[LINK] tsunami risk to Sydney

Danny Yee danny at anatomy.usyd.edu.au
Fri Dec 1 18:11:57 AEDT 2006

Stewart Fist wrote:
> I've never heard any more about that claim, a few years ago, that a tsunami
> hit Sydney about a hundred years before the First Fleet arrived.  They
> claimed at the time that it threw very large boulders up on top of coastal
> cliffs -- so if it was that size, it would have taken out most of Sydney at
> a guess.
> The story appears to have just disappeared, or been ignored.
> I wonder whether the claim is generally accepted in the scientific
> community, or whether it was considered fallacious/ridiculous.
I had a quick look with Google Scholar and found what I think is the
paper that sparked the idea: "Geological Indicators of Large Tsunami
in Australia", E. A. Bryant and J. Nott, _Natural Hazards_ Volume 24
Number 3, November 2001.

The abstract to that is:
	Tsunami waves can produce four general categories of
	depositional and erosional signatures that differentiate them
	from storm waves. Combinations of items from these categories
	uniquely define the impact of palaeo-tsunami on the coastal
	landscape. The largest palaeo-tsunami waves in Australia
	swept sediment across the continental shelf and obtained flow
	depths of 1520 m at the coastline with velocities in excess of
	10 m -1. In New South Wales, along the cliffs of Jervis Bay,
	waves reachedelevations of more than 80 m above sea-level with
	evidence of flow depths in excess of 10 m. These waves swept 10
	km inland over the Shoalhaven delta. In northern Queensland,
	boulders more than 6 m in diameter and weighing 286 tonnes
	were tossed alongshore above cyclone storm wave limits inside
	the Great Barrier Reef. In Western Australia waves overrode
	and breached 60 m high hills up to 5 km inland. Shell debris
	and cobbles can be found within deposits mapped as dunes,
	30 km inland. The array of signatures provide directional
	information about the origin of the tsunami and, when combined
	with radiocarbon dating, indicate thatat least one and maybe
	two catastrophic events have occurred during the last 1000
	years along these three coasts. Only the West Australian coast
	hashistorically been affected by notable tsunami with maximum
	run-up elevations of 46 m. Palaeo-tsunami have been an order of
	magnitude greater than this. These palaeo-tsunami are produced
	most likely by large submarine slides on the continental
	slope or the impactof meteorites with the adjacent ocean.

And there's a followup about Western Australian coasts:
	Along 2500 km of the Western Australian coast, prehistoric
	ephemeral marine inundations (storm surges or tsunamis)
	were much larger than those that occurred since European
	settlement. The evidence is in the form of shell and coral
	deposits atop 30-m-high headlands, sand deposits containing
	large boulders, shell and coral several kilometers inland,
	and fields of large imbricated boulders across shore
	platforms. The size of transported boulders and the altitude
	of these deposits suggest that tsunamis were responsible,
	not large storm waves. The orientation of boulders reveals
	paleowave directions. Radiocarbon dating of the deposits
	suggest three very large tsunamis along this coast during
	the past millennium.

There have been a few responses:

Perhaps the most interesting is this PDF
	The tsunami hypothesis proposes that prehistoric tsunamis may
	have been larger than historic ones along coasts normally
	(historically) not associated with major tsunamis. The
	evidence for the hypothesis rests with the types of unusual
	sedimentary deposits and erosional forms along coasts where
	the largest historic and prehistoric storm waves do not appear
	capable of forming the features. This is especially the case
	at locations where boundary conditions, i.e. offshore water
	depth, coastal geomorphology and meteorological limitations,
	are not conducive to the propagation of sufficiently large
	storm waves at the shore. The tsunami hypothesis has been
	barely debated in the literature. This is despite the view
	of some, who suggest that storms have been overlooked, or
	underestimated, as a cause. Few comparisons have been made
	of the supposed tsunami generated features and the impacts on
	coasts of extreme intensity storms. Four of the most powerful
	tropical cyclones anywhere in the world in recent times struck
	the Western Australian coast between 1999 and 2002. The results
	of post-event surveys of these storms showed that none of them
	produced the enigmatic forms attributed elsewhere to tsunamis.

	This paper documents the deposition of large boulders on the
	Beecroft Peninsula, New South Wales, during a one in four-year
	swell event in October 1999. The size of the boulders in
	relation to the magnitude of the swell suggests that the
	boulder field at Whale Point on the Beecroft Peninsula is
	explicable in terms of storm swells alone.

And so forth.

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