Coastal Creationism - Part 9: Creating perfect sand-bottom points that peel forever
Why are sand-bottom point breaks only found in northern NSW and south-east Queensland?
We all know that on the East Coast from Crescent Head to the Sunshine Coast is point break heaven, but what is it about this coastline that creates epic points? Why, for instance, are there no sand-bottom point breaks south of the Hunter River? The wave climate is not that different but the coastal landforms certainly are.
The simple answer is summed up in this equation: Sediment Supply + Wave Climate = Sand-bottom Points.
If you compare the coast south of the Hunter River with northern NSW and south-east QLD, there are significant differences in the coastal landforms. To the south there are steep valleys, deep harbours, and multiple lagoons and coastal lakes. All are a function of the same process. The coast of Sydney and the south coast is what is called a 'Flooded Ria' of estuaries filling steep sided valleys, almost like a fjord.
In these locations most sediment coming down the rivers goes toward infilling the valleys that create the lakes and estuaries. Fresh sediment does not make it as far as the ocean. However, northern NSW and south-east QLD is dominated by large rivers with extensive floodplains. The deep valleys in the bedrock are separated from the ocean by extensive coastal plains, therefore these rivers deliver most of their sediment load directly into the ocean. This sand is then transported by longshore drift to accumulate in perfect triangular banks in the lee of rocky headlands.
The flooded coastline of the Central Coast and further south - think Hawkesbury and the coastal lagoons - was created when sea level rose 120 metres at the end of the last ice age. The region is also interesting as the bedrock has been uplifted, creating the plateaus and escarpments that dominate the coast. The uplift created a gradient close to the coast. Due to the low sea level during the ice ages, the rivers carved deep V-shaped valleys to the very low base level, possibly as deep as 180 metres below the current sea level. At this time the coastal plains were exposed with extensive deposits of sand heaped in dunes.
With sea level rising from about 18,000 years ago, some of this sand was pushed inward and upward, forming coastal barriers with beaches and dune systems. When sea level reached its current level, stabilising about 6,500 years ago, all the valleys were flooded forming the harbours. Where coastal sand barriers had been pushed in, they closed off valley mouths to form the coastal lakes and lagoons. Ever since, most of the sediment loads of these rivers has been filling in the estuaries and does not reach the sea. For this reason most of the reefs and points of Sydney and the South Coast are actually formed as a function of bedrock geology rather than sediment accumulation.
Conversely, from the Hunter and Manning Rivers north to about Agnes Waters, large rivers with considerable sediment loads have already infilled their valleys and are now delivering bedload sediments directly into the surf zone. These sediments are probably augmented by a large supply of readily transported sand that may have already been in place prior to sea level rise, ensuring that there is a mass of sand available for longshore transport.
The dominant wave climate of the east coast is of higher energy swells coming from the south through to the east. Cyclones bring occasional energy from the north-east, however these are rare yet enjoyable interruptions to the regular wave climate. The southerly origins of most swells ensure that sand in the active wave zone is inexorably transported northwards through a process of longshore drift. Whenever this sand rounds a rocky headland it is deposited in the lee, forming a nice triangular bank that can turn swell in to peeling, spitting, gurgling barrels for us to get kegged. Depending on the slope of the sandbank, these may range in shape from thin lipped peelers (shallow slope) through to thick sand filled drainers (steeper convex slope).
The best example of this process is the artificially augmented Superbank. The natural process of northward sediment transport had been interrupted by the breakwalls channeling the Tweed River. When the process was restarted with sand pumping, the Superbank was formed in the lee of Snapper Rocks. I am not sure what sort of bank existed at Snapper Rock prior to the Tweed breakwalls but as soon as the sand supply was stopped, big holes formed in the bank. Nowadays they think they understand the process enough so as to manage the sand supply and the subsequent shape of the bank.
You only need to look north to see where it all ends up. At the northern end of the Gold Coast the great sand islands start: South and North Stradbroke, Bribie, Moreton and Fraser Islands, which are the eventual repose of millennia of sand transport. Keep in mind this sand has accumulated over perhaps a couple of million years, with about 17 ice ages and warmer interglacials all doing their part. The ice ages and low sea levels allow the sand to be spread out on the coastal plains and then as sea level rises, the sand is pushed in to the surf zone, creating beaches and dunes, as well as moving the sand north due to longshore drift.
As mentioned in Coastal Creationism 5, the West Australian coast is different, there is just not the same amount of mobile sand. Yes, there is plenty of sand on the beaches, however with the generally drier climate and lack of large rivers delivering sediment to the sea, a greater proportion of the sand has been formed by biological sources - the shells of millions of organisms. In the west the sand is dominated by shelly carbonate fragments. However, in eastern Australia with its higher rainfall and steeper gradients, there is significant land based erosion, transport, and deposition of sediments on the coast. As a result, eastern Australian sands are dominated by terrestrial grains, chiefly quartz, plus some of those heavy minerals like rutile and illmenite. So in the west, the sand has lots of carbonate and readily becomes cemented and locked up in limestones. In the east, the sand remains mobile, accumulates over millions of years and eventually to be swept in to the lee of a headland and form a triangle of fun!
Coastal Creationsim is an ongoing series written by Chris Buykx. Chris is a geologist, traveller and lifelong surfer. Specialising in eco-tourism, his passion is interpreting nature and the environment. Chris is a resident of Sydney’s Northern Beaches. Read past articles:
Part 1: Basic reef shapes
Part 2: Complex curves
Part 3: The good, the bad, and the ugly of coasts
Part 4: Sedimentary sequences and superior shapes
Part 5: Sand dunes and limestone
Part 6: Slabs!
Part 7: Coral reefs are even better than you think!
Part 8: Coral reefs, living and adaptable surf zones