Sand Nourishment Of Australia's Iconic Surfing Beaches

20 Mar 2009 0 Share

Sth Narrabeen is in serious need of some sand nourishment.

Sth Narrabeen is in serious need of some sand nourishment.

It really is a matter of when not if.

- By Professor Andrew Short.

Sand nourishment is coming to a beach near you - its just a matter of when not if. I predict it will start happening on the most at risk Sydney beaches within the next decade or so.

What is sand nourishment? Why do some Sydney beaches need it right now? Why will all beaches need it in the future? Where does the sand come from and what are the environmental impacts?

The mechanical bank at Kirra the way it was back in the day.

The mechanical bank at Kirra the way it was back in the day.

The Superbank in all its glory.

The Superbank in all its glory.

Sydney’s and for that matter all Australian beaches consist of sand, composed of either rock debris (usually quartz grains) or broken up calcareous material (shells, hard algae, etc). While calcareous material is being continually produced in the sea, there is a finite amount of quartz sand lying off our beaches. Along the southeast coast including the Sydney region most beaches are composed of between 70 to 98% quartz sand, meaning we need to find more sand of this type if we are to nourish our beaches.

Big Groyne and Little Groyne as seen from the air before the sand pumping began. Check the water washing up onto the breakwall north of Little Groyne.

Big Groyne and Little Groyne as seen from the air before the sand pumping began. Check the water washing up onto the breakwall north of Little Groyne.

Kirra breaks so wide these days. You have to be tow assisted to be able to deal with the sweep. Jan 08 line up.

Kirra breaks so wide these days. You have to be tow assisted to be able to deal with the sweep. Jan 08 line up.

Why nourish? Beach nourishment refers to the supply of sand to a beach from another source, which may be from an estuary, river or the seabed off the coast. As sea level rises beaches will slowly retreat. By nourishing a beach you can retard the retreat at the same time as widening the beach and surf zone. This is what has been happening on the southern Gold Coast, which is manifest in the very wide beaches at Rainbow Bay, Greenmount and Kirra, is also replicated in the surf zone, where it resulted in the super wide surf zone known as Superbank.

So beach nourishment can be good for the beach – its wider, and good for the surf, especially if it produced better banks. The wider beach is also good for protection of any exposed property or facilities behind the beach, such as surf clubs, car parks, beachfront houses etc. All this means we have a wider more useful beach, as well as a soft (i.e. sand) form of coastal protection, which looks just like the original beach. This is why they have been pumping on the Gold coast since 1976 and to this day about 500 000 m3 per year are pumped each year from NSW under the Tweed River to supply the southern Gold Coast. It has also been happening on a smaller scale (about 50 000 m3 per year) along the Adelaide coast.

Suss the shortage of sand at Nth Kirra and also look how close to Greenmount Point the bank at Cooly used to be.

Suss the shortage of sand at Nth Kirra and also look how close to Greenmount Point the bank at Cooly used to be.

North Kirra desert thesedays.

North Kirra desert thesedays.

Sand nourishment has been proposed for the Sydney coast since the 1970’s. But to date only minimal nourishment has taken place at Cronulla and Collaroy, just a few 10 000’s m3. This is good but nowhere near enough to have a serious and visible impact. Narrabeen-Collaroy beach would require on the order of 2.5 million m3 to build the beach 30 m seaward, likewise Manly would need about 1.5 million m3 and the entire Sydney coast on the order of 20 to 30 million m3. Sounds a lot but since 2000 when pumping started approximately 5 million m3 has been pumped onto the southern Gold Coast already and it should reach 10 million m3 by 2020.

On the Gold Coast sand moves along a coastal conveyor belt, eventually making it up to Surfer Paradise and The Spit, where it is pumped at a similar rate under the Nerang entrance onto South Stradbroke Island at The Other Side, yet another man-modified surf break. This sand continues heading north to ultimately reach Fraser Island (thousands of years later) where it moves along the northern submerged Break Sea Spit to finally spill down the Continental slope to the lost to the deep ocean (but that’s another story).

The sand is so wide at Snapper these days that after a storm swell, waves actually break left back towards the point.

The sand is so wide at Snapper these days that after a storm swell, waves actually break left back towards the point.

The Sydney and most of the central-southern NSW coast is however physically very different to the Gold Coast. It has large headlands that prevent northward sand movement. The sand on all Sydney beaches is essentially locked between the boundary headlands, with the only escape directly offshore to deep water. And this is in fact where sand has been escaping for the past 12 000 years. During very big seas and swell, a series of megaips form within Sydney embayed beaches. These massive rips are big and fast enabling them to erode more beach, and transport it faster and further seaward, in fact so far seaward some of it is deposited in water depths between 30 to 80 m depth, 1 to 3 km off the coast. At these depths the sand cannot return to shore. Over the past 12 000 years this sand has been slowly accumulating, off Broken Bay and off Narrabeen, and off much of the coast between Long Reef and The Royal National Park. It has accumulated as a huge body of sand, which are called Inner Shelf Sand Bodies (ISSB). They were only discovered by members of the University of Sydney's Coastal Studies Unit in the mid-1980’s, who went on to find similar deposits off all exposed steep cliffy coasts, including off Cape Byron, Seal Rocks, Narooma-Bermagui and Cape Howe.

The much talked about sand pumping.

The much talked about sand pumping.

There have been sporadic attempts at Cronulla in the last decade to replenish the sand at Cronulla, but replacing all the lost sand from Wanda/Greenhills is unrealistic. Some sand nourishment is obviously urgently required though.

There have been sporadic attempts at Cronulla in the last decade to replenish the sand at Cronulla, but replacing all the lost sand from Wanda/Greenhills is unrealistic. Some sand nourishment is obviously urgently required though.

So we have a scenario in central NSW where we have a number of beaches already in need of nourishment including Stockton, Palm Beach, Collaroy, Manly, Cronulla, Warilla, with every beach probably needing topping up as sea level rise accelerates. We very fortuitously have off the same coast massive shelf sand bodies, orders of magnitude larger than the beaches they could protect. The simple solution is to pump sand from the sand bodies onto affected beaches. This is also where the sand came from originally.

It would cost lots – at least $1 for every 1 m3 of sand, probably more. But what price would you put on Sydneys or your favourite beach. And the alternative is eroding beaches, exposed rocks and seawall, and houses toppling into the sea, which also means no where to sit on the beach yet alone hold a the monthly surf comp. If we wish to maintain our beaches, our beach amenity and to protect the backing properties and facilities then beach nourishment is the best option.

Environmental impacts?

Brighton Le Sands - without much sand.

Brighton Le Sands - without much sand.

Moving millions of cubic metres of sand from the inner shelf to the shore will have a massive physical impact, particularly on the beaches it widens, just look at the southern Gold Coast. It will also slightly lower the seabed from where the sand is pumped, perhaps by a metre or. This will disturb the benthic marine organisms, which will take about 3 month to re-establish their habitat, and in fact it will allow a little bit more wave energy to reach the shore, i.e. slightly bigger waves. So wider beaches, slightly deeper seabed and slightly bigger waves, and after 3 months happy seabed critters.

Sand nourishment is coming to a beach near you – its just a matter of when not if. I predict it will start happening on the most at risk Sydney beaches within the next decade or so.

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