School of Earth Sciences - Theses

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    Thermochronological and structural insights into the Mesozoic-Cenozoic tectonic evolution of the eastern Tibetan Plateau
    Tian, Yuntao ( 2013)
    This thesis explores the evolution of the eastern Tibetan Plateau (TP) in select areas, i.e. Yidun Arc, eastern Songpan-Ganze terrane (SGT), Longmen Shan (LMS) and Sichuan Basin (SB) using a combination of structural and thermochronological techniques. Results highlight the role of: (i) flat subduction of the Meso-Tethys in triggering crustal refrigeration and exhumation in the Yidun Arc; (ii) crustal strength discontinuities in transferring deformation in central Asia; (iii) inherited crustal architecture in forming the eastern TP margin; (iv) crustal extrusion in forming the post-collisional stress-regime and high elevations in the eastern TP; and (v) Late Cenozoic onset of the Asian monsoon in enhancing river incision in the southeastern TP. Thermochronology data from the Yidun Arc indicate a distinct phase of Late Jurassic-Early Cretaceous crustal refrigeration and exhumation, which is interpreted as resulting from flat subduction of the Meso-Tethys and subsequent Lhasa-Qiangtang collision along the Bangong suture. Such an interpretation is consistent with lithospheric features imaged by seismic data. Thermochronology data from the eastern SGT (including a deep >7 km borehole) point to a ubiquitous phase of Late Jurassic-Early Cretaceous cooling. This cooling is best explained by regional post-orogenic denudation, as a far-field response to Lhasa-Qiangtang collision to the south and extensive sinistral shearing along major faults bounding the SGT. Projecting these aforementioned events onto an Early Cretaceous paleogeographic terrane reconstruction, results in a new tectonic model, where shearing along the faults transferred strain related to the Lhasa-Qiangtang collision into central Asia. Structural and geochronological results from the southern LMS suggest that Early Cretaceous-Early Paleogene deformation of the LMS included a phase of crustal extension along its hinterland, and a phase of crustal shortening along its front, forming ~2-3 km thick foredeep deposits in the southwestern SB. This deformation assemblage is interpreted as the combined effect of failure of the LMS crustal wedge and clockwise rotation of the SGT. These results indicate that the pre-Cenozoic LMS was underlain by a thickened crust, which was further thickened by Late Cenozoic crustal shortening along a series of listric reverse-faults merging into a detachment seated at a depth of ~20-30 km. Thermochronology data from deep boreholes across a W-dipping reverse fault in the eastern SB point to a distinctive cooling episode in the hanging wall commencing at ~28 ± 3 Ma. This age constrains the timing of a Cenozoic shortening component along this structural belt corroborating that a phase of W-E shortening occurred in the eastern TP and SB. This finding is consistent with evidence supporting extrusion of the eastern TP. Enhanced river incision in the Yidun Arc (part of the southeastern TP) was initiated in the Early Miocene (~15-22 Ma). This timing is older than previous proposals for Late Miocene plateau formation and river incision elsewhere in the same region. It is concluded that the Early Miocene inception of river incision heralds the onset of surface uplift by continental subduction and extrusion, whereas Late Miocene incision was triggered by commencement of the Asian monsoon.
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    Exploring the palaeoclimate potential of South East Australian speleothems
    Green, Helen Elizabeth ( 2013)
    The paucity of palaeoclimatic data existing for the Southern Hemisphere and the regional bias of new data from the Northern Hemisphere has meant conclusions regarding the global response to the numerous climatic events of the last 20 kyr is both widely disputed and poorly understood. Despite being one of the Southern Hemisphere’s largest landmasses, Australia in particular displays a limited pool of palaeoclimatic information and the production of a new, robust record providing an insight into the response and timing of key climatic events is paramount to generating a more comprehensive characterisation and improved understanding of palaeoclimate in this region. Speleothems (cave deposits) are valuable archives of palaeoclimate variation, characterised by their extensive growth intervals and large geographic extent. They contain a multitude of ‘proxy’ records both directly and indirectly linked to climatic fluctuations and are typically robust, displaying high preservation potential with no post-depositional alteration. Key to their success is their amenability to radiometric dating, allowing the establishment of robust and reliable chronologies to which their multi-proxy records can be anchored. Consequently speleothems provide a clear opportunity to explore and expand palaeoclimatic knowledge at sites across the globe. This thesis describes and explains the use of state of the art technology to exploit relatively recent advances in U-series dating to construct reliable and detailed records of south east Australia’s response to palaeoclimatic fluctuations over the last 50 kyr using samples collected from cave sites from across the states of Victoria and New South Wales. 28 speleothem samples have been analysed in terms of both their coincident growth intervals and stable isotope variation to provide records with palaeoclimatic implications at a range of time scales. The production of a chronological template of speleothem growth intervals has enabled the assessment of south east Australia’s response to some of the key local and global millennial scale climatic events of the last glacial to interglacial transition and detailed stable isotope analysis of selected samples have been interpreted with the aid of a thorough cave monitoring programme, identifying increased variability in the region’s climate during the late Holocene. The palaeoclimatic records developed in this thesis represent a significant step forward in Southern Hemispheric palaeoclimatology. These records offer valuable new data for both palaeoclimatologists exploring south east Australia’s past climate and those investigating climatic fluctuations at a hemispheric to global scale. The high resolution and robust chronology of the records produced means that they provide a benchmark to which future records might be anchored.
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    Hydrological extremes and consequences of climate change
    Jakob, Dörte ( 2013)
    In the design of infrastructure, risk has been – and often still is – assessed on the basis of long-term averages. Information on variation in hydrological extremes is required as the basis for informed decision-making, preparedness and possible adaptation. Long-term trends are fairly well understood for temperature but less well for precipitation. While climate models have become sophisticated tools for projecting future changes in our climate, their ability to replicate observed variations in precipitation is limited and it is therefore prudent to complement climate models through analysis of historical observations. Design rainfall is used as one of the required inputs for hydrological models in the design of structures such as dams and bridges. Design rainfall estimates are supplied in form of intensity-frequency-duration curves. Rainfall frequency analysis is almost invariably based on the assumption of a stationary climate. Sub-daily durations are of particular interest for urban applications. This thesis was strongly driven by the motivation to provide guidance to decision makers who have to account for non-stationarity in rainfall extremes. Non-stationarity in rainfall extremes comes about as a conflation of climate change and climate variability. Unlike for temperature extremes, rainfall extremes for Australia as a whole exhibit no clear increase or decrease in intensity over time but strong association with the El Niño-Southern Oscillation (ENSO). This has implications for the choice of suitable analysis techniques, e.g. sophisticated non-parametric techniques. Depending on the planning horizons both climate change and climate variability may have to be accounted for. The association of rainfall extremes with ENSO leads to an opportunity to develop statistical models to support decision-making on shorter time scales. Analysis of seasonality in frequency and magnitude of rainfall extremes revealed considerable variation across a set of sites in the southeast of Australia, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. The strongest signal for an increase in extreme precipitation is found for short durations. Changes in rainfall extremes come about through a combination of changes in thermodynamical and dynamical variables. To assess large-scale changes in circulation, a classification technique (self-organising maps, SOM) was applied and synoptic types were identified. Rainfall extremes were then related to the synoptic type under which they occurred, to assess observed changes in the frequency of rainfall extremes. Rainfall extremes are typically preceded by conditions that are much wetter (both in absolute and relative terms) and warmer than the climatological average. These anomalies tend to be larger for shorter durations, and for rarer events. Given that increase in humidity exhibits strong regional variability and that it may be counteracted by changes in dynamics, it appears simplistic to state categorically that climate change will lead to an increase in extreme rainfall events and observed trends in rainfall extremes show a picture that is more complex. In summary, the combination of changes in thermodynamic and dynamic variables will define the change in frequency and intensity of rainfall extremes. The factors that are most relevant for the effect of climate change on rainfall extremes depend on geographical location.
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    Extending the instrumental climate record of southeastern Australia
    Ashcroft, Linden Claire ( 2013)
    Southeastern Australia (SEA) is the most highly populated and agriculturally productive area of the Australian continent. The region also experiences one of the most variable climates in the world, particularly in terms of interannual rainfall. To understand the influence that anthropogenic climate change will have on future SEA climate, it is important to identify the full range of natural climate variability in the region. However, the current instrumental datasets used for SEA climate research only begin in the early 20th century, restricting efforts to identify long-term climate changes and low-frequency variability. This thesis describes the first instrumental climate record for SEA to extend from European settlement in 1788 to the end of 2012. Historical instrumental observations of air temperature, atmospheric pressure and rainfall over SEA have been located and homogenised for the period 1788 to 1909. The historical observations have then been combined with existing high-quality SEA climate data for 1910 to 2012 to examine interannual SEA climate variability over the past 225 years. The extended climate record includes a number of previously unpublished observations during 1788–1859, providing new insight into the climate experienced by early European settlers in Australia. The interannual climate variability identified using the extended record is in good agreement with SEA documentary records, palaeoclimate reconstructions and a historical reanalysis product, verifying the quality of the homogenised pre-1910 SEA climate data. Assessing rainfall variability during 1788–2012 identifies the most recent droughts in 1935–1942 and 1997–2009 as the longest periods of rainfall deficit in SEA since at least 1832, when continuous rainfall observations begin. Conversely, prolonged wet periods during the 1870s and 1890s appear to be more extreme than more recent wet conditions experienced in the 1950s and 1970s. Analysis of the extended SEA temperature record reveals that the current positive temperature trend seen in the region is the strongest and most significant since at least 1860. Long-term stability of teleconnections associated with climate variations in the SEA region are also examined, with a focus on the influence of El Niño–Southern Oscillation (ENSO) on SEA rainfall. Breakdowns in the ENSO–SEA rainfall relationship are identified during 1835–1850 and 1920–1959, in agreement with previous observational and palaeoclimate studies. The decrease in ENSO–SEA rainfall correlations appear to be associated with changes in the Southern Hemisphere mid-latitude meridional pressure gradient, possibly linked to prolonged negative phases of the Southern Annular Mode. The extended instrumental climate record developed in this thesis makes a significant contribution to the emerging field of historical climatology in Australia. It offers valuable new data for historians, climatologists and palaeoclimatologists exploring SEA’s past and present climate. As the regional impacts of anthropogenic climate change become an increasing reality, improved understanding of past climate variability is vital for future climate research.
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    Thermochronology of Tasmania and the South Tasman Rise: implications for the dynamic evolution of a complex rifted continental block
    Chung, Ling ( 2013)
    The crustal architecture of previously adjacent basement terranes in SE Australia, Tasmania and northern Victoria Land, Antarctica is a legacy of late Neoproterozoic-Paleozoic subduction along the east Gondwana margin, highlighting the Cambro-Ordovician Delamerian-Ross orogeny. Structures in this ancient crust were reactivated during late Mesozoic-Cenozoic Gondwana breakup. Tasmania and the offshore South Tasman Rise (STR) lay in a crucial location at the centre of these continental fragments and potentially contain clues regarding the nature of Gondwana dispersal. This study reports results of a systematic thermochronological study that has been carried out on STR dredge samples and Tasmanian dolerites to uncover the history of the ancient subduction system and the post-break up thermal history recorded in their crust. Oceanic investigations (ODP, IODP, magnetic and seismic surveys and swath mapping, etc) have been conducted across the STR and the western transform boundary over the past few decades. Important results have been previously reported and form the foundation towards understanding the tectonic significance of this region. In particular, the interpreted seismic profiles along with basement dredge materials provide essential structural, stratigraphic and petrological controls as in situ investigations are not possible. Nevertheless, only limited geochronological data are available and the regional thermal history is still sparse. In this study a systematic low-temperature thermochronological study has been carried out, applying 40Ar/39Ar, AFT, ZHe and AHe dating techniques to the STR dredge samples to disclose the tectonic evolution of study area. Results suggest that the western and eastern terranes of the STR (W-STR and E-STR, respectively) evolved differently prior to east-Gondwana breakup. While the 40Ar/39Ar data from the W-STR suggest late Cambrian-early Ordovician ages (~495-460 Ma), results from the E-STR suggests a much younger early Carboniferous (Mississippian) age range (325-357 Ma). The data allow temporal comparisons to be made between existing 40Ar/39Ar and K-Ar datasets obtained from S-SE Australia and NVL, and results obtained here from the thermochronologically less studied offshore STR region. The data support reconstructions, which indicate that W-STR shared provenance with Wilson Terrane prior to the continental breakup, and furthermore, refine its paleo-position. Based on the age pattern as well as petrological evidence, it is proposed that it was most likely situated west of the Lanterman Fault Zone (LFZ) and extended further west to the western flank of the pop-up structure bounded by Wilson and Exiles Thrust. E-STR,40Ar/39Ar ages are synchronous with a phase of major granite emplacement and mineralization, which occurred in western Tasmania and are correlated with deformation post Tabberabberan Orogeny in Tasmania and the Victoria. Results from this study provide more solid time-temperature constraints for late Neoproterozoic-Cambrian subduction-related processes and the more recent evolution of the transform Tasman Fracture Zone during separation between Antarctica and Australia. The thermal history models suggest a strong correlation between rapid cooling and tectonic activity in the STR block. Since mid-Cretaceous time these can be summarized as follows: (1) onset of the opening of the Tasman Sea at ~80 Ma, (2) amalgamation of the W- and E-STR blocks and a shift in the relative motion between Australia and Antarctica and (3) final clearance of continental breakup and onset of the opening of the Tasman Gateway. This study is also reports results from the first application of apatite (U-Th-Sm)/He (AHe) thermochronometry to the Tasmanian region. The data not only provide further spatial and temperature constraints, but also examine the quality of AHe ages obtained from mafic lithologies with lower U and Th content than felsic rocks and yielding less age dispersion. Mid Jurassic (~175-180 Ma) dolerite is widely distributed across onshore Tasmania making such a study possible. The dolerite forms part of the Ferrar Group continental flood basalt (CFB) emplaced prior to eastern Gondwana breakup. Its spatial/temporal significance and chemical composition make it an ideal rock-type to aid in documenting the regional post-continental breakup history and to test the influence of different parameters such as -radiation damage and U and Th zonation on AHe age dispersion from a low eU perspective. Low-temperature thermal modelling reveals two distinct cooling episodes. (1) Mid-Cretaceous cooling, which involved km-scale denudation (~3-4 km) in response to continental extension prior to the actual seafloor spreading in the Tasman Sea. (2) Late Cretaceous-early Tertiary cooling, restricted to the west margin of Tasmania in response to transform margin tectonism to the west. In addition, no correlation could be found between AHe ages and potential factors influencing age dispersion such as radiation damage, grain size, U-Th zonation. However, zircons derived from Tasmanian dolerites having high eU suggest an effective upper dosage limit (0.2-0.31018/g) for obtaining meaningful ZHe ages when studying Tasmanian dolerites or possibly other similar mafic lithologies.
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    Petrology and geochemistry of mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): new insights into lithospheric mantle fluids
    GIULIANI, ANDREA ( 2013)
    In cratonic areas the lithospheric mantle formed and stabilised during the Archean eon and has been modified through time by multiple episodes of enrichment (i.e. metasomatism) driven by fluids and melts of variable composition. The vast majority of metasomatised mantle rocks are thought to have interacted with basic/ultrabasic and carbonate melts/fluids, variably enriched in alkali and other incompatible elements. The common occurrence of volatile-dominated fluids and brines included in diamonds and mantle silicate minerals suggests that such fluids might be widespread in the Earth’s mantle; however the metasomatising impact of these fluids still require further study. This thesis provides new textural, mineralogical and geochemical data for mantle xenoliths entrained by the Bultfontein kimberlite (Kimberley, South Africa). The studied xenoliths include two mantle polymict breccias, a Ni-mineralised spinel harzburgite and a sulphate-rich MARID (mica-amphibole-rutile-ilmenite-diopside) rock. Mantle polymict breccias are complex mixtures of mantle clasts and minerals cemented together by variable amounts of olivine, phlogopite, ilmenite, rutile, orthopyroxene and sulphides. Groundmass olivine and ilmenite host carbonate-rich inclusions dominated by magnesite, dolomite, alkali-carbonates, phlogopite and kalsilite. These inclusions probably represent an alkali-carbonate melt, which was entrapped during olivine and ilmenite crystallisation in the mantle. This is the first evidence for an alkali-carbonate fluid in the lithospheric mantle above the diamond stability field. The heterogeneous mineralogy and geochemistry of polymict breccias suggest these rocks formed immediately prior to entrainment and transport by kimberlite magmas. Polymict breccias are therefore regarded as failed kimberlite intrusions frozen at depth. The alkali-carbonate melt preserved as inclusions in olivine and ilmenite could be parental to the cementing phases of polymict breccias and could either derive from silicate-carbonate liquid immiscibility of a precursor proto-kimberlite melt or represent a pristine example of primitive kimberlite melt. The Ni-rich spinel harzburgite hosts millimeter-sized mineralised areas that include native nickel, heazlewoodite and Ni-rich silicates (e.g., olivine, phlogopite). The presence of several mineral phases enriched in alkali and volatile species (e.g., phlogopite, phosphates, carbonates, chlorides, djerfisherite) indicates that the transition metal cations were introduced during metasomatism by alkali-rich C–O–H fluids or alkali-carbonate melts. The sulphate-rich MARID sample is traversed by veins dominated by Ba-rich celestine and clinopyroxene, with minor phlogopite, pectolite, sphene, apatite, barite and Sr-Ca carbonates. Celestine hosts the other metasomatic vein phases, but also occurs as inclusions in clinopyroxene, suggesting co-precipitation of these minerals. Celestine was partly replaced by serpentine during alteration by hydrous fluids after kimberlite emplacement in the upper crust. The texture and chemical composition of the metasomatic phases indicate that the MARID rock was infiltrated by a sulphate fluid enriched in Sr, Ba, Na and Ca, with lesser P, Ti, LREE, CO2 and F. A mantle origin for the sulphate fluid is supported by: (i) comparisons between the Sr–S isotopic compositions of celestine, the host kimberlite, crustal and mantle lithologies from the area, and (ii) alteration of celestine by late-stage hydrous fluids. The celestine-bearing veins provide the first evidence for the occurrence of sulphate-dominated fluids in the Earth’s mantle. In summary this thesis provides new insights into the compositions of widespread mantle metasomatic agents, namely alkali-carbonate melts, and documents some of the metasomatic processes occurring in the lithospheric mantle during ascent of primitive or precursor kimberlite magmas. My research also provides unexpected evidence for the occurrence of previously unknown fluids in the Earth’s mantle, including Ni-rich C-O-H fluids and sulphate-dominated fluids.
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    Drivers of Southern Hemisphere climate change
    Arblaster, Julie Michelle ( 2013)
    The climate of the Southern Hemisphere (SH) has undergone significant changes over recent decades, with additional warming expected under future emission scenarios. However, while temperature increases are robust across models there is more uncertainty around changes in rainfall, atmospheric circulation and extremes, all of which have a large impact on human society and ecosystems. The objective of this thesis was to increase our confidence in future projections by considering the relative importance of various drivers of past and future climate change, with a focus on the Southern Hemisphere. These drivers include sea surface temperatures (SSTs; which may or may not be anthropogenically forced), greenhouse gases and stratospheric ozone. To achieve this objective a hierarchy of model experiments were utilised, from idealised experiments to multimodel datasets. Insights were gained by exploring both the consistencies and the spread across the model results. The main results were: 1) The internal variability of the climate system, such as the El Niño-Southern Oscillation (ENSO), impacts the emerging signals of anthropogenic climate change and characterisations of this noise were explored. Opposite phases of ENSO were found to drive marked contrasts in maximum temperature extremes, with ENSO fidelity crucial in simulating the observed relationships. These patterns are unlikely to change substantially under future climate change. Over the Australian continent, future warming leads to increases in warm temperature extremes and a propensity for longer dry spells interspersed with heavy rainfall events. In general, the magnitude of changes in both temperature and precipitation extremes indices scaled with the strength of emissions. 2) Coupled model simulations were able to reproduce the large-scale features of SH climate trends since 1950, if observed changes in anthropogenic forcings were included. However, atmospheric models driven by observed SSTs and anthropogenic forcings were unable to capture wintertime trends, suggesting either deficiencies in the modelling framework, SSTs or models themselves or that internal variability has been largely responsible for these trends. The improvement of the simulated trends in experiments with partial coupling suggests the modelling framework plays some role in this deficiency. 3) Future changes in the SH atmospheric circulation will be driven by the competing effects of greenhouse gases and stratospheric ozone recovery. Climate sensitivity was found to largely explain the difference in Southern Annular Mode (SAM) projections between two coupled climate models under identical greenhouse gas and stratospheric ozone forcing. This result extended to multimodel simulations under transient carbon dioxide (CO2) conditions in all seasons, with the stronger the warming the larger the trend in the SAM. Tropical upper tropospheric warming was found to be more relevant than polar stratospheric cooling to the model spread in SAM responses to CO2. 4) Idealised SST experiments in two models showed a consistent poleward shift of the SH wintertime westerly jet under Southern Ocean cooling. However, the austral winter response to increased tropical SSTs was found to be model dependent, with opposite latitudinal shifts in the SH westerly jet in the two models. This finding was linked to different tropical rainfall and convective atmospheric heating responses in the models to identical SST increases. These results highlight the reliability of current climate model simulations as well as some of their limitations. Potential deficiencies in forcing datasets, modelling frameworks and the simulation of internal variability were identified. Understanding and improving these deficiencies is crucial for interpreting recent observed change and understanding future projections, particularly at the regional scale.
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    On the dynamics of orographic precipitation in idealised and realistic settings
    Watson, Campbell D. ( 2013)
    This thesis explores the dynamics and sensitivities of orographic precipitation to highlight the complexity of precipitation generation in the Australian Alps. The thesis is arranged in two parts. First, the influence of variations in relatively simple terrain geometries and upstream conditions using idealised model simulations are examined. Second, observations and an ensemble of high-resolution model simulations are used to examine a heavy precipitation event in the Australian Alps. The first part simulates a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. The single sounding environment is representative of a prefrontal flow that produced heavy precipitation in the Australian Alps. A variety of simulations were conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height (H) of approximately unity. The concave ridge generated substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funnelling mechanism near the ridge vertex; however, for some concave ridge configurations the results featured dual-precipitation maxima, which is an important difference from previous findings. Results from a simple ensemble of idealised simulations elucidated the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries. The findings were expanded upon by examining how variations to H and the horizontal aspect ratio (β) of a straight ridge and a concave ridge influence orographic precipitation. It was hypothesised that when the approaching flow is blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge would be negligible; however this study revealed that when H is sufficiently large to induce flow-reversal on the windward slope, a secondary circulation develops that is strengthened by the concave ridge and precipitation is subsequently enhanced. In addition, a flow regime diagram for the straight ridge and the concave ridge was constructed to illustrate the sensitivity of the critical-H for flow regime transition to changes in the terrain geometry. The second part of the thesis examined a heavy precipitation event in the Australian Alps associated with the passage of a cold front and a prefrontal trough. Precipitation accumulated predominantly on the northwestern slopes of the Alps, and observations and results from high- resolution model simulations were used to reconstruct the event. Precipitation was produced by a variety of phenomena, including the front itself, an undular bore propagating along the stable nocturnal layer ahead of the prefrontal trough, and a mesoscale convective system. The sensitivity of a modest prefrontal precipitation event to initial condition uncertainties was then explored using an ensemble of high-resolution numerical simulations. In total, only seven of the 34 simulations produced precipitation distributions comparable to what was observed. The maximum eastward extent of the prefrontal trough and the presence of sufficient moisture upstream were found to be controlling factors. A bore was generated in every simulation suggesting the occurrence of an undular bore was highly predictable, although its position varied considerably among the members.