Assorted grains: Why 2019 has been a mixed bag for sand-bottom breaks
On the night of February 13th 1898, the 220 tonne Brig ‘Amy’ sunk off Thirroul Beach killing all eight people aboard. The wreck of the Amy now sits 200 metres offshore from Thirroul Surf Club though it’s rarely seen as over the years the structure has sunk into the sandy bed. Every decade or so the wreck makes an appearance, always during times of small swell when the sand is pushed out of the surf zone and up onto the beach.
The wreck of the Amy has been fully exposed for six months now. It's possible to swim over it on a sunny day, peer down and see the bones of the old ship laying bare. The sand that usually surrounds it is has been pushed shoreward, filling gutters, and widening the beach. The end result of that sand movement has been a chronic case of the closeouts. And it’s not just Thirroul, all the beaches on this stretch of coastline are suffering the same fate. What’s happened at my local has repeated itself across the region.
And going on the chatter in the Swellnet forums it’s also happened at various regions up and down the East Coast. This should come as no surprise: the whole coast has been under the same extended pattern of small-to-medium swell and calm winds. But what has been a surprise...or at least of great interest, is that in contrast, some regions of the East Coast have reported fantastic conditions - small but high quality waves for weeks on end. Unlike my local stretch of coast, those regions responded differently to the set conditions.
And it got me thinking about the variable nature of sand flow and how on this side of the continent, our immediate wave quality isn’t necessarily dictated by storms hundreds of kilometres away, nor by winds, or even by tides, but by what lies beneath the waves: the ever-shifting bathymetry of sand.
Unlike Australia’s southern and western coasts, the East Coast is predominantly beaches. From Eden to Hervey Bay no two beaches are separated by anything more than a leisurely walk. It’s a sandy coast, especially north of Newcastle, which is roughly where a great conveyor belt of sand begins its northward journey picking up sediments from each of the coastal rivers and dusting the northern pointbreaks with its load on the way to Fraser Island, the world’s largest sand island.
From Newcastle south there’s an increase in reef breaks yet they’re still vastly outnumbered by beaches. Unlike up north, the sand on the central and southern NSW coast largely stays local, with much smaller quantities moving north via longshore drift. When sand here is lost off the beach, it’s usually moved out into the deep water beyond the beach, and will slowly return to the same beach - or a beach nearby - during periods of low swell.
Last week, Swellnet forecaster Craig Brokensha raided the Bureau of Meteorology’s records and found autumn 2019 on the East Coast was a far from average season. The season was characterised by a semi-permanent high pressure system in the Tasman Sea that directed light easterly winds into SE Queensland and northerlies down the northern NSW coast. Lots of northerlies. Weeks of them. Months even. In fact, the northerlies were so relentless they halted the flow of sand up the East Coast which is fuelled by southerly wind and swell. Where my local beach is full of sand, many of the north-facing pointbreaks that rely on the conveyor belt of sand are now denuded. Amongst them are Snapper, Cabarita, Lennox Head, and the Pass. In fact, Clarke’s Beach, just down from the Pass, has experienced its worst erosion in decades with beer bottles date-stamped from the 1950s being uncovered.
A snapshot of April 2019 compared against the average for that month shows a stationary high in the Tasman Sea directing east and north winds down the coast
One person who wasn’t surprised by the season that’s just passed is Associate Professor Ian Goodwin who in 2016 published a paper detailing the changes to the East Coast wave climate and its flow on effect to sand deposition. Goodwin, along with his fellow researchers Thomas Mortlock and Stuart Browning, is predicting a poleward expansion of the tropics, less southerly swells on the East Coast, and in turn less sand moving up the coast.
“It’s the lack of storms with a southeast wave direction that’s turned off the longshore sand transport,” explains Ian. South winds and swell are the motor that drives the great conveyor belt of sand, and when the motor stalls, as it has for most of the last six months, then the sand stops flowing. The result is a lack of connectivity in the sand transport system with the most vulnerable stretches being the lee-sides - or north sides - of rocky headlands, which includes almost every right hand pointbreak from Boulders to Burleigh.
“The East Australian longshore sand transport system is one of the largest sand transport systems on Earth,” says Ian. Though beaches may be divided by headlands, the whole system is connected, hence you’ll often hear Ian use the word ‘connectivity’, which implies a consistency across the system that can be tested by observation. When one point in the region is lacking sand, there's a good chance all the other points will lack sand too. Or when one beach in the region has good sand, the others will have good sand. They’re all fed by the same source, which is controlled by the same overarching system.
Recent observations on the ground match Professor Goodwin’s theory, but will Autumn 2019 be an outlier or the new normal? Ian’s answer is based on future climate projections, and northern NSW surfers should hope those projections are incorrect because if it continues, “there’ll be a long term decline in longshore sand transport,” says Ian. Worryingly, the transport rate has already declined by 25-30% over the past century according to his research.
That number should concern any surfer who lives near a sand-bottom pointbreak on the East Coast, but what about those who only surf beachbreaks? Over the last season, as the pointbreaks have lain dormant, Swellnet has heard many reports of firing beachbreaks, most commonly on the Mid North Coast and the Tweed Coast. Why have they been good when other regions have been poor? To explain this I spoke to coastal engineer, Simon Brandi Mortensen, director of Ports and Maritime at DHI.
“There’s little commercial work in sand movement so we don’t have that much opportunity to study it, “ says Simon who’s consulted on various artificial reef projects modelling wave propagation over solid bottoms, something he has a firm handle on. But what about understanding sand flow? “It’s chaotic,” says Simon cautiously.
However, it is possible to ascertain patterns within the chaos, and being a surfer, Simon is good at recognising them. Like Ian, Simon agrees that the East Coast’s sand transport system requires regular south swells to maintain the movement of sand. Yet he also notes that regularity doesn’t necessarily mean consistency. Often the best results are when the system works in bursts.
Simon explains that the best sand formations at Burleigh Heads, just for example, is when a large southeast storm moves a load of sand around the point, and the storm is followed by a period of ambient conditions that align the sand’s curvature to the headland. A scenario that unfortunately hasn’t happened for a while.
A similar exercise in pattern recognition can explain why some beachbreaks have fared so well during Autumn. “In February we had a large storm when Tropical Cyclone Oma hit the coast,” says Simon. “It created a storm bar on many of the open beaches - a long, straight sandbank sitting a hundred or so metres off the shoreline.”
“The longshore current stretches out the sand load like a tail,” explains Simon. “Yet if we get periods of calm afterwards then the bar starts to break down, it gets deeper and less defined.”
Storm bar on the far north coast of NSW (Photo: Professor Andy Short/OzCoasts)
There’s a stage of this process when the sand forms “crescentic bars” which are as the name implies, bars shaped like crescents, and unsurprisingly they make for the best quality beachbreaks. How long the crescentic bars form after the storm, and how long they last is open to many variables - this is where the chaos aspect comes into play. The storm bar is the foundation for good banks yet it can differ greatly. For instance, the bigger the storm, the further offshore it will form, and this will affect the quality of banks that form later.
Another pattern that can be discerned is that, at least at a regional level, when good banks form on one beach, the likelihood is that good banks will form on each beach nearby. Sure, you may find a stellar bank whose quality surpasses all others but the general form across the region should be consistent - and this is what was reported on the Tweed beaches and further south on the Mid North Coast.
When I mention the beachbreaks of the Mid North Coast to Ian he’s not the least surprised they performed well under the current conditions. After months of small swell most open beaches on the East Coast have an abundance of sand which can lead to closeouts when the swell is mainly coming from one direction - as it’s been doing from Sydney south, where small south swells have persisted. Yet the Mid North Coast also receives swells from the east and north-east, and this is just enough to break up what would otherwise be straight banks. “The surf zones there are described as rhythmic bars,” says Ian. “Where you’ve got rip channels running out from the beach.”
Rhythmic bars on the Mid North Coast (Photo: Professor Andy Short/OzCoasts)
The Mid North Coast, and places north of it, have received just enough energy, and from various directions, to maintain rips cutting through the sand build up. If the swell direction was to lock onto pure southerly for weeks at a time this would change the pattern, but it hasn’t been the case. The semi-permanent high pressure system in the Tasman has ensured lots of short-period east swell amongst the small, long-period south swells.
Those small easterly trade swells that sculpted the banks of the Mid North Coast are simply too weak by the time they make it to Sydney, the Illawarra, and the South Coast - if they make it at all. Down here the pattern has been relentless: small and clean from the south, and it’s created what Ian calls “oblique welded bars” and they’re every bit as dull as they sound.
Oblique welded bars are what’s plaguing my local right now, and also many other beaches from Sydney south. Much like the regional patterns seen on the Tweed and Mid North Coasts, the beaches south of Sydney are mostly responding to the calm conditions in a predictable, uniform way. The sand from the area that would otherwise be the surf zone has been pushed shoreward, and the sand banks are ‘welded’ to the beach - meaning there’s no gutter (or very little gutter) between the beach and the bank.
What’s become apparent is that this configuration is terrible for surf quality. The small clean south swells that surfers have enjoyed elsewhere are simply passing over the deep outside banks, then slamming down on the inside bank in a manner that contradicts the old saying that nature abhors a straight line. A T-square couldn’t fault the closeouts I’ve seen.
Also, we‘ve had one or two days of big south swell during the season but they’ve done nothing to shift the sand. Welded bars are stubborn critters and for them to move, to be reshaped, we need to take note of what’s caused the rhythmic bars up north: that being swell from various directions. One big storm could do it, lashing the coast with short range energy that moves about as it strikes, stirring the sand into patterns not sensed since it settled six months ago. Or a sequence of storms from varying directions that stimulate sand flow breaking the tight grip of the welded bars.
As I type this, the East Coast is on the cusp of an extended run of southerly energy with a week’s worth of fronts queuing to drive swell up the Tasman Sea, all of them acutely south. When this cycle of swell is over the sand will, by degrees, be shifted. But whether the wreck of the Amy is covered up again and we can put Autumn 2019 behind us remains to be seen.
PS: Apologies for the East Coast-centric article. At least you surfers from the southern and western states can console yourselves with the certainty of swell over reef.