Bêifāng Hâitān Cháng-chéng 長城
Collaroy-Narrabeen’s Great Wall of Sand!

Collaroy-Narrabeen’s 長城

Sth Narrabeen Surf Club peering over the Wall of Sand!

The photos (L + R) taken five days ago show the cumulative effects of successive series of storms on the one kilometre plus stretch of coast where Collaroy Beach merges seamlessly with South Narrabeen Beach. The foredune in the pictures (the high ridge of sand) was created from countless poundings by severe storms over the last 100 years, but the gouging out of a phenomenally large quantity of sand to create the current steepness of the ridge owes itself to the most recent destructive event, a massive storm action which relentlessly blitzed the narrow beachfront at Collaroy and Narrabeen in June 2016. For three days huge waves and king tides pummelled the beach, the cost in landform terms was the lost of an estimated 50 metres of beach on what was already a precariously thin strip of coast [M Levy, A Benny-Morrison, D Dumas, ‘Sydney storm: erosion swallows 50 metres of Collaroy, Narabeen beaches’, Sydney Morning Herald, 07-Jun-2016, www.smh.com.au].

Just one sample of the damage to Collaroy Beach & property in 2016

The damage to coastal infrastructure last year was extensive – back fences, balconies, sunrooms, whole backyards and most spectacularly in-ground swimming pools were uprooted – but came as no surprise to locals, coastal geologists or environmentalists. The beach here has had numerous precedents over the years (such as 1913-14, 1920, 1944, 1945, 1967, 1974, 1978, 1986, 1995, 2007), and has been witness to the undermining of foundations, damage to clubhouses, the washing away of houses and the demolition of some homes. The storm havoc in 1967 undermined the foundations of the (then) recently built, high-rise Flight Deck apartment block [J Morcombe, ‘Collaroy beachfront has been an erosion hotspot for a hundred years’, Manly Daily, 07-Jun-2016].

Urban development and sand bank vulnerability
The present predicament of waves swallowing up beachfront properties had its genesis in the early 1900s when people, attracted by the sea and the views, started constructing their houses on Pittwater Road on the edges of the beach. Solutions to the ongoing onslaught from nature of storms was from the start consistently ad hoc. In the 1920 “Great Storm” locals desperately tried to shore up their beachfront cottages with sandbags✧. Warringah Council’s response to each new threat was largely reactive rather than proactive. The frequency of recurrence clearly called for preventative measures…one of the few early attempts by Council to pre-empt the threat was its purchase and demolition of storm-damaged houses between Arlington Amusement Hall and Jenkins Street, to be replaced by a public reserve.

Flight Deck too close to ground turbulence!

By the early 1960s Council’s planning scheme had polarised the community. The Collaroy-South Narrabeen Progress Association lobbied Council to halt its policy of building flats and resume as many houses as possible along the beachfront, demolishing them to create open space. This prompted an opposition group to spring up, demanding that more flats be constructed on the beach edge. This division was mirrored within Warringah Council itself, Councillors were split, some were in favour of increasing the number of flats, some were opposed to the development. A compromise was reached in 1964 which still permitted construction of flats and houses on the fragile edge of the sand bank [J Morcombe, ‘Milestone for Collaroy’s landmark Flight Deck’, Manly Daily, 12-Feb-2016].

Beach erosion and reinforcement measures
More storms in the eighties and nineties prompted Council to try to come up with better thought-out strategies to cope with the burgeoning threat. Earlier construction of rock walls had been haphazard. In 2002 it opted to construct a 1.1km long sea wall along the most pregnable part of the Collaroy-South Narrabeen beach❖. The plan was shelved after several thousand people staged a beach protest against the proposed sea wall.

Again public opinion has been divided – many different views have been voiced on the issue…from those whose properties would not be protected by the wall or by individual walls covering single properties (eg, water can be pushed on to other neighbouring properties which don’t have a sea wall – sometimes because they were denied approval by Council to build one!). Who bears the cost is another issue – coastal engineer Angus Gordon has raised the thorny prospect of “councils building walls using public money to protect private property” (current Northern Beaches Council policy dictates that individual homeowners must pay for the construction of an approved wall for their properties). Beach-goers too, tend to have a differing perspective to those of beachfront homeowners, many surfers point to the way changes to the nature of the beach can affect the wave patterns and thus the quality of surfing (altering the breaks, etc) [C Chang, ‘Bitter battle over Collaroy beachfront has raged for years’, (News) 07-Jun-2016, www.news.com.au

‘Flight Deck’

Given the alarming extent of beach erosion gorged out of Collaroy and South Narrabeen beaches, supplementing the lost sand is vital. Northern Beaches Council tackles what it calls “Sand Replenishment and Beach Nourishment” by sourcing sand from local building sites, and it is also “dredged periodically from the entrance of Narrabeen Lagoon … to replenish Collaroy-Narrabeen Beach” [‘Coastal erosion’, (Northern Beaches Council), www.northernbeaches.nsw.gov.au/.] One of the concerns about using building site materials is the issue of ensuring clean fill…rubbish, waste matter or other, undesirable pollutants may inadvertently end up in the bolstering mix on the beach (risk of litter and impurities, aesthetic considerations).

Collaroy-South Narrabeen beach is considered to be Australia’s third most at-risk area¤ when it comes to the deleterious effects of coastal processes. Thus maintenance of sand bank stability remains a crucial civic objective against a backdrop of rising seas and unpredictable and extreme weather patterns. This is also an extremely costly matter for both Council and the state Liberal government. In 2013 Waringah Council forked out almost $3M to acquire a (potentially endangered?) luxury beachfront property to demolish it and use the land for a public park, and the troubled beach will figure significantly in the allocation of the $69M the state government announced for councils to address beach erosion, coastal inundation and cliff instability [Chang, op.cit.]

Collaroy Beach dune reinforcement

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Endnote: Longshore drift, an agent of beach erosion
A beach is inherently dynamic, motion and change are constant factors in its composition. Lateral movement of sediment (sand, clay, silt and shingle) along the shoreline is known as Longshore drift (or sometimes called Littoral drift). The geological process occurs thus – the prevailing wind powers the waves, directing them towards the coast. When they hit the surf zone they break at an angle to the shoreline (oblique wave action). Sand and other sediments are propelled along the surf zone in a zig-zagging motion, the swash (onshore-rushing water moves the beach materials along), followed by a corresponding backwash (the water returns offshore – if the wave is constructive, it will do so with reduced energy)[‘Longshore Drift’, (Revision World), www.revisionworld.com]

See related post King Canute Battling the Relentless Tide*: The Dilemma of Rampant Beach Erosion – Gold Coast and Adelaide (Nov. 2017)(Cartoon: Clive Goddard)

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* The title is of course a symbolic nod to King Canute (Cnut the Great) 11th century Anglo-Saxon ruler of the North Sea Empire, and the apocryphal anecdote of his futile but persistent efforts to turn back the tide on the seashore
✧ sand-bagging’s value is as an immediate response to flooding and is considered ineffective in countering sand erosion
❖ Council had earlier erected groynes (low walls or sturdy timber barrier) on the beach which had become largely ineffective over time
¤ after the Gold Coast and Adelaide’s north-western suburban beaches