Report Reveals Location Of 400,000 Cubic Metres Of Eroded Sand From Collaroy Is Revealed

13 Dec 2016 8

The sandless Collaroy-Narrabeen stretch

The sandless Collaroy-Narrabeen stretch

COASTALWATCH | ENVIRONMENTAL NEWS

The Impact & Aftermath Of The June 2016 Swell 

Story by Coastalwatch Coastal Scientist Prof. Andrew Short and Dr. Mitchell Harley, Water Research Laboratory, University of New South Wales

THE SNAPSHOT

WHAT HAPPENED? The 3.6km stretch of Collaroy-Narrabeen made international headlines after backyard pools were swept into the ocean, houses and businesses collapsed under the erosion of the beach and insurance companies refused to compensate.

THE STORM’S IMPACT: The waves peaked at 6.5m and exceeded 3m for three days between 4-7th June 2016 along the east coast of Australia.

HOW MUCH SAND MOVED? 410,000m3 of sand was moved from the Collaroy-Narrabeen beach during this time (Pasha Bulker Storm of 2007, in comparison was just 263,400m3).

WHERE IS THE SAND? The seabed was eroded out to 3m with the sand deposited 250metres offshore to a depth of 8metres and is slowly making it’s way back in.

WHY WAS IT SO BAD? Private development dating right back to the 1900s is to blame, with apartments erected in the active beach zone and right across the foredune and beach area. Every major storm over 100 years has demanded the beach’s sand back so the beach and houses are destroyed or severely threatened every time (Major storms have occurred in1920’s, in 1944, 1967, 1967, 1975, 1978 and 1986.)

The June 2016 east coast cyclone which impacted the NSW coast between 4th and 7th June, was one of the largest storms in decades and also one of the most damaging – but not the most intense, nor most damaging.

So while it got national and international headlines and media coverage with graphic photos of houses undermined and backyards destroyed at Collaroy, just how big and how bad was this storm event?

SEE ALSO: The Impact Of Coastal Erosion In Australia

We are fortunate that the storm and accompanying big waves were forecast days in advance, both by the Bureau of Meteorology and by Coastalwatch Chief Surf Forecaster Ben Macartney. Just as the surfers were prepared, so were the scientists. While surfers were checking their equipment and deciding where to tackle the incoming waves, scientists were busy measuring the beaches before the storm impacted.

In order to measure the impact of a storm event you need to know a few things –THE WAVES: their height, period, direction and duration. That was taken care of by the NSW Office of Environment and Heritage (OEH) series of wave rider buoys spread between Byron Bay and Eden. In addition knowing the storm was coming the UNSW Water Research Laboratory (WRL) installed a wave rider buoy 300 m off Collaroy Beach, just two days before the storm hit, and miraculously it survived.

SEE ALSO: Revisit The June Super Swell In Pictures 

Next the amount of erosion on each beach. Here modern technology in the form of Lidar (Light Detection and Ranging) was utilised. UNSW-WRL and the UNSW School of Aviation arranged for the coast between Coffs Harbour and Sydney to be flown and mapped with Lidar before the event (31 May 2016), and then immediately after the storm had passed (7-8 June 2016). As Lidar provides accurate 3D images of each beach, by comparing the before and after images the exact changes in beach width and volume for the each beach can be mapped and quantified. By subtracting the two the change in volume, that is, how much was eroded is known as the ‘storm demand’. That is, how much sand was removed or demanded by the waves on that particular beach. The bigger the demand the greater the erosion.

(Video above is the drone footage taken over the course of the June 2016 swells showing the damage to the beach. By the UNSW Water Research Labs.) 

Next, it would be nice to know where all the eroded sand goes - offshore no doubt but how far. To check this the OEH team ran a bathymetric survey of Narrabeen-Collaroy beach just before the event. This was followed by a post-event survey.

Finally, we need to know how big the waves were when they broke at the beach, which will change along the beach, particularly if its protected by a headland. The way the waves move from deep water into the beaches depends on a number of factors, such as the wave direction, height and period, and the seafloor bathymetry, with wave refracting or bending around headlands. In some areas the waves will be focused and bigger such as North Narrabeen, and lowered in other location. This is all important because of how the wave height and energy is spread along the shore determines the actual amount of erosion along the beach, the direction of longshore currents and the location of rip currents, and in particular the location of megarips.

SEE ALSO: Feature Film, The Biggest East Coast Swell Of A Generation

Megarips are massive rips that can drain an entire beach the size of Palm Beach or Manly. Longer beaches like Narrabeen usually have two to three megarips, while smaller beaches like Bilgola join with Newport to form one giant meagrip. Megarips flow faster (up to 3 m s-1), deeper, and further out to sea (as much as 1-2 km). This means they can erode more sand and carry it further seaward. Where they exit the beach is usually the area of greatest erosion, so their location is critical to localized erosion hot spots. Because they can flow so far out to sea depositing the eroded beach sand in deep water (10-20 m), it can take months to years for the sand to return, and in extreme cases it deposited in such deep water (>30 m) it cannot return and is lost from the system.

In order to track the movement of the June storm waves from deepwater to wave breaking a program called ‘SWAN’ is used (Fig. 1). It models the movement of the waves over the seabed and their changes in height and direction as a result of this interaction, and finally provides the height of the wave at the shore, the ‘breaker wave height’. With this information the variable behavior between different beach and even within one beach can be more clearly understood.

Fig. 1 SWAN modeling of 3 m waves moving into Narrabeen-Collaroy beach. Note the focusing (red) on the headlands

Fig. 1 SWAN modeling of 3 m waves moving into Narrabeen-Collaroy beach. Note the focusing (red) on the headlands

So what happen during the June storms?

WAVES:

The waves were big peaking at 6.5 m and exceeding 3 m for three days (3-5 June) (Fig. 2 and Table 1). 

Fig. 2. Plot of wave height during three recent large wave events, including the June 2016 event

Fig. 2. Plot of wave height during three recent large wave events, including the June 2016 event

EROSION:

Erosion of the average volume of sand eroded from the beach ranged from 12 to 130 m3 m-1, that is, for every meter length of beach 12 to 130 m3 of sand was eroded. For a beach like Narrabeen-Collaroy which is 3.6 km long this equates to 410 000 m3, a massive amount of sand. This compared with 263 400 m3 for the June 2007 (the Pasha Bulker storm) and 199 600 m3 for the April 2015 events (Figs 3 and 4).

Fig. 3 Amount of erosion along Narrabeen-Collaroy beach during three recent storm events including the June 2016 event (red)

Fig. 3 Amount of erosion along Narrabeen-Collaroy beach during three recent storm events including the June 2016 event (red)

Fig. 4 Amount of erosion at P4 in the centre of Narrabeen Beach and a time series of the beach volume at P4 between 1976 and 2016. The green to red dot indicate the amount of erosion in June 2016.

Fig. 4 Amount of erosion at P4 in the centre of Narrabeen Beach and a time series of the beach volume at P4 between 1976 and 2016. The green to red dot indicate the amount of erosion in June 2016.

DEPOSITION:


Again off Narrabeen the pre and post-storm surveys show the seabed was eroded out to a depth of 3 m, with sand then deposited up to 250 m seaward and to a depth of 8 m (Fig. 5)

Fig. 5. Pre and post storm seabed off Narrabeen Beach.

Fig. 5. Pre and post storm seabed off Narrabeen Beach.

IMPACT:

The most dramatic impact of the June storms was what happened at Collaroy, particularly between Stuart and Fraser streets, where several houses lost their yards and even a swimming pool. Less obvious was the failure and undermining of most of the exiting seawalls spread along Collaroy beach.

Just as Collaroy eroded, so did every other beach in the state erode, some experiencing even more erosion that Collaroy. But why was Collaroy so special? Why did it suffer such severe damage and not the other 754 beaches along the NSW coast? There are two reasons for this:

DIRECTION:

First was the wave direction, it was easterly (107°), whereas most major storms arrive more from the south to southeast. As a result, at Collaroy it lost the usual protection of Long Reef Point, meaning it was more exposed to the June waves then would normally be the case. Collaroy got hammered. However so did every other usually sheltered southern end of our many embayed beaches.

What else unique happened at Collaroy?

To answer that we need to go back more than 100 years when Collaroy beach was subdivided and the land sold for private development. Unfortunately the blocks went right down across the foredune and onto the beach and people built their houses on the foredune and even the beach. They did it in the 1900’s and are still doing it today. In the meantime, every major storm has demanded its sand and so the beach was eroded and houses were destroyed or severely threatened at Collaroy in the 1920’s, in 1944, 1967, 1974, 1978 and 1986. This all happened because the houses are located in the active beach and well within the storm demand zone and with climate change they are creeping further onto the beach.

RECOVERY:

Since the June 2016 event, there have been no further storm events and the generally low waves have allowed the sand taken offshore to slowly return to the beach – the beaches are recovering. In six months, the beach has recovered about 40% of the sand taken from it over the few days of the storm (Fig. 5). The substantial damage to dunes however, will take a lot longer to recover, as winds slowly transport sand to repair the large dune scarps seen all along the coast.

Fig. 6 Images from the UNSW Coastal Imaging Station taken immediately after the storm (6 June 2016, above) and six months later (6 December 2016, bottom)

Fig. 6 Images from the UNSW Coastal Imaging Station taken immediately after the storm (6 June 2016, above) and six months later (6 December 2016, bottom)

What we have seen with the June storm is a recurrent event. It is estimated that the June storm was a 1 in 20 year event. In other words we can expect a storm of this magnitude on average every 20 years. Even bigger storms occur every 50 and 100 years. The 1974 event was been placed between a 1 in 100 to 1 in 200 year event, a far bigger and more damaging event than the June storm. When the next ’74 magnitude storm arrives, not just Collaroy, but every beachfront property owner in the state, beware! It’s impact will be widespread and devastating.

What are the solutions?

Do nothing and watch the houses fall into the sea? – the UK approach. Build seawalls and replace our public sandy beaches with piles of rock to protect a few private houses? Resume the properties and turn them into parkland and public open space (imagine Collaroy without the houses!)?

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