocks our beaches was coming from. Sure, we knew that, north of roughly Sydney, sand moved northwards in a longshore transport system that created all of Queensland’s great sand islands: North and South Stradbroke, Morton, Bribie, and Fraser (K’Gari) before expiring in an abyss north of Fraser.
(That abyss, by the way, is what allowed the corals of the Great Barrier Reef to grow. If the sand continued north it would smother any foundation where coral tried to take hold.)
Meanwhile, south of Sydney, no sand transport system exists and sand is held in embayments between prominent headlands that sometimes ‘leak’ during big swells but generally cycle the same sand around - offshore during a storm, back onshore during periods of small swell.
But what’s the source of it all?
Scientists have traced it all to our river systems. However, what scientists such as Ian are now finding is that river sand - such as that which washed down the Northern Rivers during La Nina floods - doesn’t join the sand transport system. Instead it sits around the rivers’ ebb tidal delta, and sometimes even washes back into the rivers themselves.
Yes, the sand that’s on our beaches comes from the rivers, however between washing down the rivers and ending up on our beaches, much of that sand has spent long periods of time - centuries, millenia - on our continental shelf, sometimes far from shore. I ask Ian to elaborate.
“Think about the last million years of history,” says Ian, “for 90% of that time the sea level has been about 50 to 60 metres lower than it is now, and all our river systems deposited their sediment through river mouths that sat sometimes tens of kilometres further out on the continental shelf.”
“This whole process has been operating for close to a billion years,” adds Ian and I hope readers are now appreciating the breadth of Ian’s worldview. If nothing else, understand that these ancient rivers left great sand reservoirs on the continental shelf.
“Then for 10% of the time,” continues Ian, “the warm climate periods, such as we’re in now when the sea level is at its landward extent, wave energy - specifically long period wave energy - transports that sand back onto the coast.”
So yes, the sand originates in our rivers, but there’s a huge lag between when it was deposited through a river mouth and when it stacked up on our beaches. During periods of short range energy - such as the La Nina years - it may not move much at all, but when swell periods lift over, say, twelve seconds, the grains of sand cycle ever so slowly towards shore.
Add that slow moving sand from the ancient rivermouths to the sand that was moved offshore during La Nina’s erosive storms, and you’ve got one almighty bounce back. Ian, as you’d expect from someone who thinks a century is short-term, isn’t surprised.
“This beach situation was typical in the period 1910 to 1940, and between 1830 and 1850, and even earlier between 1300 and 1550 AD.” How he knows all that is a conversation for another time.
So most East Coast beaches are full to brimming with sand, and it’s not helping the surf quality. The worst case scenario is that we go through spring and summer - traditionally our quieter seasons - without enough swell activity to shake up the pattern. I asked Ian what might result from six months more sand buildup. Just how wide can our beaches get?
"I think they're pretty well at their maximum now," answered Ian. "Once the sandbars are welded to the shoreline, that's pretty well a limit."
I guess that's good news. Though even contemplating the status quo - another six months of what we’ve just had - doesn’t bear thinking about.
One bright spot, countered Ian, "is that during the last twelve months we haven't had a single East Coast Low." We're moving out of the season for ECL's, which in the weather world are industrial-strength sand removalists, however it's still possible to get one.
As I type this we're on the backside of a north-east swell event; one that many people had hoped would provide good waves and shake the foundations up a bit. First impressions are that we got the former and not the latter - the sand is as straight and plentiful as it was before the swell.
The wave models show a south swell coming in a few days. It doesn't appear significant, but there's hope it'll stir things up, maybe gouge channels and gutters while depositing the sand into new shapes and formations. Those wonderful and mysterious things we call sandbanks - or used to call them back when they existed on this coastline.
// STU NETTLE
Special thanks to Manly Hydraulics Laboratory on behalf of the NSW Department of Planning and Environment Biodiversity and Conservation Division.