August 15 - 21, 2021: Issue 506

 

Drone Gives A New View On Coastal Stability

Turimetta: Narrabeen Lagoon entrance to Bungan Beach

The first of a series of five videos taken by a drone flying just offshore from Long Reef to Barrenjoey was released this week. All of the imagery was taken in 2021 and we believe that repeated flights every year or so will reveal a great deal about the rapidity of coastal change. 

With the latest IPCC report also released this week we have scientific confirmation that climate change is with us.  One on the issues has always been sea level rise. Everyone is familiar with the threats posed by severe storms along Collaroy – Narrabeen Beach, but we hear little about what might happen to the smaller beaches, and almost nothing about the stability of the cliffs. The unique view that the drone has provided not only identifies geological features that are otherwise very hard to see but also has some significant warnings about coastal change.

From this first video several points emerge:

  • The big beaches such as Narrabeen and Mona Vale are backed by dunes and lagoons and the embayments occupy former valley floors where we can expect the sand to be quite deep. As sea level rises these beaches will erode in severe storms like 2016 but in time some of the sand will return. Over decades there will be a slow episodic retreat landwards. In these locations some form of beach protection can be expected to work to slow the process.
  • The smaller beaches such as Turrimetta, Basin and Bungan however have not formed across former valleys but bury coastal shore platforms and are backed by vegetated cliffs. The volume of sand in these is a very small wedge and they have almost no dune backing them. Even a small amount of sea level rise will wipe these beaches out and the cliffs will once again be subject to wave attack.
  • About half of the cliffs are vegetated and only have a few small rock falls. But the cliffs without vegetation and especially the headlands, have many rockfalls, a number of which are fairly recent. In 2007 the former Pittwater Council commissioned a study of rockfall frequency and volume and drew the conclusions that falls ranged in volume from 1 to 400m3 and that on average we could expect 10 m3 of rockfall per km per year. That study relied on knowledge of falls that had been reported to Council and it was not able to inspect about 25% of the entire cliffed coast.  This drone view will give us a better record and we have some preliminary figures for Turrimetta Headland that show that the largest recent falls are about three times the volume previously described, and ten times the average annual volume per kilometre.  With a rise in sea level and more wave attack at the cliff base we should be concerned.

Which set of numbers is right? Well, both are ballpark figures but they are probably both ‘right’ because the drone view also shows that some sections of the coast, Turrimetta Headland being one of them, are subject to larger and more frequent falls than elsewhere simply because that headland lies on a zone of close spaced rock joints and minor faults that can be identified as an extension of the Luna Park Fault Zone.  This major structural weakness has not been projected so far north before and there is a lesson here that says we should undertake a structural survey of all the cliffs in order to identify the critical zones.

In short the drone has given us new eyes with which to view details of the geology right under our noses which were previously almost invisible.

At:

2.56 A nice rockfall from about early 2015

4.34 Clean section with Bald Hill claystone on the shore platform, about 5m of Garie Formation and then the Newport Formation.

5.18 Significant cliff instability on the headland with earlier falls sitting on the concrete of the Warriewood outfall circa 1980. The cliff here is very cracked and ready to go.

7.38 A nice dyke

8.45 The blowhole sea caves

18.09 The rounded pool at the edge of the platform where a large joint block has been plucked out. Not sure what rock unit this, probably Bald Hill Claystone but just might be a bit of Bulgo Sandstone, needs to be visited.

Dr. Peter Mitchell

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