Jonathan Nott

Waves, coastal boulder deposits and the importance of the pre-transport setting

The pre-transport environment of a coastal boulder along with its shape, size and density determines the height of wave required for it to be transported. Different forces act on sub-aerial boulders as opposed to submerged boulders when struck by a wave. Boulders derived from joint bounded blocks on shore platforms predominantly experience lift force and require a wave of greater height to be transported than boulders in other environments. No one equation is applicable to determine the height of palaeo-waves responsible for depositing a field or ridge of imbricated coastal boulders. A range of equations and their derivation is presented here which can be applied to the respective pre-transport environment of a boulder. Such an approach is necessary when attempting to reconstruct the frequency and magnitude of past coastal wave hazards and for differentiating between tsunami and storm wave deposited boulder fields.

EXTREMELY HIGH-ENERGY WAVE DEPOSITS INSIDE THE GREAT BARRIER REEF, AUSTRALIA : DETERMINING THE CAUSE: TSUNAMI OR TROPICAL CYCLONE

Abstract A hydrodynamic approach is used to determine whether tsunami- or cyclone-generated waves were responsible for the deposition of fields of well-imbricated rock boulders (up to 290 tonnes) along the coast of Cairns inside the Great Barrier Reef, Australia. Calculations of the overturning moments show that only tsunami are capable of moving such large boulders in this environment. It is hypothesised that large tsunami (> 11 m) have been able to penetrate the Great Barrier Reef through wide (5–10 km) 50–70 m deep passages between individual reefs. Three such passages each approximately 35 km apart and oriented in the same direction exist in the Cairns region. It is possible that these passages have funnelled and amplified palaeotsunamis. The preferential location of eroded coral boulders up to 3 m in length on reef flats alongside these passages and their absence on other reefs throughout the region provide further evidence that extremely high-energy waves have been able to penetrate the Great Barrier Reef into the inner channel adjacent to the mainland. Carbon-14 ages of the coral boulders on these reef flats matches closely the ages of coral fragments pinned below the very large rock boulders along the coast. These ages suggest that the Cairns region has experienced large tsunami twice over the last millennium.

Geological Indicators of Large Tsunami in Australia

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 15–20 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 4–6 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.

Wearing Down, Wearing Back, and Gorge Extension in the Long-Term Denudation of a Highland Mass: Quantitative Evidence from the Shoalhaven Catchment, Southeast Australia

The long standing issue of the dominance of scarp retreat versus summit lowering in the denudation of a highland mass is investigated with supporting evidence provided by Tertiary basalts throughout the Shoalhaven catchment in southeast Australia. Both of these forms of denudation are found to be insignificant compared to the role of fluvial gorge extension over the last 30 m.y. Headward advancement of the Shoalhaven Gorge has been occurring at approximately 15 times the rate of major escarpment retreat, 250 times the average rate of summit lowering, and over 500 times the rate of interfluve consumption. These estimates show that the headward erosion of gorges is the most important process denuding the highlands in the Shoalhaven region and possibly elsewhere in the highlands of eastern Australia. Over the long term, the highlands in this region will become considerably more dissected well before they decrease substantially in height or are narrowed. This conclusion has important implications for models predicting isostatic rebound from assumed character and rates of denudation.

Australian tropical cyclone activity lower than at any time over the past 550–1,500 years

The assessment of changes in tropical cyclone activity within the context of anthropogenically influenced climate change has been limited by the short temporal resolution of the instrumental tropical cyclone record (less than 50 years). Furthermore, controversy exists regarding the robustness of the observational record, especially before 1990. Here we show, on the basis of a new tropical cyclone activity index (CAI), that the present low levels of storm activity on the mid west and northeast coasts of Australia are unprecedented over the past 550 to 1,500 years. The CAI allows for a direct comparison between the modern instrumental record and long-term palaeotempest (prehistoric tropical cyclone) records derived from the 18O/16O ratio of seasonally accreting carbonate layers of actively growing stalagmites. Our results reveal a repeated multicentennial cycle of tropical cyclone activity, the most recent of which commenced around ad 1700. The present cycle includes a sharp decrease in activity after 1960 in Western Australia. This is in contrast to the increasing frequency and destructiveness of Northern Hemisphere tropical cyclones since 1970 in the Atlantic Ocean and the western North Pacific Ocean. Other studies project a decrease in the frequency of tropical cyclones towards the end of the twenty-first century in the southwest Pacific, southern Indian and Australian regions. Our results, although based on a limited record, suggest that this may be occurring much earlier than expected.

Tropical cyclones and the evolution of the sedimentary coast of northern Australia

Abstract A considerable portion of the sedimentary coast of northern Australia is dominated by ridge plains (beach ridges) where the ridges are composed of coarse-grained sands and/or sand and beds of marine shells that rise above the limits of normal (fair weather and noncyclonic storms) wave run-up. Elsewhere, there exist ridge plains composed of lithic gravel, coral shingle, shell (cheniers), and, in one location, a ridge of pumice. These ridge sequences also lie above the zone of normal wave (noncyclonic) processes. There is little doubt that these ridges are deposited by waves and it is likely that only tropical cyclone-generated marine inundations are able to cause the necessary ephemeral rise in sea level in order to emplace them. Tropical cyclones also cause substantial erosion of the coast. When the marine inundation (surge + tide + wave set-up + waves + wave run-up) or just wave run-up alone overtops coastal dunes (eolian) or ridges where they are unconsolidated, those dunes are eroded vertically and removed. At times, this can result in the deposition of sand sheets that extend inland for several hundreds of meters and taper in thickness landward. The sedimentary coast of northern Australia is composed therefore of a mosaic of landforms that represent the constant interplay between high-intensity, low-frequency events and processes and high-frequency, lower energy processes. The presence of numerous coastal landforms generated by tropical cyclones highlights the importance of recognizing the role of these events in policies concerning the management of coastal landscapes and also the reduction of hazard risks in this region.

Intensity of prehistoric tropical cyclones

Prediction of future tropical cyclone climate scenarios requires identification of quasi-periodicities at a variety of temporal scales. Extension of records to identify trends at century and millennial scales is important, but to date the emerging field of paleotempestology has been hindered by the lack of a suitable methodology to discern the intensity of prehistoric storms. Here a technique to quantify the central pressure of prehistoric tropical cyclones is presented in detail and demonstrated for the tropical southwest Pacific region. The importance of extending records to century time scales is highlighted for northeast Australia, where a virtual absence of category 5 cyclones during the 20th century stands in contrast to an active period of severe cyclogenesis during the previous century. Several land crossing storms during the 19th century achieved central pressures lower than that ever recorded historically and close to the theoretical thermodynamic limit of storms for the region. This technique can be applied to all tropical and subtropical regions globally and will assist in obtaining more realistic predictions for future storm scenarios with implications for insurance premiums, urban and infrastructural design, and emergency planning.

Alluvial fans, landslides and Late Quaternary climatic change in the wet tropics of northeast Queensland

Extensive alluvial‐fan and debris‐flow deposits occur along the base of the escarpment of the east Australian highlands in the wet tropics of northeast Queensland. Luminescence and radiocarbon dating show that these deposits accumulated between 27 ka and 14 ka, which was the driest phase of climate during the last full glacial cycle. Climatic desiccation and reduced plant cover, along with a continuation of discrete high‐magnitude rainfall events, were the principle causes of this phase of enhanced slope instability. Landslide activity and alluvial‐fan development have continued throughout the Holocene, but probably to a lesser extent and magnitude because of the amelioration of climate and the re‐establishment of forests throughout the region.

Extreme Marine Inundations (Tsunamis?) of Coastal Western Australia

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.

Time and process rates over the past 100 m.y.: A case for dramatically increased landscape denudation rates during the late Quaternary in northern Australia

A central question in the earth sciences is how rates and styles of landscape evolution have varied over time in response to climate and sea-level change, tectonic and isostatic uplift, and human disturbance. The Quaternary has been a period of major landscape evolution in many glaciated regions of the world, but few data sets of sufficient length are available to assess its significance to the long-term development of landscapes in nonglaciated regions. Analysis of denudation rates over the past 0.5 m.y. and 100 m.y. for upland and lowland surfaces in western Arnhem Land, tropical northern Australia, shows that denudation rates have increased by at least an order of magnitude in the late Quaternary compared to the previous 100 m.y., despite tectonic stability and the absence of glaciation in this region over the past 120 m.y. The lowlands have undergone denudation rates up to an order of magnitude higher than the uplands, so that the landscape has increased in relief independently of the absolute rate of denudation. This result is counter to several prominent theories of landscape development that postulate relief reduction over millions of years. At least 3%–7% of the post-Cretaceous denudation of this landscape occurred during the most recent 0.5 m.y. We suggest that the Quaternary here was a period of greatly accelerated erosion resulting from changes in climate and sea level accompanying glacial-interglacial cycles. General sea-level regression since the mid-Cretaceous also likely initiated substantial lowland erosion in this region. It appears, therefore, that the past 100 m.y. have been punctuated by at least two episodes of accelerated landscape denudation.