Displacement of mega boulders across coastal rock platforms south of Sydney, Australia, by big storm waves generated by East Coast Lows

Bann, Dr Glen1, Lau, Dr Annie2

1University of Wollongong, Wollongong, Australia, 2University of Queensland, St Lucia, Australia

This research reports on a number of large boulders, or blocks, displaced across coastal rock platforms between Kiama and Snapper Point, south of Sydney, Australia. All boulders are derived from underlying fine to coarse-grained sandstones. A boulder described previously (Bann 2012), weighing  ~10 tonnes, had been transported 42m across the rock platform and rotated 110o from its original in-situ position. Before and after photos show evidence of more recent movements, ~1m sideways and rotating it back 50o in 2016, coinciding with an ‘East Coast Low’ that impacted the whole NSW shoreline in June 2016. It was moved again in 2017 a further 30o and ~30cm sideways with one end lifted ~15cm to rest it on a ledge, then in July 2020 following another East Coast Low, another 10o rotation and ~20cm movement occurred off and away from the ledge. Three smaller boulders nearby of ~1 to 2 tonne each also get tossed around with these events. Two other much larger boulders weighing ~100 and ~180 tonnes also reported movement previously from 2016 (Bann et al. 2018), to the north of Jervis Bay moved ~10m upslope and rotated ~15 degrees for the smaller one and a few metres up slope for the larger one. The smaller boulder has also been flipped over at some stage given by reversed cross-bedding and inverted sedimentary grading. Slight movement of these two has also been detected since 2016. Examples from other localities include the placement of blocks weighing a few tonnes onto an elevated rock platform, then removed at a later date, a block weighing about 1 tonne lifted and moved onto a higher ledge on a cliff about 20m from the platform sea edge and 5m elevation, and a boulder weighing about 10 tonnes on a rock platform lifted with tree logs emplaced and wedged firmly beneath it before settling on them. Satellite imagery was used to identify approximate movement times and then correlated with storm wave data obtained from wave buoys located offshore at Port Kembla and Batemans Bay. This provided timing in addition to wave heights, allowing approximate wave height constraints for wave height formulas. None of the boulders appear to have been pushed or slid across the actual platform, rather, all boulders show evidence of being lifted during displacement, hence by saltation or suspension. Calculations suggest that waves of considerable height and wave length are required to displace these boulders across the platform, either in the one event, or in incremental steps, larger than the actual heights from the wave buoy data show. The findings indicate that large storm waves are capable of shifting very large boulders, such as those generated by East Coast Lows which frequent the coast during the winter months. Although a few previous authors use boulder movements as evidence that the east coast experiences occasional large tsunamis, tsunamis are not required to transport these large boulders on coastal platforms. Clearly, coastal lowland environments south of Sydney are prone to occasional very large storm waves and needs to be prioritized in coastal hazard and sustainable management plans, particularly applicable to climate change and future sea level rise.


Biography

This is ongoing research which originated in 2012 when looking for intrusions and tuffs along the coast –  noticing big boulders in places they shouldn’t be, then, noticing these boulders are moving around regularly, without leaving scrape marks, meaning, the boulders are effectively floating…

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